See J. H. Westphal,Leben, Studien, und Schriften des Astronomen Johann Hevelius(1820); C. B. Lengnich,Anekdoten und Nachrichten(1780);Allgemeine deutsche Biographie(C. Bruhns); J. B. J. Delambre,Histoire de l’astronomie moderne, ii. 471; J. F. Weidler,Historia astronomiae, p. 486; F. Baily’s edition of the Catalogue of Hevelius,Memoirs Roy. Astr. Society, xiii. (1843); R. Wolf,Geschichte der Astronomie, p. 396; J. C. Poggendorff,Biog.-lit. Handwörterbuch. For an account of the epistolary remains of Hevelius, see C. G. Hecker,Monatl. Correspondenz, viii. 30; alsoAstr. Nachrichten, vols. xxiii., xxiv.
See J. H. Westphal,Leben, Studien, und Schriften des Astronomen Johann Hevelius(1820); C. B. Lengnich,Anekdoten und Nachrichten(1780);Allgemeine deutsche Biographie(C. Bruhns); J. B. J. Delambre,Histoire de l’astronomie moderne, ii. 471; J. F. Weidler,Historia astronomiae, p. 486; F. Baily’s edition of the Catalogue of Hevelius,Memoirs Roy. Astr. Society, xiii. (1843); R. Wolf,Geschichte der Astronomie, p. 396; J. C. Poggendorff,Biog.-lit. Handwörterbuch. For an account of the epistolary remains of Hevelius, see C. G. Hecker,Monatl. Correspondenz, viii. 30; alsoAstr. Nachrichten, vols. xxiii., xxiv.
(A. M. C.)
HEWETT, SIR PRESCOTT GARDNER,Bart. (1812-1891), British surgeon, was born on the 3rd of July 1812, being the son of a Yorkshire country gentleman. He lived for some years in early life in Paris, and started on a career as an artist, but abandoned it for surgery. He entered St George’s Hospital, London (where his half-brother, Dr Cornwallis Hewett, was physician from 1825 to 1833) becoming demonstrator of anatomy and curator of the museum. He was the pupil and intimate friend of Sir B. C. Brodie, and helped him in much of his work. Eventually he rose to be anatomical lecturer, assistant-surgeon and surgeon to the hospital. In 1876 he was president of the College of Surgeons; in 1877 he was made serjeant-surgeon extraordinary to Queen Victoria, in 1884 serjeant-surgeon, and in 1883 he was created a baronet. He was a very good lecturer, but shrank from authorship; his lectures onSurgical Affections of the Headwere, however, embodied in his treatise on the subject in Holmes’sSystem of Surgery. As a surgeon he was always extremely conservative, but hesitated at no operation, however severe, when convinced of its expediency. He was a perfect operator, and one of the most trustworthy of counsellors. He died on the 19th of June 1891.
HEWITT, ABRAM STEVENS(1822-1903), American manufacturer and political leader, was born in Haverstraw, New York, on the 31st of July 1822. His father, John, a Staffordshire man, was one of a party of four mechanics who were sent by Boulton and Watt to Philadelphia about 1790 to set up a steam engine for the city water-works and who in 1793-1794 built at Belleville, N.J., the first steam engine constructed wholly in America; he made a fortune in the manufacture of furniture, but lost it by the burning of his factories. The boy’s mother was of Huguenot descent. He graduated with high rank from Columbia College in 1842, having supported himself through his course. He taught mathematics at Columbia, and in 1845 was admitted to the bar, but, owing to defective eyesight, never practised. With Edward Cooper (son of Peter Cooper, whom Hewitt greatly assisted in organizing Cooper Union, and whose daughter he married) he went into the manufacture of iron girders and beams under the firm name of Cooper, Hewitt & Co. His study of the making of gun-barrel iron in England enabled him to be of great assistance to the United States government during the Civil War, when he refused any profit on such orders. The men in his works never struck—indeed in 1873-1878 his plant was run at an annual loss of $100,000. In politics he was a Democrat. In 1871 he was prominent in the re-organization of Tammany after the fall of the “Tweed Ring”; from 1875 until the end of 1886 (except in 1879-1881) he was a representative in Congress; in 1876 he left Tammany for the County Democracy; in the Hayes-Tilden campaign of that year he was chairman of the Democratic National Committee, and in Congress he was one of the House members of the joint committee which drew up the famous Electoral Count Act providing for the Electoral Commission. In 1886 he was elected mayor of New York City, his nomination having been forced upon the Democratic Party by the strength of the other nominees, Henry George and Theodore Roosevelt; his administration (1887-1888) was thoroughly efficient and creditable, but he broke with Tammany, was not renominated, ran independently for re-election, and was defeated. In 1896 and 1900 he voted the Republican ticket, but did not ally himself with the organization. He died in New York City on the 18th of January 1903. In Congress he was a consistent defender of sound money and civil service reform; in municipal politics he was in favour of business administrations and opposed to partisan nominations. He was a leader of those who contended for reform in municipal government, was conspicuous for his public spirit, and exerted a great influence for good not only in New York City but in the state and nation. His most famous speech was that made at the opening of the Brooklyn Bridge in 1883. He was a terse, able and lucid speaker, master of wit and sarcasm, and a fearless critic. He gave liberally to Cooper Union, of which he was trustee and secretary, and which owes much of its success to him; was a trustee of Columbia University from 1901 until his death, chairman of the board of trustees of Barnard College, and was one of the original trustees, first chairman of the board of trustees, and a member of the executive committee of the Carnegie Institution.
HEWLETT, MAURICE HENRY(1861- ), English novelist, was born on the 22nd of January 1861, the eldest son of Henry Gay Hewlett, of Shaw Hall, Addington, Kent. He was educated at the London International College, Spring Grove, Isleworth, and was called to the bar in 1891. From 1896 to 1900 he was keeper of the land revenue records and enrolments. He published in 1895 two books on Italy,Earthwork out of Tuscany, and (in verse)The Masque of Dead Florentines.Songs and Meditationsfollowed in 1897, and in 1898 he won an immediate reputation by hisForest Lovers, a romance of medieval England, full of rapid movement and passion. In the same year he printed the pastoral and pagan drama ofPan and the Young Shepherd, shortened for purposes of representation and produced at the Court Theatre in March 1905, when it was followed by theYoungest of the Angels, dramatized from a chapter in hisFool Errant. InLittle Novels of Italy(1899), a collection of brilliant short stories, he showed again his power of literary expression together with a close knowledge of medieval Italy. The new and vivid portraits of Richard Cœur de Lion in hisRichard Yea-and-Nay(1900), and of Mary, queen of Scots, inThe Queen’s Quair(1904) showed the combination of fiction with real history at its best.The New Canterbury Tales(1901) was another volume of stories of English life, but he returned to Italian subjects withThe Road in Tuscany(1904); inFond Adventures, Tales of the Youth of the World(1905), two are Italian tales, andThe Fool Errant(1905) purports to be the memoirs of Francis Antony Stretley, citizen of Lucca. Later works were the novelThe Stooping Lady(1907), and a volume of poems,Artemision(1909).
HEXAMETER,the name of the earliest and most important form of classical verse in dactylic rhythm. The word is due to each line containing six feet or measures (μέτρα), the last of which must be a spondee and the penultimate a dactyl, though occasionally, for some special effect, a spondee may be allowed in the fifth foot, when the line is said to be spondaic. The four other feet may be either spondees or dactyls. All the great heroic and epic verse of the Greek and Roman poets is in this metre, of which the finest examples are to be found in Homer and in Virgil. Varied cadences and varied caesura are essential to this form of verse, otherwise the monotony is wearying to the ear. The most usual places for the caesura are at the middle of the third, or the middle of the fourth foot: the former is known as the penthemimeral and the latter as hepthemimeral caesura. There are several more or less successful examples of English poems in this metre, for example Longfellow’sEvangeline, Kingsley’sAndromedaand Clough’sBothie of Tober-na-Vuoilich, but it does not really suit the genius of the English language. In English the lack of true spondees is severely felt, even though the English metre depends, not, as in Greek and Latin, on the distinction between long and short syllables, but on that between accented and unaccented syllables. The accent must always (or it sounds very ugly) fall on the first syllable, whatever may have been the case in Greek and Latin—Voss, Klopstock and Goethe have written hexameter poemsof varying merit and the metre suits the German language distinctly better than the English. The customary form of hexameter in English verse is exemplified by Coleridge’s descriptive line:—
“In the hex | ameter | rises the | fountain’s | silvery | column.”
Several modern poets, and in particular Robert Browning, and Lord Bowen (1835-1894) have used with effect a truncated hexameter consisting of the usual verse deprived of its last syllable. Thus Browning:—
“Well, it is I gone at | last, the | palace of | music I | reared.”
It is not sufficiently observed that even the classic Greek poets introduced considerable variations into their treatment of the hexameter. These have been treated with erudition in G. Hermann’sDe aetate scriptoris Argonauticorum. The differences in the hexameters of the Latin poets were not so remarkable, but even these varied, in various epochs, their treatment of the separate feet, and the position of the caesura. The satirists in particular allowed themselves an extraordinary licence: these hexameters, from Persius, are as far removed from the rhythm of Homer, or even of Virgil, as possible, if they are to remain hexameters:—
“Mane piger stertis. ‘Surge!’ inquit Avaritia, ‘heiaSurge!’ negas; instat ‘Surge!’ inquit ‘Non queo.’ ‘Surge!’‘Et quid agam?’ ‘Rogitas? en saperdam advehe Ponto.’”
“Mane piger stertis. ‘Surge!’ inquit Avaritia, ‘heia
Surge!’ negas; instat ‘Surge!’ inquit ‘Non queo.’ ‘Surge!’
‘Et quid agam?’ ‘Rogitas? en saperdam advehe Ponto.’”
It is also to be noted that various prosodical liberties, due originally to the extreme antiquity of the hexameter, and long reformed and repressed by the culture of poets, were apt to be revived in later ages, by writers who slavishly copied the most antique examples of the art of verse.
See Wilhelm Christ,Metrik der Griechen und Römer, 2te Aufl. (1879).
See Wilhelm Christ,Metrik der Griechen und Römer, 2te Aufl. (1879).
HEXAPLA(Gr. for “sixfold”), the term for an edition of the Bible in six versions, and especially the edition of the Old Testament compiled by Origen, which placed side by side (1) Hebrew, (2) Hebrew in Greek character, (3) Aquila, (4) Symmachus, (5) Septuagint, (6) Theodotion. SeeBible:Old Testament, Texts and Versions.
HEXAPODA(Gr.ἕξ, six, andπούς, foot), a term used in systematic zoology for that class of theArthropoda, popularly known as insects. Linnaeus in hisSystema naturae(1735) grouped under the class Insecta all segmented animals with firm exoskeleton and jointed limbs—that is to say, the insects, centipedes, millipedes, crustaceans, spiders, scorpions and their allies. This assemblage is now generally regarded as a great division (phylum or sub-phylum) of the animal kingdom and known by K. T. E. von Siebold’s (1848) name of Arthropoda. For the class of the true insects included in this phylum, Linnaeus’s old term Insecta, first used in a restricted sense by M. J. Brisson (1756), is still adopted by many zoologists, while others prefer the name Hexapoda, first used systematically in its modern sense by P. A. Latreille in 1825 (Familles naturelles du règne animal), since it has the advantage of expressing, in a single word, an important characteristic of the group. The terms “Hexapoda” and “hexapod” had already been used by F. Willughby, J. Ray and others in the late 17th century to include the active larvae of beetles, as well as bugs, lice, fleas and other insects with undeveloped wings.
Characters.
A true insect, or member of the class Hexapoda, may be known by the grouping of its body-segments in three distinct regions—a head, a thorax and an abdomen—each of which consists of a definite number of segments. In the terminology proposed by E. R. Lankester the arrangement is “nomomeristic” and “nomotagmic.” The head of an insect carries usually four pairs of conspicuous appendages—feelers, mandibles and two pairs of maxillae, so that the presence of four primitive somites is immediately evident. The compound eyes of insects resemble so closely the similar organs in Crustaceans that there can hardly be reasonable doubt of their homology, and the primitively appendicular nature of the eyes in the latter class suggests that in the Hexapoda also they represent the appendages of an anterior (protocerebral) segment. Behind the antennal (or deutocerebral) segment an “intercalary” or tritocerebral segment has been demonstrated by W. M. Wheeler (1893) and others in various insect embryos, while in the lowest insect order—the Aptera—a pair of minute jaws—the maxillulae—in close association with the tongue are present, as has been shown by H. J. Hansen (1893) and J. W. Folsom (1900). Distinct vestiges of the maxillulae exist also in the earwigs and booklice, according to G. Enderlein and C. Börner (1904), and they are very evident in larval may-flies. The number of limb-bearing somites in the insectan head is thus seen to be seven. All of these are to be regarded as primitively post-oral, but in the course of development the mouth moves back to the mandibular segment, so that the first three somites—ocular, antennal and intercalary—lie in front of it. In Lankester’s terminology, therefore, the head of an insect is “triprosthomerous.” The maxillae of the hinder pair become more or less fused together to form a “lower lip” or labium, and the segment of these appendages is, in some insects, only imperfectly united with the head-capsule.
