Sexual Differences.

Fig. 5.—Egg-capsule ofP. orientalis(magnified). A, external view; B, opened; C, end view.

The larvæ of the Cockroach hardly differ outwardly from the adult, except in the absence of wings. The tenth tergum is notched in both sexes, as in the adult female. The sub-anal styles of the male are developed in the larva.

Cornelius, in his Beiträge zur nähern Kenntniss von Periplaneta orientalis (1853), gives the following account of the moultsof the Cockroach. The first change of skin occurs immediately after escape from the egg-capsule, the second four weeks later, the third at the end of the first year, and each succeeding moult after a year’s interval. At the sixth moult the insect becomes a pupa,25and at the seventh (being now four years old) it assumes the form of the perfect Insect. The changes of skin are annual, and like fertilisation and oviposition, take place in the summer months only. He tells us further that the ova require about a year for their development. These statements are partly based upon observation of captive Cockroaches, and are the only ones accessible; but they require confirmation by independent observers, especially as they altogether differ from Hummel’s account of the life-history ofBlatta germanica, and are at variance with the popular belief that new generations of the Cockroach are produced with great rapidity.

Fig. 6.—Young nymph (male). × 6.Fig. 7.—Older nymph (male) with rudiments of wings. ×21/2.

Fig. 6.—Young nymph (male). × 6.

Fig. 7.—Older nymph (male) with rudiments of wings. ×21/2.

The antennæ of the male nymph resemble those of the adult female. Wings and wing-covers appear first in the later larvalstages, but are then rudimentary, and constitute a mere prolongation of the margins of the thoracic rings. Cornelius says that the round white spot internal to the antenna first appears plainly in the pupa, but we have readily found it in a very young larva. The Insect is active in all its stages, and is therefore, with other Orthoptera, described as undergoing “incomplete metamorphosis.” After each moult it is for a few hours nearly pure white. Of the duration of life in this species we have no certain information, and there is great difficulty in procuring any.

Male Cockroaches are readily distinguished from the females by the well-developed wings and wing-covers. They are also slighter and weaker than the females; their terga and sterna are not so much thickened; their alimentary canal is more slender, and they feed less greedily (the crop of the male is usually only half-full of food). They stand higher on their legs than the females, whose abdomen trails on the ground. The external anatomical differences of the sexes may be tabulatedthus:—

Female.Male.Antenna shorter than the body, the third joint longer than the second.Antenna rather longer than the body, the third joint about as long as the second.Wings and wing-covers rudimentary.Wings and wing-covers well developed.Mesosternum divided.Mesosternum entire.Abdomen broader.Abdomen narrower.Terga 8 and 9 not externally visible.Terga 8 and 9 externally visible.The 10th tergum notched.The 10th tergum hardly notched.The 7th sternum divided behind.The 7th sternum undivided.The external outlet of the rectum and vulva between the 10th tergum and the 7th sternum.The outlet between the 10th tergum and the 9th sternum.No sub-anal styles.Sub-anal styles.

We have before us a long list ofparasites26which infest the Cockroach. There is a conferva, an amœba, several infusoria, nematoid worms (one of which migrates to and fro between the rat and the Cockroach), a mite, as well as hymenopterous and coleopterous Insects. The Cockroach has a still longer array of foes, which includes monkeys, hedgehogs, pole-cats, cats, rats, birds, chamæleons, frogs, and wasps, but no single friend, unless those are reckoned as friends which are the foes of its foes.

A few lines must be added upon the popular and scientific names of this insect. Etymologists have found it hard to explain the common English name, which seems to be related tocockandroach, but has really nothing to do with either. The lexicographers usually hold their peace about it, or give derivations which are absurd. Mr. James M. Miall informs us that “Cockroachcan be traced to the Spanishcucarácha, a diminutive form ofcucoorcoco(Lat.coccum, a berry).Cucaráchais used also of the woodlouse, which, when rolled up, resembles a berry. The termination-ácha(Ital.-accio,-accia) signifiesmeanorcontemptible. The Spanish word has also taken a French form; at leastcoqueracheshas some currency (see, for example, Tylor’s Anahuac, p. 325).” In provincial EnglishBlack Clockis a common name. The German wordSchabe, often turned intoSchwabe, means perhapsSuabian, as Moufet, quoting Cordus, seems to explain.27FranzoseandDäneare other German words for the insect, applied specially toBlatta germanica; and all seem to imply some popular theory as to the native country of the Cockroach.28This etymology ofSchabeis not free from suspicion, particularly as the same term is commonly applied to the clothes-moth.Kakerlac, much used in France and French-speaking colonies, is a Dutch word of unknown signification.P. Americanais usually namedKakerlacorCancrelatby the French; whileorientalishas many names, such asCafard,Ravet, andBête noire.29The nameBlattawas applied by the ancients to quite different insects, of which Virgil and Pliny make mention;Periplanetais a modern generic term, coined by Burmeister.

Of the uses to which Cockroaches have been put we have little to say. They constitute a popular remedy for dropsy in Russia, and both cockroach-tea and cockroach-pills are known in the medical practice of Philadelphia. Salted Cockroaches are said to have an agreeable flavour which is apparent in certain popular sauces.

The Outer Skeleton.

SPECIAL REFERENCES.Krukenberg.Vergleichend-Physiologische Vorträge. IV.—Vergl. Physiologie der Thierischen Gerüstsubstanzen. (1885.) [Chemical Relations of Chitin.]Graber.Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut. Arch. f. mikr. Anat. Bd. X. (1874.) [Minute Structure of Integument.] Also,Viallanes.Recherches sur l’Histologie des Insectes. Ann. Sci. Nat., Zool. VIeSérie, Tom. XIV. (1882).Audouin.Recherches anatomiques sur le thorax des Insectes, &c. Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect Segments.] Also,Milne-Edwards.Leçons sur la Physiologie et l’Anatomie Comparée. Tom. X. (1874.)Savigny.Mémoires sur les animaux sans vertèbres. Partie Ie.Théorie des organes de la bouche des Crustacées et des Insectes. (1816.) [Comparative Anatomy of the Mouth-parts.]Muhr.Ueber die Mundtheile der Orthopteren. Prag. 1877. [Mouth-parts of Orthoptera.]

SPECIAL REFERENCES.

Krukenberg.Vergleichend-Physiologische Vorträge. IV.—Vergl. Physiologie der Thierischen Gerüstsubstanzen. (1885.) [Chemical Relations of Chitin.]

Graber.Ueber eine Art fibrilloiden Bindegewebes der Insectenhaut. Arch. f. mikr. Anat. Bd. X. (1874.) [Minute Structure of Integument.] Also,

Viallanes.Recherches sur l’Histologie des Insectes. Ann. Sci. Nat., Zool. VIeSérie, Tom. XIV. (1882).

Audouin.Recherches anatomiques sur le thorax des Insectes, &c. Ann. Sci. Nat. Tom. I. (1824.) [Theoretical Composition of Insect Segments.] Also,

Milne-Edwards.Leçons sur la Physiologie et l’Anatomie Comparée. Tom. X. (1874.)

Savigny.Mémoires sur les animaux sans vertèbres. Partie Ie.Théorie des organes de la bouche des Crustacées et des Insectes. (1816.) [Comparative Anatomy of the Mouth-parts.]

Muhr.Ueber die Mundtheile der Orthopteren. Prag. 1877. [Mouth-parts of Orthoptera.]