The thorax is composed of three segments; each bears a pair of jointed legs, and in the vast majority of insects the two hindmost bear each a pair of wings. From these three pairs of thoracic legs comes the name—Hexapoda—which distinguishes the class. And the wings, though not always present, are highly characteristic of the Hexapoda, since no other group of the Arthropoda has acquired the power of flight. In the more generalized insects the abdomen evidently consists of ten segments, the hindmost of which often carries a pair of tail-feelers, (cerci or cercopods) and a terminal anal segment. In some cases, however, it can be shown that the cerci really belong to an eleventh abdominal segment which usually becomes fused with the tenth. With very few exceptions the abdomen is without locomotor limbs. Paired processes on the eighth and ninth abdominal segments may be specialized as external organs of reproduction, but these are probably not appendages. The female genital opening usually lies in front of the eighth abdominal segment, the male duct opens on the ninth.
In all main points of their internal structure the Hexapoda agree with other Arthropoda. Specially characteristic of the class, however, is the presence of a complex system of air-tubes (tracheae) for respiration, usually opening to the exterior by a series of paired spiracles on certain of the body segments. The possession of a variable number of excretory tubes (Malpighian tubes), which are developed as outgrowths of the hind-gut and pour their excretion into the intestine, is also a distinctive character of the Hexapoda.
The wings of insects are, in all cases, developed after hatching, the younger stages being wingless, and often unlike the parent in other respects. In such cases the development of wings and the attainment of the adult form depend upon a more or less profound transformation or metamorphosis.
With this brief summary of the essential characters of the Hexapoda, we may pass to a more detailed account of their structure.
ExoskeletonThe outer cellular layer (ectoderm or “hypodermis”) of insects as of other Arthropods, secretes a chitinous cuticle which has to be periodically shed and renewed during the growth of the animal. The regions of this cuticle have a markedly segmental arrangement, and the definite hardened pieces (sclerites) of the exoskeleton are in close contact with one another along linear sutures, or are united by regions of the cuticle which are less chitinous and more membranous, so as to permit freedom of movement.Head.—The head-capsule of an insect (figs. 1, 2) is composed of a number of sclerites firmly sutured together, so that the primitive segmentation is masked. Above is the crown (vertexorepicranium), on which or on the “front” may be seated three simple eyes (ocelli). Below this comes the front, and then the face or clypeus, to which a very distinct upper lip (labrum) is usually jointed. Behind the labrum arises a process—theepipharynx—which in some blood-sucking insects becomes a formidable piercing-organ. On either side a variable amount of convex area is occupied by the compound eye; in many insects of acute sense and accurate flight these eyes are very large and sub-globular, almost meeting on the middle line of thehead. Below each eye is a cheek area (gena), often divided into an anterior and a posterior part, while a distinct chin-sclerite (gula) is often developed behind the mouth.From Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 1.—Head and Jaws of Cockroach (Blatta). Magnified 10 times. A, Front; B, side; C, back;v, vertex;f, frons;cl, clypeus;lbr, labrum;oc, compound eye;ge, gena;mn, mandible;ca,st,pa,ga,la, cardo, stipes, palp, galea, lacinia of first maxilla;sm,m,pa″,pg, sub-mentum, mentum, palp, galea of 2nd maxilla.Feelers.—Most conspicuous among the appendages of the head are the feelers or antennae, which correspond to the anterior feelers (antennules) of Crustacea. In their simpler condition they are long and many-jointed, the segments bearing numerous olfactory and tactile nerve-endings. Elaboration in the form of the feelers, often a secondary sexual character in male insects, may result from a distal broadening of the segments, so that the appendage becomes serrate, or from the development of processes bearing sensory organs, so that the structure is pinnate or feather-like. On the other hand, the number of segments may be reduced, certain of them often becoming highly modified in form.After Marlatt,Entom. Bull.14, n. s. (U.S. Dept. Agric.).Fig. 2.—Head of Cicad, front view. Ia, frons;b, clypeus (the pointed labrum beneath it); II, mandible; III, first maxilla; (a, base;b, sheath;c, piercer), III′, inner view of sheath; IV, second maxillae forming rostrum (b, mentum;c, ligula).Jaws.—The mandibles of the Hexapoda are usually strong jaws with one or more teeth at the apex (fig. 1, A, B,mn), articulating at their bases with the head-capsule by sub-globular condyles, and provided with abductor and adductor muscles by means of which they can be separated or drawn together so as to bite solid food, or seize objects which have to be carried about. They never bear segmented limbs (palps) and only exceptionally (as in the chafers) is the skeleton composed of more than one sclerite. The mandibles often furnish a good example of “secondary sexual characters,” being more strongly developed in the male than in the female of the same species. In most insects that feed by suction the mandibles are modified. In bugs (Heteroptera) and many flies, for example, they are changed into needle-like piercers (fig. 2, II), while in moths and caddis-flies they are reduced to mere vestiges or altogether suppressed.As previously mentioned, a pair of minute jaws—themaxillulae—are present in the lowest order of insects, between the mandibles and the first maxillae. They usually consist of an inner and an outer lobe arising from a basal piece, which bears also in some genera a small palp (seeAptera).In their typical state of development, thefirst maxillaeoffer a striking contrast to the mandibles, being composed of a two-segmented basal piece (cardoandstipes, fig. 1, C,ca,st) bearing a distinct inner and outer lobe (laciniaandgalea, fig. 1, C,la,ga) and externally a jointed limb or palp (fig. 1, C,pa). Such maxillae are found in most biting insects. In insects whose mouths are adapted for sucking and piercing, remarkable modifications may occur. In many blood-sucking flies, for example, the galea is absent, while the lacinia becomes a strong knife-like piercer and the palp is well developed. In bugs and aphids the lacinia is a slender needle-like piercer (fig. 2, III), while the palp is wanting. In butterflies and moths the lacinia is absent while the galea becomes a flexible process, grooved on its inner face, so as to make with its fellow a hollow sucking-trunk, and the palp is usually very small.Thesecond pair of maxillaeare more or less completely fused together to form what is known as thelabiumor “lower lip.” In generalized biting insects, such as cockroaches and locusts (Orthoptera), the parts of a typical maxilla can be easily recognized in the labium. The fused cardines form a broad basal plate (sub-mentum) and the stipites a smaller plate (mentum)—see fig. 1, C,sm,m—jointed on to the sub-mentum, while the galeae, laciniae and palps remain distinct. In specialized biting insects, such as beetles (Coleoptera), the labium tends to become a hard transverse plate bearing the pair of palps, a median structure—known as theligula—formed of the conjoined laciniae, and a pair of small rounded processes—the reduced galeae—often called the “paraglossae,” a term better avoided since it has been applied also to the maxillulae of Aptera, entirely different structures. The long sucking “tongue” of bees is probably a modification of the ligula. In bugs and aphids (Hemiptera), the fused second maxillae form a jointed grooved beak or rostrum (fig. 2, IV) in which the slender piercers (mandibles and first maxillae) work to and fro.This second pair of maxillae (or labium) form then the hinder or lower boundary of the mouth. In front or above the mouth is bounded by the labrum, while the mandibles and first maxillae lie on either side of it. A median process, known as thehypopharynxor tongue, arises from the floor of the mouth in front of the labium, and becomes most variously developed or specialized in different insects. The salivary duct opens on its hinder surface. It does not appear to represent a pair of appendages, but the maxillulae of the Aptera become closely associated with it. According to the view of R. Heymons, the hypopharynx represents the sterna of all the jaw-bearing somites, but other students consider that it belongs to the mandibular and first maxillary segments, or entirely to the segment of the first maxillae.Neck.—The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (cervical sclerites). These have been interpreted as indicating one or more primitive segments between the head and thorax. Probably, however, as suggested by T. H. Huxley (Anat. Invert. Animals, 1877), they really belong to the labial segment which has not become completely fused with the head-capsule. It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J. H. Comstock and C. Kochi (1902) and W. A. Riley (1904).Thorax.—The three segments which make up the thorax or fore-trunk are known as theprothorax,mesothoraxandmetathorax(see fig. 3). The dorsal area of the prothorax is occupied by a single sclerite, thepronotum(fig. 3,d), which is large and conspicuous in those insects, such as cockroaches, bugs (Heteroptera) and beetles, which have the prothorax free—i.e.readily movable on the segment (mesothorax) immediately behind—smaller and of less importance where the prothorax is fixed to the mesothorax, as in bees and flies. The dorsal area of the mesothorax, and also of the metathorax, may be made up of a series of sclerites arranged one behind the other—prescutum,scutum,scutellumandpost-scutellum(fig. 3,e,f,g,h), the scutellum of the mesothorax being often especially conspicuous. Ventrally, each segment of the thorax has asternumwith which a medianpre-sternumand pairedepisternaandepimeraare often associated (see figs. 3, 4). The recent suggestion of K. W. Verhoeff (1904) that the hexapodan thorax in reality contains six primitive segments is entirely without embryological support.Legs.—Each segment of the thorax carries a pair of legs. In most insects the leg is built up of nine segments: (1) a broad triangular, sub-globular, conical or cylindrical haunch (coxa); (2) a smalltrochanter; (3) an elongate stout thigh (femur); (4) a more slender shin (tibia); and (5-9) a foot consisting of fivetarsal segments. The fifth (distal) tarsal segment carries a median adhesive pad—thepulvillus—on either side of which is a claw. The pulvillus isprobably to be regarded as a true terminal (tenth) segment of the leg, while the claws are highly modified bristles. Numerous bristles are usually present on the thighs, shins and feet of insects, some of them so delicate as to be termed “hairs,” others so stout and hard that they are named “spines” or “spurs.” In the relative development and shape of the various segments of the leg there is almost endless variety, dependent on the order to which the insect belongs, and the special function—walking, running, climbing, digging or swimming—for which the limb is adapted. The walking of insects has been carefully studied by V. Graber (1877) and J. Demoor (1890), who find that the legs are usually moved in two sets of three, the first and third legs of one side moving with the second leg of the other. One tripod thus affords a firm base of support while the legs of the other tripod are brought forward to their new positions.After Marlat,Ent. Bull.3, n.s. (U.S. Dept. Agr.).Fig. 3.—Thorax of Saw-Fly (Pachynematus).I, Dorsal view.II, Ventral view.III, Lateral view.IV, Lateral view with segments separated.Prothorax:a, Episternum.b, Sternum.c, Coxa of fore-leg.d, Pronotum.Mesothorax:e, Prescutum.f, Scutum.g, Scutellum.h, Post-scutellum.i, Mesophragma.j,Epimeron.k,Episternum.l, Coxa of middle leg.Metathorax:m, Scutum.o, Epimeron.p, Coxa of hind leg.n,First Abdominal Segment.t, Tegula at base of fore-wing.After Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 4.—Legs and Ventral Thoracic Sclerites of Female Cockroach (Blatta).I, Fore-leg and pro-sternum (S) in front of which are the ventral cervical sclerites (c).cx, Coxa.tr, Trochanter.fe, Thigh.tb, Shin.ta, Tarsal segments.II, Middle leg and mesosternum.III, Hind-leg and metasternum.In IIIa, the episternum (a) and epimeron (b) are slightly separated.Wings.—Two pairs of wings are present in the vast majority of insects, borne respectively on the mesothorax and metathorax. At the base of the wing,i.e.its attachment to the trunk, we find a highly complex series of small sclerites adapted for the varied movements necessary for flight. Those of the dragon-flies (Odonata) have been described in detail by R. von Lendenfeld (1881). The long axis of the wings, when at rest, lies parallel to the body axis. In this position the outer margin of the wing is thecosta, the inner thedorsum, and the hind-margin thetermen. The angle between the costa and termen is theapex. When the wing is spread, its long axis is more or less at a right angle to the body axis. A wing is an outgrowth from the dorsal and pleural regions of the thoracic segment that bears it, and microscopic examination shows it to consist of a double layer of cuticularized skin, the two layers being in contact except where they are thickened and folded to form the firm tubular nervures, which serve as a supporting framework for the wing membrane, enclose air-tubes, and convey blood. These nervures consist of a series of trunks radiating from the wing-base and usually branching as they approach the wing-margins, the branches being often connected by short transverse nervures, so that the wing-area is marked off into a number of “cells” or areolets.After Quail,Natural Science, vol. xiii., J. M. Dent & Co.Fig. 5.