When the skin of an Insect is boiled successively in acids, alkalies, alcohol, and ether, an insoluble residue known as Chitin (C15H26N2O10) is obtained. It may be recognised and sufficiently separated by its resistance to boiling liquor potassæ. Chitin forms less than one-half by weight of the integument, but it is so coherent and uniformly distributed that when isolated by chemical reagents, and even when cautiously calcined, it retains its original organised form. The colour which it frequently exhibits is not due to any essential ingredient; it may be diminished or even destroyed by various bleaching processes. The colouring-matter of the chitin of the Cockroach, which is amber-yellow in thin sheets and blackish-brown in dense masses, is particularly stable and difficult of removal. Its composition does not appear to have been ascertained; it is white when first secreted, but darkens on exposure to air. Fresh-moulted Cockroaches are white, but gradually darken in three or four hours.Lowne30observes that in the Blow-fly the pigment is “first to be met with in the fat-bodies of the larvæ. These are perfectly white, but when cut from the larva, and exposed to the air, they rapidly assume an inky blackness.... When the perfect insect emerges from the pupa, and respiration again commences, the integument is nearly white, or a faint ashy colour prevails. This soon gives place to the characteristic blue or violet tint, first immediately around those portions most largely supplied with air vessels.” ProfessorMoseley31tells us that, thinking it just within the limits of possibility that the brown coloration of the Cockroach might be due to the presence of silver, he analysed one pound weight of Blatta. He found no silver, but plenty of iron, and a remarkable quantity of manganese. That light has some action upon the colouring matter seems to be indicated by the fact that in a newly-moulted Cockroach the dorsal surface darkens first.

Chitin is not peculiar to Insects, nor even to Arthropoda. The pen of cuttle-fishes and the shell of Lingula contain the same substance,32which is also proved, or suspected, to occur in many other animals.

The chemical stability of chitin is so remarkable that we might well expect it to accumulate like the inorganic constituents of animal skeletons, and form permanent deposits.Schlossberger33has, however, shown that it changes slowly under the action of water. Chitin kept for a year under water partially dissolved, turned into a slimy mass, and gave off a peculiar smell. This looks as if it were liable to putrefaction. The minute proportion of nitrogen in its composition may explain the complete disappearance of chitin in nature.

Fig. 8.—Diagram of Insect integument, in section.bm, basement membrane;hyp, hypodermis, or chitinogenous layer;ct,ct′, chitinous cuticle;s, a seta.

The chitinous exoskeleton is rather an exudation than a true tissue. It is not made up of cells, but of many superposedlaminæ, secreted by an underlying epithelium, or “chitinogenous layer.” This consists of a single layer of flattened cells, resting upon a basement membrane. A cross-section of the chitinous layer, or “cuticle,” examined with a high power shows extremely close and fine lines perpendicular to the laminæ. The cells commonly form a mosaic pattern, as if altered in shape by mutual pressure. The free surface of the integument of the Cockroach is divided into polygonal, raised spaces. Here and there an unusually long, flask-shaped, epithelial cell projects through the cuticle, and forms for itself an elongate chitinous sheath, commonly articulated at the base; such hollow sheaths form the hairs or setæ of Insects—structures quite different histologically from the hairs of Vertebrates.

The polygonal areas of the cuticle correspond each to a chitinogenous cell. Larger areas, around which the surrounding ones are radiately grouped, are discerned at intervals, and these carry hairs, or give attachment to muscular fibres.

Viallanes (loc. cit.) has added some interesting details to what was previously known of Insect-hairs. There are, he points out, two kinds of hairs, distinguished by their size and structure. The smaller spring from the boundary between contiguous polygonal areas, and have no sensory character. The larger ones occupy unusually large areas, surmount chitinogenous cells of corresponding size, and receive a special nervous supply.Thenerve34expands at the base of the hair into a spindle-shaped, nucleated mass (bipolar ganglion-cell), from which issues a filament which traverses the axis of the hair, piercing the chitinogenous cell, whose protoplasm surrounds it with a sheath which is continued to the tip of the hair. Such sensory hairs are abundant in parts which are endowed with special sensibility.

Fig. 9.—Nerve-ending in skin of Stratiomys larva.h, hairs;b, their chitinous base;c, nucleus of generating cell;g, ganglion cell. × 250. Copied from Viallanes.Fig. 10.—Diagram of sensory hair of Insect.Cc, chitinous cuticle;h, hair;c, its generating cell;g, ganglion cell;bm, basement-membrane.

Fig. 9.—Nerve-ending in skin of Stratiomys larva.h, hairs;b, their chitinous base;c, nucleus of generating cell;g, ganglion cell. × 250. Copied from Viallanes.