—Wing-Neuration in a Cossid Moth. 2, sub-costal; 3, radial; 4, median; 5, cubital; 6, 7, 8, anal nervures.The details of the nervuration vary greatly in the different orders, but J. H. Comstock and J. G. Needham have lately (1898-1899) shown that a common arrangement underlies all, six series of longitudinal or radiating nervures being present in the typical wing (see fig. 5). Along the costa runs a costal nervure. This is followed by a sub-costal which sometimes shows two main branches. Then comes the radial—usually the most important nervure of the wing—typically with five branches, and the median with four. These sets arise from a main trunk towards the front region of the wing-base. From another hinder trunk arise the two-branched cubital nervure and three separate anal nervures. In the hind-wing of many insects the number of radial branches becomes reduced, while the anal area is especially well developed and undergoes a fan-like folding when the wings are closed. Great diversity exists in the texture and functions of fore and hind-wings in different insects; these differences are discussed in the descriptions of the various orders. The wings often afford secondary sexual characters, being not infrequently absent or reduced in the female when well developed in the male (see fig. 6). Rarely the male is the wingless sex.In addition to the wings there are smaller dorsal outgrowths of the thorax in many insects. Paired erectile plates (patagia) are borne on the prothorax in moths, while in moths, sawflies, wasps, bees and other insects there are small plates (tegulae)—see Fig. 3,t—on the mesothorax at the base of the fore-wings.Abdomen.—In the abdominal exoskeleton the segmental structure is very clearly marked, a series of sclerites—dorsal terga and abdominal sterna—being connected by pale, feebly chitinized cuticle, so that considerable freedom of movement between the segments is possible. The first and second abdominal sterna are often suppressed or reduced, on account of the strong development of the hind-legs. In many insects ten, and in a few eleven, abdominal segments can be clearly distinguished in addition to a small terminal anal segment. The female genital opening usually lies between the seventh and eighth segments, the male on the ninth. Prominent paired limbs are often borne on the tenth segment, the elongate tail-feelers (cerci) of bristle-tails and may-flies, or the forceps of earwigs, for example. In the Embiidae, a family of Isoptera, it has been shown by G. Enderlein (1901) that these cerci clearly belong to a partially suppressed eleventh segment, and R. Heymons (1895-1896) has proved by embryological study that in all cases they really belong to this eleventh segment, which in the course of development becomes fused with the tenth. Smaller appendages (such as the stylets of male cockroaches) may be carried on the ninth segment. Pairs of processes carried on the eighth and ninth segments often become specialized to form the ovipositor of the female (see fig. 14) and the genital armature of the male. A marked modification of the hinder abdominal segments may be noticed in most insects,the sclerites of the eighth and ninth being frequently hidden by those of the seventh. In the higher orders several of the hinder segments may be altogether suppressed.From Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 6.—Outline of Male (♂) and Female (♀) Cockroaches (Blatta) from the side, showing Abdominal Segments (numbered 1-10).Internal OrgansFrom Miall and Denny (after Newton),The Cockroach, Lovell Reeve & Co.Fig. 7.—Brain of Cockroach from side.oe, Gullet;op, optic nerve;sb, sub-oesophageal ganglion;mn,mx,mx′, nerves to jaws;t, tentorium.Nervous System.—The nervous system in the Hexapoda is built up on the typical arthropodan plan of a double ventral nerve-cord with a pair of ganglia in each segment, the cords passing on either side of the gullet and connecting with an anterior nerve-centre or brain (fig. 7) in the head. The brain innervates the eyes and feelers, and must be regarded as a “syncerebrum” representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes. Behind the gullet lies the sub-oesophageal nerve-centre (fig. 7,sb), composed of the ganglia of the four hinder head-somites and sending nerves to the jaws. A pair of ganglia in each thoracic segment is usual (fig. 8), and as many as eight distinct pairs of abdominal ganglia may often be distinguished, the hindmost of which represents the fused ganglia of the last four segments. But in many highly organized insects a remarkable concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax. The legs, wings and other organs of the trunk receive their nerves from the thoracic and abdominal ganglia, and the fusion of several pairs of these ganglia may be regarded as corresponding to a centralization of individuality. A special “sympathetic” system arises by paired nerves from the oesophageal connectives; these nerves unite, and send back a median recurrent nerve associated with ganglia on the gullet and crop, whence proceed cords to various parts of the digestive system.In connexion with the central nervous system there are usually numerous organs of special sense. Most insects possess a pair of compound eyes, and many have, in addition, three simple eyes or ocelli on the vertex. The nature of these organs is described in the articleArthropoda. The surface of a compound eye is seen to be covered with a large number of hexagonal corneal facets, each of which overlies an ommatidium or series of cell elements (fig. 9, A, B). There are over 25,000 ommatidia in the eye of a hawk moth.After Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 8.—Ventral Muscles and Nerve Cord of Cockroach.Auditory organs of a simple type are present in most insects. These consist of fine rods suspended between two points of the cuticle, and connected with nerve-fibres; they are known as chordotonal organs. In many cases a more complex ear is developed, which may be situated in strangely diverse regions of the insect’s body. In locusts (Acridiidae) a large ovate, tympanic membrane (fig. 9, G) is conspicuous on either side of the first abdominal segment; on the inner surface of this membrane are two horn-like processes in contact with a delicate sac containing fluid, connected with which are the actual nerve-endings. In the nearly-related crickets and long-horned grasshoppers (Locustidae) the ears are situated in the shins of the fore-legs (see fig. 9, F). Just below the knee-joint there is a swelling, along which two narrow slits run lengthwise. They lead into chambers, formed by inpushing of the cuticle, whose delicate inner walls are in contact with air-tubes; on the outer surface of these latter are ridges, along which the special nerve-endings are arranged. An ear of another type is found in the swollen second segment of the feeler in many male gnats and midges, the cuticle between this segment and the third forming an annular drum which is connected with numerous nerve-endings, while the fine bristles on the more distal segments vibrate in response to the note produced by the humming of the female.From Ridley,Insect Life, vol. 7 (U.S. Dept. Agr.).Fig. 9.—Single Ommatidium of Cockroach’s Eye (after Grenacher). B, Section through compound eye (after Miall and Denny); C, organs of smell in cockchafer (after Kraepelin); D,a,b, sensory pits on cercopods of golden-eye fly;c, sensory pit on palp of stone-fly (after Packard); E, sensory hair (after Miall and Denny); F, ear of long-horned grasshopper;a, Front shin showing outer opening and air-tube;b, section (after Graber); G, ear of locust from within (after Graber). All highly magnified.Many of the numerous hairs (fig. 9, E) that cover the body of an insect have a tactile function. The sense of smell resides chiefly in the feelers, on whose segments occur tiny pits, often guarded by peg-like or tooth-like structures and containing rod-like cells (fig. 9, C) in connexion with large nerve-cells. It is said that 13,000 such olfactory organs are present on the feeler of a wasp, and 40,000 on the complex antennae of a male cockchafer. Organs of similar type on the maxillae and epipharynx appear to exercise the function of taste.After Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 10.—Dorsal Muscles, Heart and Pericardial Tendons of Cockroach.Muscular System.—The muscles in the Hexapoda are striated, as in Arthropods generally, the large fibres being associated in bundles which are attached from point to point of the cuticle, so as to move adjacent sclerites with respect to one another (see figs. 8, 10). For example, the contraction of the tergo-sternal muscles, connecting the dorsal with the ventral sclerites of the abdomen, lessens the capacity of the abdominal region, while the contraction of the powerful muscles arising from the thoracic walls, and inserted into the proximal ends of the thighs, flexes or extends the legs.Circulatory System.—Insects afford an excellent illustration of the remarkable type of blood-system characterizing the Arthropoda. The dorsal vessel is an elongate tube, whose abdominal portion is usually chambered, forming a contractile heart (fig. 10). At the constrictions between the chambers are paired slits, through which the blood passes from the surrounding pericardial sinus. The dorsal vessel is prolonged anteriorly into an aorta, through which the blood is propelled into the great body-cavity or haemocoel. After bathing the various tissues and organs, the blood returns dorsalwards into the pericardial sinus through fine perforations of its floor, and so makes its way into the heart again. Some water-bugs,e.g.of the familiesBelostomatidae,Nepidae,CorixidaeandHydrometridaehave a pulsating sac at each knee-joint to assist the flow of blood through the legs, while in dragon-flies and locusts (Acridiidae) there is a ventral pulsating diaphragm, which forms the roof of a sinus enclosing the nerve-cords.After Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 11.—Ventral Portion of Air-Tubes in Cockroach.Respiratory System.—As mentioned above, respiration by means of air-tubes (tracheae) is a most characteristic feature of the Hexapoda. An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and a chitinous layer internally, the last-named being continuous with the outer cuticle. The chitinous layer is usually strengthened by thread-like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube. The tracheal system in Hexapods is very complex, forming a series of longitudinal trunks with transverse anastomosing connexions (fig. 11), and extending by the finest sub-division and by repeated branching into all parts of the body. In insects of active flight the tubes swell out into numerous air-sacs, by which the breathing capacity is much increased.Atmospheric air gains access to the air-tubes through pairedspiraclesorstigmata, which usually occur laterally on most of the body-segments. These spiracles have firm chitinous edges, and can be closed by valves moved by special muscles. When the spiracles are open and the body contracts, air is expired. The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place. The physiology of respiration has been carefully studied by F. Plateau (1884). In aquatic insects various devices for obtaining or entangling air are found; these modifications are described in the special articles on the various orders of insects (Coleoptera,Hemiptera, &c.). Many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills. These modifications are mentioned below in the section on metamorphosis.From Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 12.—Food Canal of Cockroach.s, Salivary glands and reservoir.c, Crop (the gizzard below it).coe, Caecal tubes (below them the stomach).k, Kidney tubes.i, Intestine.r, Rectum.Digestive System.—A striking feature in the food-canal of the Hexapoda, as in other Arthropods, is the great extent of the “fore-gut” and “hind-gut,” lined with a chitinous cuticle, continuous with the exoskeleton. The fore-gut is composed of a tubular gullet, a large sac-like crop (fig. 12,c) and a proventriculus or “gizzard,” whose function is to strain the food-substances before they pass on into the tubular stomach, which has no chitinous lining. This organ, usually regarded as a “mid-gut,” gives off a number of secretory caecal tubes (fig. 12,coe). At its hinder end it is continuous with the hind-gut, which is usually differentiated into a tubular coiled intestine (fig. 12,i) and a swollen rectum (fig. 12,r). From the fore-end of the hind-gut arise the slender Malpighian tubes (fig. 12,k), which have a renal function.On either side of the gullet are from one to ten pairs of salivary glands (fig. 12,s) whose ducts open into the mouth. Some of these glands may be modified for special purposes—as silk-producing glands in caterpillars or as poison-glands in blood-sucking flies and bugs. The food passing into the crop is there acted on by the saliva and also by an acid gastric juice which passes forwards from the stomach through the proventriculus. As the various portions of the food undergo digestion, they are allowed to pass through the proventriculus into the stomach, where the nutrient substances are absorbed.Excretory System.—Nitrogenous waste-matter is removed from the body by the Malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. These tubes vary in number from four to over a hundred in different orders of insects. The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. This cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion. The fat-body is therefore the seat of important metabolic processes in the hexapod body.Reproductive System.—All the Hexapoda are of separate sexes. The ovaries (fig. 13) in the female are paired, each ovary consisting of a variable number of tubes (one in the bristle-tailCampodeaand fifteen hundred in a queen termite) in which the eggs are developed. From each ovary an oviduct (fig. 13,od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. The vagina usually opens in front of the eighth abdominal sternite. Behind it is situated a spermatheca (fig. 14,sp)and the ovipositor previously mentioned, with its three pairs of processes (Fig. 14, G,g).From Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 13.—Ovaries of Cockroach, with OviductsOdand Colleterial GlandsCG.From Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 14.—Hinder Abdominal Segment and Ovipositor of Female Cockroach. Magnified.T8&c. Tergites.S7, 7th Sternite.S8, Sclerite between 7th and 8th sterna.S9, 8th Sclerite.Od, Vagina.sp, Spermatheca.G, Anterior, andg, posterior gonapophyses.The paired testes of the male consist of a variable number of seminal tubes, those of each testis opening into avas deferens. In some bristle-tails and may-flies, the twovasa deferentiaopen separately, but usually they lead into a sperm-reservoir, whence issues a median ejaculatory duet. The male opening is on the ninth abdominal segment, to which belong the processes that form the claspers or genital armature. Accessory glands are commonly present in connexion both with the male and the female reproductive organs. The poison-glands of the sting in wasps and bees are well-known examples of these.EmbryologyThe Egg.—Among the Hexapoda, as in Arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo. Most insect eggs are of an elongate oval shape; some are globular, others flattened, while others again are flask-shaped, and the outer envelope (chorion) is often beautifully sculptured (figs. 20,d; 21,a,b). Various devices are adopted for the protection of the eggs from mechanical injury or from the attacks of enemies, and for fixing them in appropriate situations. For example, the egg may be raised above the surface on which it is laid by an elongate stalk; the eggs may be protected by a secretion, which in some cases forms a hard protective capsule or “purse”; or they may be covered with shed hairs of the mother, while among water-insects a gelatinous envelope, often of rope-like form, is common. In various groups of the Hexapoda—aphids and some flesh-flies (Sarcophaga), for example—the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be “viviparous.”Parthenogenesis.—A number of cases are known among the Hexapoda of the development of young from the eggs of virgin females. In insects so widely separated as bristle-tails and moths this occurs occasionally. In certain gall-flies (Cynipidae) no males are known to exist at all, and the species seems to be preserved entirely by successive parthenogenetic generations. In other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations. The offspring of the virgin females are in most of these instances females; but among the bees and wasps parthenogenesis occurs normally and always results in the development of males, the “queen” insect laying either a fertilized or unfertilized egg at will.Maturation, Fertilization and Segmentation.—Polar bodies were first observed in the eggs of Hexapoda by F. Blochmann in 1887. The two nuclei are successively divided from the egg nucleus in the usual way, but they frequently become absorbed in the peripheral protoplasm instead of being extruded from the egg-cell altogether. It appears that in parthenogenetic eggs two polar nuclei are formed. According to A. Petrunkevich (1901-1903), the second polar nucleus uniting with one daughter-nucleus of the first polar body gives rise to the germ-cells of the parthenogenetically-produced male. There is no reunion of the second polar nucleus with the female pronucleus, but, according to the recent work of L. Doncaster (1906-1907) on the eggs of sawflies, the number of chromosomes is not reduced in parthenogenetic egg-nuclei, while, in eggs capable of fertilization, the usual reduction-divisions occur. Fertilization takes place as the egg is laid, the spermatozoa being ejected from the spermatheca of the female and making their way to the protoplasm of the egg through openings (micropyles) in its firm envelope. The segmentation of the fertilized nucleus results in the formation of a number of nuclei which arrange themselves around the periphery of the egg and, the protoplasm surrounding them becoming constricted, a blastoderm or layer of cells, enclosing the central yolk, is formed. Within the yolk the nuclei of some “yolk cells” can be distinguished.From Nussbaum in Miall and Denny’s,The Cockroach, Lovell, Reeve & Co.Fig. 15.—Diagram showing Formation of Germinal Layers. E, ectoderm; M, inner layer. Magnified.Germinal Layers and Food-Canal.—The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15). In some cases the inner layer is formed not by invagination but by proliferation or by delamination. The outer of these two layers (fig. 15, E) is the ectoderm. With regard to the inner layer (endoblastof some authors, fig. 15, M) much difference of opinion has prevailed. It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula. No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it. Further, there is general agreement that the chitin-lined fore-gut and hind-gut, which formthe greater part of the digestive tract, arise from ectodermal invaginations (stomodaeum and proctodaeum respectively) at the positions of the future mouth and anus. The origin of the mid-gut (mesenteron), that has no chitinous lining in the developed insect, is the disputed point. According to the classical researches of A. Kowalevsky (1871 and 1887) on the embryology of the water-beetleHydrophilusand of the muscid flies, an anterior and a posterior endoderm-rudiment both derived from the “endoblast” become apparent at an early stage, in close association with the stomodaeum and the proctodaeum respectively. These two endoderm-rudiments ultimately grow together and give rise to the epithelium of the mid-gut. These results were confirmed by the observations of K. Heider and W. M. Wheeler (1889) on the embryos of two beetles—HydrophilusandDoryphorarespectively. V. Graber, however (1889), stated that in theMuscidae, while the anterior endoderm-rudiment arises as Kowalevsky had observed, the posterior part of the “mid-gut” has its origin as a direct outgrowth from the proctodaeum. The recent researches of R. Heymons (1895) on the Orthoptera, and of A. Lécaillon (1898) on various leaf beetles, tend to show that the whole of the “mid-gut” arises from the proliferation of cells at the extremity of the stomodaeum and of the proctodaeum. On this view the entire food-canal in most Hexapoda must be regarded as of ectodermal origin, the “endoblast” represents mesoderm only, and the median furrow whence it arises can be no longer compared with the blastopore. According to Heymons, the yolk-cells must be regarded as the true endoderm in the hexapod embryo, for he states (1897) that in the bristle-tailLepismaand in dragon-flies they give rise to the mid-gut. These views are not, however, supported by other recent observers. J. Carrière’s researches (1897) on the embryology of the mason bee (Chalicodoma) agree entirely with the interpretations of Kowalevsky and Heider, and so on the whole do those of F. Schwangart, who has studied (1904) the embryonic development of Lepidoptera. He finds that the endoderm arises from an anterior and a posterior rudiment derived from the “endoblast,” that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901), after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. On the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some Hexapoda the whole digestive tract may be ectodermal. It must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the “germ-layer theory.” Heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a “latent endoderm-group” in those invaginations.From Nussbaum in Miall and Denny,The Cockroach, Lovell Reeve & Co.Fig. 16.—Cross section of Embryo of German Cockroach (Phyllodromia). S, serosa; A, amnion; E, ectoderm; N, rudiment of nerve-cord; M, mesodermal pouches.Embryonic Membranes.—A remarkable feature in the embryonic development of most Hexapoda is the formation of a protective membrane analogous to the amnion of higher Vertebrates and known by the same term. Usually there arises around the edge of the germ band a double fold in the undifferentiated blastoderm, which grows over the surface of the embryo, so that its inner and outer layers become continuous, forming respectively theamnionand theserosa(fig. 16, A, S). The embryo of a moth, a dragon-fly or a bug is invaginated into the yolk at the head end, the portion of the blastoderm necessarily pushed in with it forming the amnion. The embryo thus becomes transferred to the dorsal face of the egg, but at a later stage it undergoes reversion to its original ventral position. In some parasitic Hymenoptera there is only a single embryonic membrane formed by delamination from the blastoderm, while in a few insects, including the wingless spring-tails, the embryonic membranes are vestigial or entirely wanting. In the bristle-tailsLepismaandMachilis, an interesting transitional condition of the embryonic membranes has lately been shown by Heymons. The embryo is invaginated into the yolk, but the surface edges of the blastoderm do not close over, so that a groove or pore puts the insunken space that represents the amniotic cavity into communication with the outside. Heymons believes that the “dorsal organ” in the embryos of the lower Arthropoda corresponds with the region invaginated to form the serosa of the hexapod embryo. Wheeler, however, compares with the “dorsal organ” the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopperXiphidium.Metameric Segmentation.—The segments are perceptible at a very early stage of the development as a number of transverse bands arranged in a linear sequence. The first segmentation of the ventral plate is not, however, very definite, and the segmentation does not make its appearance simultaneously throughout the whole length of the plate; the anterior parts are segmented before the posterior. In Orthoptera and Thysanura, as well as some others of the lower insects, twenty-one of these divisions—not, however, all similar—may be readily distinguished, six of which subsequently enter into the formation of the head, three going to the thorax and twelve to the abdomen. In Hemiptera only eleven and in Collembola only six abdominal segments have been detected. The first and last of these twenty-one divisions are so different from the others that they can scarcely be considered true segments.Head Segments.—In the adult insect the head is insignificant in size compared with the thorax or abdomen, but in the embryo it forms a much larger portion of the body than it does in the adult. Its composition has been the subject of prolonged difference of opinion. Formerly it was said that the head consisted of four divisions, viz. three segments and the procephalic or prae-oral lobes. It is now ascertained that the procephalic lobes consist of three divisions, so that the head must certainly be formed from at least six segments. The first of these, according to the nomenclature of Heymons (see fig. 17), is the mouth or oral piece; the second, the antennal segment; the third, the intercalary or prae-mandibular segment; while the fourth, fifth, and sixth are respectively the segments of the mandibles and of the first and second maxillae. These six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined. The labrum and clypeus are developed as a single prolongation of the oral piece, not as a pair of appendages. The antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae. The intercalary segment has no appendages, nor rudiments thereof, except, according to H. Uzel (1897), in the thysanuranCampodea, and probably entirely disappears, though J. H. Comstock and C. Kochi believe that the labrum belongs to it. The appendages of the posterior three or trophal segments become the parts of the mouth. The appendages of the two maxillary segments arise as treble instead of single projections, thus differing from other appendages. From these facts it appears that the anterior three divisions of the head differ strongly from the posterior three, which greatly resemble thoracic segments; hence it has been thought possible that the anterior divisions may represent a primitive head, to which three segments and their leg-like appendages were subsequently added to form the head as it now exists. This is, however, very doubtful, and an entirely different inference is possible. Besides the five limb-bearing somites just enumerated, two others must now be recognized in the head. One of these is the ocular segment, in front of the antennal, and behind the primitive pre-oral segment. The other is the segment of the maxillulae (see above, underJaws), behind the mandibular somite; the presence of this in the embryo of the collembolanAnuridahas been lately shown (1900) by J. W. Folsom (fig. 18, v. 5), who terms the maxillulae “superlinguae” on account of their close association with the hypopharynx or lingua. In reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. According to W. A. Riley (1904) the epicranium or “vertex,” the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging to the ocular segment), while the mandibular and maxillary segments form the hinder parts of the genae and the hypopharynx.After Heymons.Fig. 17.—Morphology of an Insect: the embryo ofGryllotalpa, somewhat diagrammatic. The longitudinal segmented band along the middle line represents the early segmentation of the nervous system and the subsequent median field of each sternite; the lateral transverse unshaded bands are the lateral fields of each segment; the shaded areas indicate the more internally placed mesoderm layer. The segments are numbered 1-21; 1-6 will form the head, 7-9 the thorax, 10-21 the abdomen.A, anus;Abx1Abx11, appendage of 1st and of 11th abdominal segments;Ans, anal piece = telson or 12th abdominal segment;Ant, antenna;De, deuterencephalon;Md, mandible;Mx1, first maxilla;Mx2, second maxilla or labium;O, mouth;Obcl, rudimentary labrum and clypeus;Pre, protencephalon;St1St10, stigmata 1 and 10;Terg, tergite;Thx1, appendage of first thoracic segment;Tre, tritencephalon;Ul, a thickening at hinder margin of the mouth.Great difference of opinion exists as to the hypopharynx, which has even been thought to represent a distinct segment, or the pair of appendages of a distinct segment. Heymons considers that it represents the sternites of the three trophal segments, and that the gula is merely a secondary development. Folsom looks on the hypopharynx as a secondary development. Riley holds that the hypopharynx belongs to the mandibular and maxillary segments, while the cervical sclerites or gula represent the sternum of the labial segment. The ganglia of the nervous system offer some important evidence as to the morphology of the head, and are alluded to below.Thoracic Segments.—These are always three in number. The three pairs of legs appear very early as rudiments. Though the thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, nor even, in many cases, till much later. The thoracic segments, as seen in an early stage of the ventral plate, display in a well-marked manner the essential elements of the insect segment. These elements are a central piece or sternite, and a lateral field on each side bearing the leg-rudiment. The external part of the lateral field subsequently grows up, and by coalescence with its fellow forms the tergite or dorsal part of the segment.Abdominal Segments and Appendages.