Fig. 10.—Diagram of sensory hair of Insect.Cc, chitinous cuticle;h, hair;c, its generating cell;g, ganglion cell;bm, basement-membrane.

The chitinous cuticle is often folded in so as to form a deep pit, which, looked at from the inside of the body, resembles a lever, or a hook. Such inward-directed processes serve chiefly for the attachment of muscles, and are termedapodemes(apodemata). A simple example is afforded by the two glove-tips which lie in the middle line of the under-surface of the thorax (p. 58, and fig.27). In other cases the pit is closed from thefirst, and the apodeme is formed in the midst of a group of chitinogenous cells distant from the superficial layer, though continuous therewith. Many tendons of insertion are formed in this way. The two forked processes in the floor of the thorax (p. 58, and fig.27) are unusually large and complex structures of the same kind. In the tentorium of the head (p. 39, and fig.17) a pair of apodemes are supposed to unite and form an extensive platform which supports the brain and gullet.

Fig. 11.—Nymph (in last larval stage) escaping from old skin. ×21/2.

Like other Arthropoda, Insects shed their chitinous cuticle from time to time. A new cuticle, at first soft and colourless, is previously secreted, and from it the old one gradually becomes detached. The setæ probably serve the same purpose as the “casting-hairs” described by Braun in the crayfish, and by Cartier in certain reptiles,35that is, they mechanically loosen the old skin by pushing beneath it. In many soft-bodied nymphs the new skin can be gathered up into a multitude of fine wrinkles, which facilitate separation, but we have not found such wrinkles in the Cockroach, except in the wings. The integument about to be shed splits along the back of the Cockroach, from the head to the end of the thorax,36and the animal draws its limbs out of their discarded sheaths with much effort. It is remarkable that the long, tapering, and many-jointed antennæ are drawn out from an entire sheath. At thesame time the chitinous lining of the tracheal tubes is cast, while that of the alimentary canal is broken up and passed through the body.

Fig. 12.—Cast skin of older nymph (“pupa”). ×21/2.

Prolonged boiling in caustic potash, though it dissolves the viscera, does not disintegrate the exoskeleton. This shows that the segments of the integument are not separate chitinous rings, but thickenings of a continuous chitinous investment. Nevertheless, their constancy in position and their conformity in structure often enable us to trace homologies between different segments and different species as certainly as between corresponding elements of the osseous vertebrate skeleton.

Audouin’s laborious researches into the exoskeleton ofInsects37resulted in a nomenclature which has been generally adopted. He divides each somite (segment) into eight pieces, grouped in pairs—viz.,terga(dorsal plates),sterna(ventral plates),epimera(adjacent to the terga), andepisterna(adjacent to the sterna).

While admitting the usefulness of these terms, we must warn the reader not to suppose that this subdivision is either normal or primitive. The eight-parted segment exists in no singlelarval or adult Arthropod. Lower forms and younger stages take us further from such a type, instead of nearer to it; and Audouin’s theoretical conception is most fully realised in the thorax of an adult Insect with well-developed legs and wings.

The morphologist would derive all the varieties of Arthropod segments from the very simple and uniform chitinous cuticle found in Annelids and many Insect-larvæ. This becomes differentiated by unequal thickening and folding in, and a series of rings connected by flexible membranes is produced. Locomotive and respiratory activity commonly lead to the definition of terga and sterna, which are similarly attached to each other by flexible membranes. A pair of limbs may next be inserted between the terga and sterna, and the simple segment thus composed occurs so extensively in the less modified regions and in early stages that it may well be considered the typical Arthropod somite.

Special needs may lead to the division of the sterna into lateral halves, but this is purely an adaptive change. The third thoracic sternum of the male Cockroach, and the second and third of the female are thus divided, as is also the hinder part of the seventh abdominal sternum of the female.

In an early stage every somite has its tergal region divided into lateral halves, owing to the late completion of the body on this side. Traces of this division may survive even in the imago. There is often a conspicuous dorsal groove in the thoracic terga, and at the time of moult the terga split along an accurately median line (see fig.12).