—We have already seen that in numerous lower insects the abdomen is formed from twelve divisions placed in linear fashion. Eleven of these may perhaps be considered as true segments, but the twelfth or terminal one is different, and is called by Heymons a telson; in it is placed the anal orifice, and the mass subsequently becomes the upper and lower laminae anales. In Hemiptera this telson is absent, and the anal orifice is placed quite at the termination of the eleventh segment. Moreover, in this order the abdomen shows at first a division into only nine segments and a terminal mass, which last subsequently becomes divided into two. The appendages of the abdomen are called cerci, stylets and gonapophyses. They differ much according to the kind of insect, and in the adult according to sex. Difference of opinion as to the nature of the abdominal appendages prevails. The cerci, when present, appear in the mature insect to be attached to the tenth segment, but according to Heymons they are really appendages of the eleventh segment, their connexion with the tenth being secondary and the result of considerable changes that take place in the terminal segments. It has been disputed whether any true cerci exist in the higher insects, but they are probably represented in the Diptera and in the scorpion-flies (Mecaptera). In those insects in which a median terminal appendage exists between the two cerci this is considered to be a prolongation of the eleventh tergite. The stylets, when present, are placed on the ninth segment, and in some Thysanura exist also on the eighth segment; their development takes place later in life than that of the cerci. The gonapophyses are the projections near the extremity of the body that surround the sexual orifices, and vary extremely according to the kind of insect. They have chiefly been studied in the female, and form the sting and ovipositor, organs peculiar to this sex. They are developed on the ventral surface of the body and are six in number, one pair arising from the eighth ventral plate and two pairs from the ninth. This has been found to be the case in insects so widely different as Orthoptera and Aculeate Hymenoptera. The genital armature of the male is formed to a considerable extent by modifications of the segments themselves. The development of the armature has been little studied, and the question whether there may be present gonapophyses homologous with those of the female is open.A. After Wheeler,Journ. Morph.vol. viii., and Folsom,Bull. Mus. Harvard, xxxvi.B. After Folsom.Fig. 18.—Embryos of Springtail (Anuridamaritima). Magnified. A, Head-region of germ band. B, Section through head and thorax. The neuromeres are shown in Arabic, the appendages in Roman numerals.1, Ocular segment.2, Antennal.3, Trito-cerebral.4, Mandibular.5, Maxillular.6, Maxillary.7, Labial.8, Prothoracic.9, Mesothoracic.10, Metathoracic.In the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many Thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs. In the embryos of many insects there are projections from the segments of the abdomen similar, to a considerable extent, to the rudimentary thoracic legs. The question whether these projections can be considered an indication of former polypody in insects has been raised. They do not long persist in the embryo, but disappear, and the area each one occupied becomes part of the sternite. In some embryos there is but a single pair of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature. Whether cerci, stylets and gonapophyses are developed from these rudiments has been much debated. It appears that it is possible to accept cerci and stylets as modifications of the temporary pseudopods, but it is more difficult to believe that this is the case with the gonapophyses, for they apparently commence their development considerably later than cerci and stylets and only after the apparently complete disappearance of the embryonic pseudopods. The fact that there are two pairs of gonapophyses on the ninth abdominal segment would be fatal to the view that they are in any way homologous with legs, were it not that there is some evidence that the division into two pairs is secondary and incomplete. But another and apparently insuperable objection may be raised—that the appendages of the ninth segment are the stylets, and that the gonapophyses cannot therefore be appendicular. The pseudopods that exist on the abdomen of numerous caterpillars may possibly arise from the embryonic pseudopods, but this also is far from being established.Nervous System.—The nervous system is ectodermal in origin, and is developed and segmented to a large extent in connexion with the outer part of the body, so that it affords important evidence as to the segmentation thereof. The continuous layer of cells from which the nervous system is developed undergoes a segmentation analogous with that we have described as occurring in the ventral plate; there is thus formed a pair of contiguous ganglia for each segment of the body, but there is no ganglion for the telson. The ganglia become greatly changed in position during the later life, and it is usually said that there are only ten pairs of abdominal ganglia even in the embryo. In Orthoptera, Heymons has demonstrated the existence of eleven pairs, the terminal pair becoming, however, soon united with the tenth. The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments. In front of the three masses that will form the sub-oesophageal ganglion the mass of cells that is to form the nervous system is very large, and projects on each side; this anterior or “brain” mass consists of three lobes (the prot-, deut-, and tritencephalon of Viallanes and others), each of which might be thought to represent a segmental ganglion. But the protocerebrum contains the ganglia of the ocular segment in addition to those of the procephalic lobes. These three divisions subsequently form the supra-oesophageal ganglion or brain proper. There are other ganglia in addition to those of the ventral chain, and Janet supposes that the ganglia of the sympathetic system indicate the existence of three anterior head-segments; the remains of the segments themselves are, in accordance with this view, to be sought in thestomodaeum. Folsom has detected in the embryo ofAnuridaa pair of ganglia (fig. 18, 5) belonging to the maxillular (or superlingual) segment, thus establishing seven sets of cephalic ganglia, and supporting his view as to the composition of the head.Air-tubes.—The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs. The pits leading from these lengthen into tubes, and undergo repeated branching as development proceeds.Dorsal Closure.—The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. The method of this dorsal closure varies in different insects. In the Colorado beetle (Doryphora), whose development has been studied by W. M. Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. V. Graber has described a similar method of dorsal closure in the saw-flyHylotoma.After Heymons,Zeit. Wiss. Zoolog.vol. 53.Fig. 19.—Cross sections through Abdomen of German Cockroach Embryo. A (later than fig. 16) magnified. B (still more advanced, dorsal closure complete) magnified.ec, Ectoderm.en, Endoderm.sp, Splanchnic layer of mesoderm.y, Yolk.h, Heart.p, Pericardial septum.c, Coelom.g, Germ-cells surrounded by rudiment-cells of ovarian tubes.m, Muscle-rudiment.n, Nerve-chain.f, Fat body.s, Inpushing of ectoderm to form air-tubes.x, Secondary body-cavity.Mesoderm, Coelom and Blood-System.—From the mesoderm most of the organs of the body—muscular, circulatory, reproductive—take their origin. The mass of cells undergoes segmentation corresponding with the outer segmentation of the embryo, and a pair of cavities—the coelomic pouches (fig. 16, M)—are formed in each segment. Each coelomic pouch—as traced by Heymons in his study on the development of the cockroach (Phyllodromia)—divides into three parts, of which the most dorsal contains the primitive germ-cells, the median disappears, and the ventral loses its boundaries as it becomes filled up with the growing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlargement and coalescence of the blood channels and by the splitting of the fat body. It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.Reproductive Organs.—In the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. At a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19,g). In the male ofPhyllodromiathe rudiment of a vestigial ovary becomes separated from the developing testis, indicating perhaps an originally hermaphrodite condition. An exceedingly early differentiation of the primitive germ-cells occurs in certain Diptera. E. Metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midgeCecidomyiathat a large “polar-cell” appeared at one extremity during the primitive cell-segmentation. This by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes. E. G. Balbiani and R. Ritter (1890) have since observed a similar early origin for the germ-cells in the midgeChironomusand in theAphidae.The paired oviducts and vasa deferentia are, as we have seen, mesodermal in origin. The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. The classical researches of J. A. Palmén (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera. The presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward.
Exoskeleton
The outer cellular layer (ectoderm or “hypodermis”) of insects as of other Arthropods, secretes a chitinous cuticle which has to be periodically shed and renewed during the growth of the animal. The regions of this cuticle have a markedly segmental arrangement, and the definite hardened pieces (sclerites) of the exoskeleton are in close contact with one another along linear sutures, or are united by regions of the cuticle which are less chitinous and more membranous, so as to permit freedom of movement.
Head.—The head-capsule of an insect (figs. 1, 2) is composed of a number of sclerites firmly sutured together, so that the primitive segmentation is masked. Above is the crown (vertexorepicranium), on which or on the “front” may be seated three simple eyes (ocelli). Below this comes the front, and then the face or clypeus, to which a very distinct upper lip (labrum) is usually jointed. Behind the labrum arises a process—theepipharynx—which in some blood-sucking insects becomes a formidable piercing-organ. On either side a variable amount of convex area is occupied by the compound eye; in many insects of acute sense and accurate flight these eyes are very large and sub-globular, almost meeting on the middle line of thehead. Below each eye is a cheek area (gena), often divided into an anterior and a posterior part, while a distinct chin-sclerite (gula) is often developed behind the mouth.
Feelers.—Most conspicuous among the appendages of the head are the feelers or antennae, which correspond to the anterior feelers (antennules) of Crustacea. In their simpler condition they are long and many-jointed, the segments bearing numerous olfactory and tactile nerve-endings. Elaboration in the form of the feelers, often a secondary sexual character in male insects, may result from a distal broadening of the segments, so that the appendage becomes serrate, or from the development of processes bearing sensory organs, so that the structure is pinnate or feather-like. On the other hand, the number of segments may be reduced, certain of them often becoming highly modified in form.
Jaws.—The mandibles of the Hexapoda are usually strong jaws with one or more teeth at the apex (fig. 1, A, B,mn), articulating at their bases with the head-capsule by sub-globular condyles, and provided with abductor and adductor muscles by means of which they can be separated or drawn together so as to bite solid food, or seize objects which have to be carried about. They never bear segmented limbs (palps) and only exceptionally (as in the chafers) is the skeleton composed of more than one sclerite. The mandibles often furnish a good example of “secondary sexual characters,” being more strongly developed in the male than in the female of the same species. In most insects that feed by suction the mandibles are modified. In bugs (Heteroptera) and many flies, for example, they are changed into needle-like piercers (fig. 2, II), while in moths and caddis-flies they are reduced to mere vestiges or altogether suppressed.
As previously mentioned, a pair of minute jaws—themaxillulae—are present in the lowest order of insects, between the mandibles and the first maxillae. They usually consist of an inner and an outer lobe arising from a basal piece, which bears also in some genera a small palp (seeAptera).
In their typical state of development, thefirst maxillaeoffer a striking contrast to the mandibles, being composed of a two-segmented basal piece (cardoandstipes, fig. 1, C,ca,st) bearing a distinct inner and outer lobe (laciniaandgalea, fig. 1, C,la,ga) and externally a jointed limb or palp (fig. 1, C,pa). Such maxillae are found in most biting insects. In insects whose mouths are adapted for sucking and piercing, remarkable modifications may occur. In many blood-sucking flies, for example, the galea is absent, while the lacinia becomes a strong knife-like piercer and the palp is well developed. In bugs and aphids the lacinia is a slender needle-like piercer (fig. 2, III), while the palp is wanting. In butterflies and moths the lacinia is absent while the galea becomes a flexible process, grooved on its inner face, so as to make with its fellow a hollow sucking-trunk, and the palp is usually very small.
Thesecond pair of maxillaeare more or less completely fused together to form what is known as thelabiumor “lower lip.” In generalized biting insects, such as cockroaches and locusts (Orthoptera), the parts of a typical maxilla can be easily recognized in the labium. The fused cardines form a broad basal plate (sub-mentum) and the stipites a smaller plate (mentum)—see fig. 1, C,sm,m—jointed on to the sub-mentum, while the galeae, laciniae and palps remain distinct. In specialized biting insects, such as beetles (Coleoptera), the labium tends to become a hard transverse plate bearing the pair of palps, a median structure—known as theligula—formed of the conjoined laciniae, and a pair of small rounded processes—the reduced galeae—often called the “paraglossae,” a term better avoided since it has been applied also to the maxillulae of Aptera, entirely different structures. The long sucking “tongue” of bees is probably a modification of the ligula. In bugs and aphids (Hemiptera), the fused second maxillae form a jointed grooved beak or rostrum (fig. 2, IV) in which the slender piercers (mandibles and first maxillae) work to and fro.
This second pair of maxillae (or labium) form then the hinder or lower boundary of the mouth. In front or above the mouth is bounded by the labrum, while the mandibles and first maxillae lie on either side of it. A median process, known as thehypopharynxor tongue, arises from the floor of the mouth in front of the labium, and becomes most variously developed or specialized in different insects. The salivary duct opens on its hinder surface. It does not appear to represent a pair of appendages, but the maxillulae of the Aptera become closely associated with it. According to the view of R. Heymons, the hypopharynx represents the sterna of all the jaw-bearing somites, but other students consider that it belongs to the mandibular and first maxillary segments, or entirely to the segment of the first maxillae.
Neck.—The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (cervical sclerites). These have been interpreted as indicating one or more primitive segments between the head and thorax. Probably, however, as suggested by T. H. Huxley (Anat. Invert. Animals, 1877), they really belong to the labial segment which has not become completely fused with the head-capsule. It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J. H. Comstock and C. Kochi (1902) and W. A. Riley (1904).
Thorax.—The three segments which make up the thorax or fore-trunk are known as theprothorax,mesothoraxandmetathorax(see fig. 3). The dorsal area of the prothorax is occupied by a single sclerite, thepronotum(fig. 3,d), which is large and conspicuous in those insects, such as cockroaches, bugs (Heteroptera) and beetles, which have the prothorax free—i.e.readily movable on the segment (mesothorax) immediately behind—smaller and of less importance where the prothorax is fixed to the mesothorax, as in bees and flies. The dorsal area of the mesothorax, and also of the metathorax, may be made up of a series of sclerites arranged one behind the other—prescutum,scutum,scutellumandpost-scutellum(fig. 3,e,f,g,h), the scutellum of the mesothorax being often especially conspicuous. Ventrally, each segment of the thorax has asternumwith which a medianpre-sternumand pairedepisternaandepimeraare often associated (see figs. 3, 4). The recent suggestion of K. W. Verhoeff (1904) that the hexapodan thorax in reality contains six primitive segments is entirely without embryological support.
Legs.—Each segment of the thorax carries a pair of legs. In most insects the leg is built up of nine segments: (1) a broad triangular, sub-globular, conical or cylindrical haunch (coxa); (2) a smalltrochanter; (3) an elongate stout thigh (femur); (4) a more slender shin (tibia); and (5-9) a foot consisting of fivetarsal segments. The fifth (distal) tarsal segment carries a median adhesive pad—thepulvillus—on either side of which is a claw. The pulvillus isprobably to be regarded as a true terminal (tenth) segment of the leg, while the claws are highly modified bristles. Numerous bristles are usually present on the thighs, shins and feet of insects, some of them so delicate as to be termed “hairs,” others so stout and hard that they are named “spines” or “spurs.” In the relative development and shape of the various segments of the leg there is almost endless variety, dependent on the order to which the insect belongs, and the special function—walking, running, climbing, digging or swimming—for which the limb is adapted. The walking of insects has been carefully studied by V. Graber (1877) and J. Demoor (1890), who find that the legs are usually moved in two sets of three, the first and third legs of one side moving with the second leg of the other. One tripod thus affords a firm base of support while the legs of the other tripod are brought forward to their new positions.
I, Dorsal view.
II, Ventral view.
III, Lateral view.
IV, Lateral view with segments separated.
Prothorax:
a, Episternum.
b, Sternum.
c, Coxa of fore-leg.
d, Pronotum.
Mesothorax:
e, Prescutum.
f, Scutum.
g, Scutellum.
h, Post-scutellum.
i, Mesophragma.
j,Epimeron.
k,Episternum.
l, Coxa of middle leg.
Metathorax:
m, Scutum.
o, Epimeron.
p, Coxa of hind leg.
n,First Abdominal Segment.
t, Tegula at base of fore-wing.
I, Fore-leg and pro-sternum (S) in front of which are the ventral cervical sclerites (c).
cx, Coxa.tr, Trochanter.
fe, Thigh.tb, Shin.
ta, Tarsal segments.
II, Middle leg and mesosternum.
III, Hind-leg and metasternum.
In IIIa, the episternum (a) and epimeron (b) are slightly separated.
Wings.—Two pairs of wings are present in the vast majority of insects, borne respectively on the mesothorax and metathorax. At the base of the wing,i.e.its attachment to the trunk, we find a highly complex series of small sclerites adapted for the varied movements necessary for flight. Those of the dragon-flies (Odonata) have been described in detail by R. von Lendenfeld (1881). The long axis of the wings, when at rest, lies parallel to the body axis. In this position the outer margin of the wing is thecosta, the inner thedorsum, and the hind-margin thetermen. The angle between the costa and termen is theapex. When the wing is spread, its long axis is more or less at a right angle to the body axis. A wing is an outgrowth from the dorsal and pleural regions of the thoracic segment that bears it, and microscopic examination shows it to consist of a double layer of cuticularized skin, the two layers being in contact except where they are thickened and folded to form the firm tubular nervures, which serve as a supporting framework for the wing membrane, enclose air-tubes, and convey blood. These nervures consist of a series of trunks radiating from the wing-base and usually branching as they approach the wing-margins, the branches being often connected by short transverse nervures, so that the wing-area is marked off into a number of “cells” or areolets.
The details of the nervuration vary greatly in the different orders, but J. H. Comstock and J. G. Needham have lately (1898-1899) shown that a common arrangement underlies all, six series of longitudinal or radiating nervures being present in the typical wing (see fig. 5). Along the costa runs a costal nervure. This is followed by a sub-costal which sometimes shows two main branches. Then comes the radial—usually the most important nervure of the wing—typically with five branches, and the median with four. These sets arise from a main trunk towards the front region of the wing-base. From another hinder trunk arise the two-branched cubital nervure and three separate anal nervures. In the hind-wing of many insects the number of radial branches becomes reduced, while the anal area is especially well developed and undergoes a fan-like folding when the wings are closed. Great diversity exists in the texture and functions of fore and hind-wings in different insects; these differences are discussed in the descriptions of the various orders. The wings often afford secondary sexual characters, being not infrequently absent or reduced in the female when well developed in the male (see fig. 6). Rarely the male is the wingless sex.
In addition to the wings there are smaller dorsal outgrowths of the thorax in many insects. Paired erectile plates (patagia) are borne on the prothorax in moths, while in moths, sawflies, wasps, bees and other insects there are small plates (tegulae)—see Fig. 3,t—on the mesothorax at the base of the fore-wings.
Abdomen.—In the abdominal exoskeleton the segmental structure is very clearly marked, a series of sclerites—dorsal terga and abdominal sterna—being connected by pale, feebly chitinized cuticle, so that considerable freedom of movement between the segments is possible. The first and second abdominal sterna are often suppressed or reduced, on account of the strong development of the hind-legs. In many insects ten, and in a few eleven, abdominal segments can be clearly distinguished in addition to a small terminal anal segment. The female genital opening usually lies between the seventh and eighth segments, the male on the ninth. Prominent paired limbs are often borne on the tenth segment, the elongate tail-feelers (cerci) of bristle-tails and may-flies, or the forceps of earwigs, for example. In the Embiidae, a family of Isoptera, it has been shown by G. Enderlein (1901) that these cerci clearly belong to a partially suppressed eleventh segment, and R. Heymons (1895-1896) has proved by embryological study that in all cases they really belong to this eleventh segment, which in the course of development becomes fused with the tenth. Smaller appendages (such as the stylets of male cockroaches) may be carried on the ninth segment. Pairs of processes carried on the eighth and ninth segments often become specialized to form the ovipositor of the female (see fig. 14) and the genital armature of the male. A marked modification of the hinder abdominal segments may be noticed in most insects,the sclerites of the eighth and ninth being frequently hidden by those of the seventh. In the higher orders several of the hinder segments may be altogether suppressed.
Internal Organs
Nervous System.—The nervous system in the Hexapoda is built up on the typical arthropodan plan of a double ventral nerve-cord with a pair of ganglia in each segment, the cords passing on either side of the gullet and connecting with an anterior nerve-centre or brain (fig. 7) in the head. The brain innervates the eyes and feelers, and must be regarded as a “syncerebrum” representing the ganglia of the three foremost limb-bearing somites united with the primitive cephalic lobes. Behind the gullet lies the sub-oesophageal nerve-centre (fig. 7,sb), composed of the ganglia of the four hinder head-somites and sending nerves to the jaws. A pair of ganglia in each thoracic segment is usual (fig. 8), and as many as eight distinct pairs of abdominal ganglia may often be distinguished, the hindmost of which represents the fused ganglia of the last four segments. But in many highly organized insects a remarkable concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax. The legs, wings and other organs of the trunk receive their nerves from the thoracic and abdominal ganglia, and the fusion of several pairs of these ganglia may be regarded as corresponding to a centralization of individuality. A special “sympathetic” system arises by paired nerves from the oesophageal connectives; these nerves unite, and send back a median recurrent nerve associated with ganglia on the gullet and crop, whence proceed cords to various parts of the digestive system.
In connexion with the central nervous system there are usually numerous organs of special sense. Most insects possess a pair of compound eyes, and many have, in addition, three simple eyes or ocelli on the vertex. The nature of these organs is described in the articleArthropoda. The surface of a compound eye is seen to be covered with a large number of hexagonal corneal facets, each of which overlies an ommatidium or series of cell elements (fig. 9, A, B). There are over 25,000 ommatidia in the eye of a hawk moth.
Auditory organs of a simple type are present in most insects. These consist of fine rods suspended between two points of the cuticle, and connected with nerve-fibres; they are known as chordotonal organs. In many cases a more complex ear is developed, which may be situated in strangely diverse regions of the insect’s body. In locusts (Acridiidae) a large ovate, tympanic membrane (fig. 9, G) is conspicuous on either side of the first abdominal segment; on the inner surface of this membrane are two horn-like processes in contact with a delicate sac containing fluid, connected with which are the actual nerve-endings. In the nearly-related crickets and long-horned grasshoppers (Locustidae) the ears are situated in the shins of the fore-legs (see fig. 9, F). Just below the knee-joint there is a swelling, along which two narrow slits run lengthwise. They lead into chambers, formed by inpushing of the cuticle, whose delicate inner walls are in contact with air-tubes; on the outer surface of these latter are ridges, along which the special nerve-endings are arranged. An ear of another type is found in the swollen second segment of the feeler in many male gnats and midges, the cuticle between this segment and the third forming an annular drum which is connected with numerous nerve-endings, while the fine bristles on the more distal segments vibrate in response to the note produced by the humming of the female.
Many of the numerous hairs (fig. 9, E) that cover the body of an insect have a tactile function. The sense of smell resides chiefly in the feelers, on whose segments occur tiny pits, often guarded by peg-like or tooth-like structures and containing rod-like cells (fig. 9, C) in connexion with large nerve-cells. It is said that 13,000 such olfactory organs are present on the feeler of a wasp, and 40,000 on the complex antennae of a male cockchafer. Organs of similar type on the maxillae and epipharynx appear to exercise the function of taste.
Muscular System.—The muscles in the Hexapoda are striated, as in Arthropods generally, the large fibres being associated in bundles which are attached from point to point of the cuticle, so as to move adjacent sclerites with respect to one another (see figs. 8, 10). For example, the contraction of the tergo-sternal muscles, connecting the dorsal with the ventral sclerites of the abdomen, lessens the capacity of the abdominal region, while the contraction of the powerful muscles arising from the thoracic walls, and inserted into the proximal ends of the thighs, flexes or extends the legs.
Circulatory System.—Insects afford an excellent illustration of the remarkable type of blood-system characterizing the Arthropoda. The dorsal vessel is an elongate tube, whose abdominal portion is usually chambered, forming a contractile heart (fig. 10). At the constrictions between the chambers are paired slits, through which the blood passes from the surrounding pericardial sinus. The dorsal vessel is prolonged anteriorly into an aorta, through which the blood is propelled into the great body-cavity or haemocoel. After bathing the various tissues and organs, the blood returns dorsalwards into the pericardial sinus through fine perforations of its floor, and so makes its way into the heart again. Some water-bugs,e.g.of the familiesBelostomatidae,Nepidae,CorixidaeandHydrometridaehave a pulsating sac at each knee-joint to assist the flow of blood through the legs, while in dragon-flies and locusts (Acridiidae) there is a ventral pulsating diaphragm, which forms the roof of a sinus enclosing the nerve-cords.
Respiratory System.—As mentioned above, respiration by means of air-tubes (tracheae) is a most characteristic feature of the Hexapoda. An air-tube consists of an epithelium of large polygonal cells with a thin basement-membrane externally and a chitinous layer internally, the last-named being continuous with the outer cuticle. The chitinous layer is usually strengthened by thread-like thickenings which, in the region close to the outer opening of the tube, form a network enclosing polygonal areas, but which, through most of the tracheal system, are arranged spirally, the strengthening thread not forming a continuous spiral, but being interrupted after a few turns around the tube. The tracheal system in Hexapods is very complex, forming a series of longitudinal trunks with transverse anastomosing connexions (fig. 11), and extending by the finest sub-division and by repeated branching into all parts of the body. In insects of active flight the tubes swell out into numerous air-sacs, by which the breathing capacity is much increased.
Atmospheric air gains access to the air-tubes through pairedspiraclesorstigmata, which usually occur laterally on most of the body-segments. These spiracles have firm chitinous edges, and can be closed by valves moved by special muscles. When the spiracles are open and the body contracts, air is expired. The subsequent expansion of the body causes fresh air to enter the tracheal system, and if the spiracles be then closed and the body again contracted, this air is driven to the finest branches of the air-tubes, where a direct oxygenation of the tissues takes place. The physiology of respiration has been carefully studied by F. Plateau (1884). In aquatic insects various devices for obtaining or entangling air are found; these modifications are described in the special articles on the various orders of insects (Coleoptera,Hemiptera, &c.). Many insects have aquatic larvae, some of which take in atmospheric air at intervals, while others breathe dissolved air by means of tracheal gills. These modifications are mentioned below in the section on metamorphosis.
s, Salivary glands and reservoir.
c, Crop (the gizzard below it).
coe, Caecal tubes (below them the stomach).
k, Kidney tubes.
i, Intestine.
r, Rectum.