Additional pieces may be developed between the terga and sterna, and these have long been termedpleural.38There may be, for example, single stigmatic plates, as in the abdomen of the Cockroach, pieces to support the thoracic legs, and pieces to support the wings; but the number and position of these plates depends upon their immediate use, and their homologies become very uncertain when Insects of different orders are compared. In general, the pleural elements of the segment are late in development, variable, and highly adaptive.

The exoskeleton of the Cockroach is divisible into about seventeen segments, which are grouped into three regions, asfollows:—

HeadProcephalic lobes3Post-oral segmentsThorax339Abdomen11—17—

It is a strong argument in favour of this estimate that many Insects, at the time when segmentation first appears, possess seventeen segments.40The procephalic lobes, from which a great part of the head, including the antennæ, is developed, are often counted as an additional segment.41

The limbs, which in less specialised Arthropoda are carried with great regularity on every segment of the body, are greatly reduced in Insects. Those borne by the head are converted into sensory and masticatory organs; those on the abdomen are either totally suppressed, or extremely modified, and only the thoracic limbs remain capable of aiding in locomotion.

The primitive structure of the Arthropod limb is adapted to locomotion in water, and persists, with little modification, in most Crustacea. Here we find in most of theappendages42a basal stalk (protopodite), often two-jointed, an inner terminal branch (endopodite), and an outer terminal branch (exopodite), each of the latter commonly consisting of several joints. It does not appear that the appendages of Insects conform to the biramous Crustacean type, though the ends of the maxillæ are often divided into an outer and an inner portion.

We shall now proceed to describe, in some detail, the regions of the body of the adult Cockroach.

Fig. 13.—Front of Head. × 10.

The head of the Cockroach, as seen from the front, is pear-shaped, having a semi-circular outline above, and narrowing downwards. A side-view shows that the front and back are flattish, while the top and sides are regularly rounded. In the living animal the face is usually inclined downwards, but it can be tilted till the lower end projects considerably forward. The mouth, surrounded by gnathites or jaws, opens below. On the hinder surface is the occipital foramen, by which the head communicates with the thorax. A rather long neck allows the head to be retracted beneath the pronotum (first dorsal shield of the thorax) or protruded beyond it.

On the front of the head we observe the clypeus, which occupies a large central tract, extending almost completely across the widest part of the face. It is divided above by a sharply bent suture from the two epicranial plates, which form the top of the head as well as a great part of its back and sides. The labrum hangs like a flap from its lower edge. A little above the articulation of the labrum the width of the clypeus is suddenly reduced, as if a squarish piece had been cut out of each lower corner. In the re-entrant angle so formed, the ginglymus, or anterior articulation of the mandible, is situated.

The labrum is narrower than the clypeus, and of squarish shape, the lower angles being rounded. It hangs downwards,with a slight inclination backwards towards the mouth, whose front wall it forms. On each side, about halfway between the lateral margin and the middle line, the posterior surface of the labrum is strengthened by a vertical chitinous slip set with large setæ. Each of these plates passes above into a ring, from the upper and outer part of which a short lever passes upwards, and gives attachment to a muscle (levator menti).

Fig. 14.—Top of Head.ep, epicranial plate;oc, eye;ge, gena. × 10.

The top and back of the head are defended by the two epicranial plates, which meet along the middle line, but diverge widely as they descend upon the posterior surface, thus enclosing a large opening, the occipital foramen. Beyond the foramen, they pass still further downwards, their inner edges receding in a sharp curve from the vertical line, and end below in cavities for the articulation of the mandibular condyles.43

The sides of the head are completed by the eyes and the genæ. The large compound eye is bounded above by the epicranium; in front by a narrow band which connects the epicranium with the clypeus; behind, by the gena. The gena passes downwards between the eye and the epicranial plate, then curves forwards beneath the eye, and just appears upon the front of the face, being loosely connected at this point with the clypeus. Its lower edge overlaps the base of the mandible, and encloses the extensor mandibulæ.