Digestive System.—A striking feature in the food-canal of the Hexapoda, as in other Arthropods, is the great extent of the “fore-gut” and “hind-gut,” lined with a chitinous cuticle, continuous with the exoskeleton. The fore-gut is composed of a tubular gullet, a large sac-like crop (fig. 12,c) and a proventriculus or “gizzard,” whose function is to strain the food-substances before they pass on into the tubular stomach, which has no chitinous lining. This organ, usually regarded as a “mid-gut,” gives off a number of secretory caecal tubes (fig. 12,coe). At its hinder end it is continuous with the hind-gut, which is usually differentiated into a tubular coiled intestine (fig. 12,i) and a swollen rectum (fig. 12,r). From the fore-end of the hind-gut arise the slender Malpighian tubes (fig. 12,k), which have a renal function.
On either side of the gullet are from one to ten pairs of salivary glands (fig. 12,s) whose ducts open into the mouth. Some of these glands may be modified for special purposes—as silk-producing glands in caterpillars or as poison-glands in blood-sucking flies and bugs. The food passing into the crop is there acted on by the saliva and also by an acid gastric juice which passes forwards from the stomach through the proventriculus. As the various portions of the food undergo digestion, they are allowed to pass through the proventriculus into the stomach, where the nutrient substances are absorbed.
Excretory System.—Nitrogenous waste-matter is removed from the body by the Malpighian tubes which open into the food-canal, usually where the hind-gut joins the stomach. These tubes vary in number from four to over a hundred in different orders of insects. The cells which line them and also the cavities of the tubes contain urates, which are excreted from the blood in the surrounding body-cavity. This cavity contains an irregular mass of whitish tissue, the fat-body, consisting of fat-cells which undergo degradation and become more or less filled with urates. When the worn-out cells are broken down, the urates are carried dissolved in the blood to the Malpighian tubes for excretion. The fat-body is therefore the seat of important metabolic processes in the hexapod body.
Reproductive System.—All the Hexapoda are of separate sexes. The ovaries (fig. 13) in the female are paired, each ovary consisting of a variable number of tubes (one in the bristle-tailCampodeaand fifteen hundred in a queen termite) in which the eggs are developed. From each ovary an oviduct (fig. 13,od) leads, and in some of the more primitive insects (bristle-tails, earwigs, may-flies) the two oviducts open separately direct to the exterior. Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin. The vagina usually opens in front of the eighth abdominal sternite. Behind it is situated a spermatheca (fig. 14,sp)and the ovipositor previously mentioned, with its three pairs of processes (Fig. 14, G,g).
T8&c. Tergites.
S7, 7th Sternite.
S8, Sclerite between 7th and 8th sterna.
S9, 8th Sclerite.
Od, Vagina.
sp, Spermatheca.
G, Anterior, andg, posterior gonapophyses.
The paired testes of the male consist of a variable number of seminal tubes, those of each testis opening into avas deferens. In some bristle-tails and may-flies, the twovasa deferentiaopen separately, but usually they lead into a sperm-reservoir, whence issues a median ejaculatory duet. The male opening is on the ninth abdominal segment, to which belong the processes that form the claspers or genital armature. Accessory glands are commonly present in connexion both with the male and the female reproductive organs. The poison-glands of the sting in wasps and bees are well-known examples of these.
Embryology
The Egg.—Among the Hexapoda, as in Arthropods generally, the egg is large, containing an accumulation of yolk for the nourishment of the growing embryo. Most insect eggs are of an elongate oval shape; some are globular, others flattened, while others again are flask-shaped, and the outer envelope (chorion) is often beautifully sculptured (figs. 20,d; 21,a,b). Various devices are adopted for the protection of the eggs from mechanical injury or from the attacks of enemies, and for fixing them in appropriate situations. For example, the egg may be raised above the surface on which it is laid by an elongate stalk; the eggs may be protected by a secretion, which in some cases forms a hard protective capsule or “purse”; or they may be covered with shed hairs of the mother, while among water-insects a gelatinous envelope, often of rope-like form, is common. In various groups of the Hexapoda—aphids and some flesh-flies (Sarcophaga), for example—the egg undergoes development within the body of the mother, and the young insect is born in an active state; such insects are said to be “viviparous.”
Parthenogenesis.—A number of cases are known among the Hexapoda of the development of young from the eggs of virgin females. In insects so widely separated as bristle-tails and moths this occurs occasionally. In certain gall-flies (Cynipidae) no males are known to exist at all, and the species seems to be preserved entirely by successive parthenogenetic generations. In other gall-flies and in aphids we find that a sexual generation alternates with one or with many virgin generations. The offspring of the virgin females are in most of these instances females; but among the bees and wasps parthenogenesis occurs normally and always results in the development of males, the “queen” insect laying either a fertilized or unfertilized egg at will.
Maturation, Fertilization and Segmentation.—Polar bodies were first observed in the eggs of Hexapoda by F. Blochmann in 1887. The two nuclei are successively divided from the egg nucleus in the usual way, but they frequently become absorbed in the peripheral protoplasm instead of being extruded from the egg-cell altogether. It appears that in parthenogenetic eggs two polar nuclei are formed. According to A. Petrunkevich (1901-1903), the second polar nucleus uniting with one daughter-nucleus of the first polar body gives rise to the germ-cells of the parthenogenetically-produced male. There is no reunion of the second polar nucleus with the female pronucleus, but, according to the recent work of L. Doncaster (1906-1907) on the eggs of sawflies, the number of chromosomes is not reduced in parthenogenetic egg-nuclei, while, in eggs capable of fertilization, the usual reduction-divisions occur. Fertilization takes place as the egg is laid, the spermatozoa being ejected from the spermatheca of the female and making their way to the protoplasm of the egg through openings (micropyles) in its firm envelope. The segmentation of the fertilized nucleus results in the formation of a number of nuclei which arrange themselves around the periphery of the egg and, the protoplasm surrounding them becoming constricted, a blastoderm or layer of cells, enclosing the central yolk, is formed. Within the yolk the nuclei of some “yolk cells” can be distinguished.
Germinal Layers and Food-Canal.—The embryo begins to develop as an elongate, thickened, ventral region of the blastoderm which is known as the ventral plate or germ band. Along this band a median furrow appears, and a mass of cells sinks within, the one-layered germ band thus becoming transformed into a band of two cell-layers (fig. 15). In some cases the inner layer is formed not by invagination but by proliferation or by delamination. The outer of these two layers (fig. 15, E) is the ectoderm. With regard to the inner layer (endoblastof some authors, fig. 15, M) much difference of opinion has prevailed. It has usually been regarded as representing both endoderm and mesoderm, and the groove which usually leads to its formation has been compared to the abnormally elongated blastopore of a typical gastrula. No doubt can be entertained that the greater part of the inner layer corresponds to the mesoderm of more ordinary embryos, for the coelomic pouches, the germ-cells, the musculature and the vascular system all arise from it. Further, there is general agreement that the chitin-lined fore-gut and hind-gut, which formthe greater part of the digestive tract, arise from ectodermal invaginations (stomodaeum and proctodaeum respectively) at the positions of the future mouth and anus. The origin of the mid-gut (mesenteron), that has no chitinous lining in the developed insect, is the disputed point. According to the classical researches of A. Kowalevsky (1871 and 1887) on the embryology of the water-beetleHydrophilusand of the muscid flies, an anterior and a posterior endoderm-rudiment both derived from the “endoblast” become apparent at an early stage, in close association with the stomodaeum and the proctodaeum respectively. These two endoderm-rudiments ultimately grow together and give rise to the epithelium of the mid-gut. These results were confirmed by the observations of K. Heider and W. M. Wheeler (1889) on the embryos of two beetles—HydrophilusandDoryphorarespectively. V. Graber, however (1889), stated that in theMuscidae, while the anterior endoderm-rudiment arises as Kowalevsky had observed, the posterior part of the “mid-gut” has its origin as a direct outgrowth from the proctodaeum. The recent researches of R. Heymons (1895) on the Orthoptera, and of A. Lécaillon (1898) on various leaf beetles, tend to show that the whole of the “mid-gut” arises from the proliferation of cells at the extremity of the stomodaeum and of the proctodaeum. On this view the entire food-canal in most Hexapoda must be regarded as of ectodermal origin, the “endoblast” represents mesoderm only, and the median furrow whence it arises can be no longer compared with the blastopore. According to Heymons, the yolk-cells must be regarded as the true endoderm in the hexapod embryo, for he states (1897) that in the bristle-tailLepismaand in dragon-flies they give rise to the mid-gut. These views are not, however, supported by other recent observers. J. Carrière’s researches (1897) on the embryology of the mason bee (Chalicodoma) agree entirely with the interpretations of Kowalevsky and Heider, and so on the whole do those of F. Schwangart, who has studied (1904) the embryonic development of Lepidoptera. He finds that the endoderm arises from an anterior and a posterior rudiment derived from the “endoblast,” that many of the cells of these rudiments wander into the yolk, and that the mesenteric epithelium becomes reinforced by cells that migrate from the yolk. K. Escherich (1901), after a new research on the embryology of the muscid Diptera, claims that the fore and hind endodermal rudiments arise from the blastoderm by invagination, and are from their origin distinct from the mesoderm. On the whole it seems likely that the endoderm is represented in part by the yolk, and in part by those anterior and posterior rudiments which usually form the mesenteron, but that in some Hexapoda the whole digestive tract may be ectodermal. It must be admitted that some or the later work on insect embryology has justified the growing scepticism in the universal applicability of the “germ-layer theory.” Heider has suggested, however, that the apparent origin of the mid-gut from the stomodaeum and proctodaeum may be explained by the presence of a “latent endoderm-group” in those invaginations.
Embryonic Membranes.—A remarkable feature in the embryonic development of most Hexapoda is the formation of a protective membrane analogous to the amnion of higher Vertebrates and known by the same term. Usually there arises around the edge of the germ band a double fold in the undifferentiated blastoderm, which grows over the surface of the embryo, so that its inner and outer layers become continuous, forming respectively theamnionand theserosa(fig. 16, A, S). The embryo of a moth, a dragon-fly or a bug is invaginated into the yolk at the head end, the portion of the blastoderm necessarily pushed in with it forming the amnion. The embryo thus becomes transferred to the dorsal face of the egg, but at a later stage it undergoes reversion to its original ventral position. In some parasitic Hymenoptera there is only a single embryonic membrane formed by delamination from the blastoderm, while in a few insects, including the wingless spring-tails, the embryonic membranes are vestigial or entirely wanting. In the bristle-tailsLepismaandMachilis, an interesting transitional condition of the embryonic membranes has lately been shown by Heymons. The embryo is invaginated into the yolk, but the surface edges of the blastoderm do not close over, so that a groove or pore puts the insunken space that represents the amniotic cavity into communication with the outside. Heymons believes that the “dorsal organ” in the embryos of the lower Arthropoda corresponds with the region invaginated to form the serosa of the hexapod embryo. Wheeler, however, compares with the “dorsal organ” the peculiar extra embryonic membrane or indusium which he has observed between serosa and amnion in the embryo of the grasshopperXiphidium.
Metameric Segmentation.—The segments are perceptible at a very early stage of the development as a number of transverse bands arranged in a linear sequence. The first segmentation of the ventral plate is not, however, very definite, and the segmentation does not make its appearance simultaneously throughout the whole length of the plate; the anterior parts are segmented before the posterior. In Orthoptera and Thysanura, as well as some others of the lower insects, twenty-one of these divisions—not, however, all similar—may be readily distinguished, six of which subsequently enter into the formation of the head, three going to the thorax and twelve to the abdomen. In Hemiptera only eleven and in Collembola only six abdominal segments have been detected. The first and last of these twenty-one divisions are so different from the others that they can scarcely be considered true segments.
Head Segments.—In the adult insect the head is insignificant in size compared with the thorax or abdomen, but in the embryo it forms a much larger portion of the body than it does in the adult. Its composition has been the subject of prolonged difference of opinion. Formerly it was said that the head consisted of four divisions, viz. three segments and the procephalic or prae-oral lobes. It is now ascertained that the procephalic lobes consist of three divisions, so that the head must certainly be formed from at least six segments. The first of these, according to the nomenclature of Heymons (see fig. 17), is the mouth or oral piece; the second, the antennal segment; the third, the intercalary or prae-mandibular segment; while the fourth, fifth, and sixth are respectively the segments of the mandibles and of the first and second maxillae. These six divisions of the head are diverse in kind, and subsequently undergo so much change that the part each of them takes in the formation of the head-capsule is not finally determined. The labrum and clypeus are developed as a single prolongation of the oral piece, not as a pair of appendages. The antennal segment apparently entirely disappears, with the exception of a pair of appendages it bears; these become the antennae; it is possible that the original segment, or some part of it, may even become a portion of the actual antennae. The intercalary segment has no appendages, nor rudiments thereof, except, according to H. Uzel (1897), in the thysanuranCampodea, and probably entirely disappears, though J. H. Comstock and C. Kochi believe that the labrum belongs to it. The appendages of the posterior three or trophal segments become the parts of the mouth. The appendages of the two maxillary segments arise as treble instead of single projections, thus differing from other appendages. From these facts it appears that the anterior three divisions of the head differ strongly from the posterior three, which greatly resemble thoracic segments; hence it has been thought possible that the anterior divisions may represent a primitive head, to which three segments and their leg-like appendages were subsequently added to form the head as it now exists. This is, however, very doubtful, and an entirely different inference is possible. Besides the five limb-bearing somites just enumerated, two others must now be recognized in the head. One of these is the ocular segment, in front of the antennal, and behind the primitive pre-oral segment. The other is the segment of the maxillulae (see above, underJaws), behind the mandibular somite; the presence of this in the embryo of the collembolanAnuridahas been lately shown (1900) by J. W. Folsom (fig. 18, v. 5), who terms the maxillulae “superlinguae” on account of their close association with the hypopharynx or lingua. In reference to the structure of the head-capsule in the imago, it appears that the clypeus and labrum represent, as already said, an unpaired median outgrowth of the oral piece. According to W. A. Riley (1904) the epicranium or “vertex,” the compound eyes and the front divisions of the genae are formed by the cephalic lobes of the embryo (belonging to the ocular segment), while the mandibular and maxillary segments form the hinder parts of the genae and the hypopharynx.
Great difference of opinion exists as to the hypopharynx, which has even been thought to represent a distinct segment, or the pair of appendages of a distinct segment. Heymons considers that it represents the sternites of the three trophal segments, and that the gula is merely a secondary development. Folsom looks on the hypopharynx as a secondary development. Riley holds that the hypopharynx belongs to the mandibular and maxillary segments, while the cervical sclerites or gula represent the sternum of the labial segment. The ganglia of the nervous system offer some important evidence as to the morphology of the head, and are alluded to below.
Thoracic Segments.—These are always three in number. The three pairs of legs appear very early as rudiments. Though the thoracic segments bear the wings, no trace of these appendages exists till the close of the embryonic life, nor even, in many cases, till much later. The thoracic segments, as seen in an early stage of the ventral plate, display in a well-marked manner the essential elements of the insect segment. These elements are a central piece or sternite, and a lateral field on each side bearing the leg-rudiment. The external part of the lateral field subsequently grows up, and by coalescence with its fellow forms the tergite or dorsal part of the segment.
Abdominal Segments and Appendages.—We have already seen that in numerous lower insects the abdomen is formed from twelve divisions placed in linear fashion. Eleven of these may perhaps be considered as true segments, but the twelfth or terminal one is different, and is called by Heymons a telson; in it is placed the anal orifice, and the mass subsequently becomes the upper and lower laminae anales. In Hemiptera this telson is absent, and the anal orifice is placed quite at the termination of the eleventh segment. Moreover, in this order the abdomen shows at first a division into only nine segments and a terminal mass, which last subsequently becomes divided into two. The appendages of the abdomen are called cerci, stylets and gonapophyses. They differ much according to the kind of insect, and in the adult according to sex. Difference of opinion as to the nature of the abdominal appendages prevails. The cerci, when present, appear in the mature insect to be attached to the tenth segment, but according to Heymons they are really appendages of the eleventh segment, their connexion with the tenth being secondary and the result of considerable changes that take place in the terminal segments. It has been disputed whether any true cerci exist in the higher insects, but they are probably represented in the Diptera and in the scorpion-flies (Mecaptera). In those insects in which a median terminal appendage exists between the two cerci this is considered to be a prolongation of the eleventh tergite. The stylets, when present, are placed on the ninth segment, and in some Thysanura exist also on the eighth segment; their development takes place later in life than that of the cerci. The gonapophyses are the projections near the extremity of the body that surround the sexual orifices, and vary extremely according to the kind of insect. They have chiefly been studied in the female, and form the sting and ovipositor, organs peculiar to this sex. They are developed on the ventral surface of the body and are six in number, one pair arising from the eighth ventral plate and two pairs from the ninth. This has been found to be the case in insects so widely different as Orthoptera and Aculeate Hymenoptera. The genital armature of the male is formed to a considerable extent by modifications of the segments themselves. The development of the armature has been little studied, and the question whether there may be present gonapophyses homologous with those of the female is open.
A. After Wheeler,Journ. Morph.vol. viii., and Folsom,Bull. Mus. Harvard, xxxvi.
B. After Folsom.
1, Ocular segment.
2, Antennal.
3, Trito-cerebral.
4, Mandibular.
5, Maxillular.
6, Maxillary.
7, Labial.
8, Prothoracic.
9, Mesothoracic.
10, Metathoracic.
In the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many Thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs. In the embryos of many insects there are projections from the segments of the abdomen similar, to a considerable extent, to the rudimentary thoracic legs. The question whether these projections can be considered an indication of former polypody in insects has been raised. They do not long persist in the embryo, but disappear, and the area each one occupied becomes part of the sternite. In some embryos there is but a single pair of these rudiments (or vestiges) situate on the first abdominal segment, and in some cases they become invaginations of a glandular nature. Whether cerci, stylets and gonapophyses are developed from these rudiments has been much debated. It appears that it is possible to accept cerci and stylets as modifications of the temporary pseudopods, but it is more difficult to believe that this is the case with the gonapophyses, for they apparently commence their development considerably later than cerci and stylets and only after the apparently complete disappearance of the embryonic pseudopods. The fact that there are two pairs of gonapophyses on the ninth abdominal segment would be fatal to the view that they are in any way homologous with legs, were it not that there is some evidence that the division into two pairs is secondary and incomplete. But another and apparently insuperable objection may be raised—that the appendages of the ninth segment are the stylets, and that the gonapophyses cannot therefore be appendicular. The pseudopods that exist on the abdomen of numerous caterpillars may possibly arise from the embryonic pseudopods, but this also is far from being established.
Nervous System.—The nervous system is ectodermal in origin, and is developed and segmented to a large extent in connexion with the outer part of the body, so that it affords important evidence as to the segmentation thereof. The continuous layer of cells from which the nervous system is developed undergoes a segmentation analogous with that we have described as occurring in the ventral plate; there is thus formed a pair of contiguous ganglia for each segment of the body, but there is no ganglion for the telson. The ganglia become greatly changed in position during the later life, and it is usually said that there are only ten pairs of abdominal ganglia even in the embryo. In Orthoptera, Heymons has demonstrated the existence of eleven pairs, the terminal pair becoming, however, soon united with the tenth. The nervous system of the embryonic head exhibits three ganglionic masses, anterior to the thoracic ganglionic masses; these three masses subsequently amalgamate and form the sub-oesophageal ganglion, which supplies the trophal segments. In front of the three masses that will form the sub-oesophageal ganglion the mass of cells that is to form the nervous system is very large, and projects on each side; this anterior or “brain” mass consists of three lobes (the prot-, deut-, and tritencephalon of Viallanes and others), each of which might be thought to represent a segmental ganglion. But the protocerebrum contains the ganglia of the ocular segment in addition to those of the procephalic lobes. These three divisions subsequently form the supra-oesophageal ganglion or brain proper. There are other ganglia in addition to those of the ventral chain, and Janet supposes that the ganglia of the sympathetic system indicate the existence of three anterior head-segments; the remains of the segments themselves are, in accordance with this view, to be sought in thestomodaeum. Folsom has detected in the embryo ofAnuridaa pair of ganglia (fig. 18, 5) belonging to the maxillular (or superlingual) segment, thus establishing seven sets of cephalic ganglia, and supporting his view as to the composition of the head.
Air-tubes.—The air-tubes, like the food-canal, are formed by invaginations of the ectoderm, which arise close to the developing appendages, the rudimentary spiracles appearing soon after the budding limbs. The pits leading from these lengthen into tubes, and undergo repeated branching as development proceeds.
Dorsal Closure.—The germ band evidently marks the ventral aspect of the developing insect, whose body must be completed by the extension of the embryo so as to enclose the yolk dorsally. The method of this dorsal closure varies in different insects. In the Colorado beetle (Doryphora), whose development has been studied by W. M. Wheeler, the amnion is ruptured and turned back from covering the germ band, enclosing the yolk dorsally and becoming finally absorbed, as the ectoderm of the germ band itself spreads to form the dorsal wall. In some midges and in caddis-flies the serosa becomes ruptured and absorbed, while the germ band, still clothed with the amnion, grows around the yolk. In moths and certain saw-flies there is no rupture of the membranes; the Russian zoologists Tichomirov and Kovalevsky have described the growth of both amnion and embryonic ectoderm around the yolk, the embryo being thus completely enclosed until hatching time by both amnion and serosa. V. Graber has described a similar method of dorsal closure in the saw-flyHylotoma.
ec, Ectoderm.
en, Endoderm.
sp, Splanchnic layer of mesoderm.
y, Yolk.
h, Heart.
p, Pericardial septum.
c, Coelom.
g, Germ-cells surrounded by rudiment-cells of ovarian tubes.
m, Muscle-rudiment.
n, Nerve-chain.
f, Fat body.
s, Inpushing of ectoderm to form air-tubes.
x, Secondary body-cavity.
Mesoderm, Coelom and Blood-System.—From the mesoderm most of the organs of the body—muscular, circulatory, reproductive—take their origin. The mass of cells undergoes segmentation corresponding with the outer segmentation of the embryo, and a pair of cavities—the coelomic pouches (fig. 16, M)—are formed in each segment. Each coelomic pouch—as traced by Heymons in his study on the development of the cockroach (Phyllodromia)—divides into three parts, of which the most dorsal contains the primitive germ-cells, the median disappears, and the ventral loses its boundaries as it becomes filled up with the growing fat body (fig. 19). This latter, as well as the heart and the walls of the blood spaces, arises by the modification of mesodermal cells, and the body cavity is formed by the enlargement and coalescence of the blood channels and by the splitting of the fat body. It is therefore a haemocoel, the coelom of the developed insect being represented only by the cavities of the genital glands and their ducts.
Reproductive Organs.—In the cockroach embryo, before the segmentation of the germ-band has begun, the primitive germ-cells can be recognized at the hinder end of the mesoderm, from whose ordinary cells they can be distinguished by their larger size. At a later stage further germ-cells arise from the epithelium of the coelomic pouches from the second to the seventh abdominal segments, and become surrounded by other mesoderm cells which form the ovarian or testicular tubes and ducts (fig. 19,g). In the male ofPhyllodromiathe rudiment of a vestigial ovary becomes separated from the developing testis, indicating perhaps an originally hermaphrodite condition. An exceedingly early differentiation of the primitive germ-cells occurs in certain Diptera. E. Metchnikoff observed (1866) in the development of the parthenogenetic eggs produced by the precocious larva of the gall-midgeCecidomyiathat a large “polar-cell” appeared at one extremity during the primitive cell-segmentation. This by successive divisions forms a group of four to eight cells, which subsequently pass through the blastoderm, and dividing into two groups become symmetrically arranged and surrounded by the rudiments of the ovarian tubes. E. G. Balbiani and R. Ritter (1890) have since observed a similar early origin for the germ-cells in the midgeChironomusand in theAphidae.
The paired oviducts and vasa deferentia are, as we have seen, mesodermal in origin. The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings. The classical researches of J. A. Palmén (1884) on these ducts have shown that in may-flies and in female earwigs the paired mesodermal ducts open directly to the exterior, while in male earwigs there is a single mesodermal duct, due either to the coalescence of the two or to the suppression of one. In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera. The presence of rudiments of the genital ducts of both sexes in the embryo of either sex is interesting and suggestive. The ejaculatory duct which opens on the ninth abdominal sternum in the adult male arises in the tenth abdominal embryonic segment and subsequently moves forward.