Fig. 15.—Side of Head.oc, eye;ge, gena;mn, mandible. × 10.Fig. 16.—Back of Head.ca, cardo;st, stipes;ga, galea;la, lacinia;pa, palp;sm, submentum;m, mentum;pg, paraglossa. × 10.

Fig. 15.—Side of Head.oc, eye;ge, gena;mn, mandible. × 10.

Fig. 16.—Back of Head.ca, cardo;st, stipes;ga, galea;la, lacinia;pa, palp;sm, submentum;m, mentum;pg, paraglossa. × 10.

The occipital foramen has the form of an heraldic shield. Its lateral margin is strengthened by a rim continuous with the tentorium, or internal skeleton of the head. Below, the foramen is completed by the upper edge of the tentorial plate, which nearly coincides with the upper edge of the submentum (basal piece of the second pair of maxillæ); a cleft, however, divides the two, through which nerve-commissures pass from the sub-œsophageal to the first thoracic ganglion. Through the occipital foramen pass the œsophagus, the salivary ducts, the aorta, and the tracheal tubes for the supply of air to the head.

Fig. 17.—Fore-half of Head, with tentorium, seen from behind. × 12.

The internal skeleton of the head consists of a nearly transparent chitinous septum, namedtentoriumby Burmeister, which extends downwards and forwards from the lower border of theoccipital foramen. In front it gives off two long crura, or props, which pass to the ginglymus, and are reflected thence upon the inner surface of the clypeus, ascending as high as the antennary socket, round which they form a kind of rim. Each crus is twisted, so that the front surface becomes first internal and then posterior as it passes towards the clypeus. The form of the tentorium is in other respects readily understood from the figure (fig. 17). Its lower surface is strengthened by a median keel which gives attachment to muscles. The œsophagus passes upwards between its anterior crura, the long flexor of the mandible lies on each side of the central plate; the supra-œsophageal ganglion rests on the plate above, and the sub-œsophageal ganglion lies below it, the nerve-cords which unite the two passing through the circular aperture. A similar internal chitinous skeleton occurs in the heads of other Orthoptera, as well as in Neuroptera and Lepidoptera.Palmén44thinks that it represents a pair of stigmata or spiracles, which have thus become modified for muscular attachment, their respiratory function being wholly lost. In Ephemera he finds that the tentorium breaks across the middle when the skin is changed, and each half is drawn out from the head like the chitinous lining of a tracheal tube.

Fig. 18.—Base of Antenna of Male (to left) and Female (to right). × 24.

A pair of antennæ spring from the front of the head. In the male of the common Cockroach they are a little longer than the body; in the female rather shorter. From seventy-five to ninety joints are usually found, and the three basal joints are larger than the rest. Up to about the thirtieth, the joints are about twice as wide as long; from this point they become more elongate. The joints are connected by flexible membranes, and provided with stiff, forward-directed bristles. The ordinary position of the antennæ is forwards and outwards.

Each antenna is attached to a relatively large socket (fig.15), which lies between the epicranium and clypeus, to the front and inner side of the compound eyes. A flexible membrane unites the antenna to the margin of the socket, from the lower part of which a chitinous pin projects upwards and supports the basal joint.

It is well known that in many Crustacea two pairs of antennæ are developed, the foremost pair (antennules) bearing two complete filaments. Some writers have suggested that both pairs may be present in Insects, though not simultaneously, the Crustacean antennule being found in the larva, and the Crustacean antenna in the adult. This view was supported by thefamiliar fact that in many larvæ the antennæ are placed further forward than in the adult. The three large joints at the base of Orthopterous antennæ have been taken to correspond with those of Crustacean antennules, and it has been inferred that in Insects with incomplete metamorphosis, only antennules or larval antennæ are developed.45This reasoning was never very cogent, and it has been impaired by further inquiry. Weismann has shown that inCorethra plumicornis, the adult antenna, though inserted much further back than that of the larva, is developed within it,46and Graber has described a still more striking case of the same thing in a White Butterfly.47There is, therefore, no reason to suppose that Insects possess more than one pair of antennæ, which is probably preoral, not corresponding with either of the Crustacean pairs.

We have already noticed (p. 26) the superficial points in which the antenna of the male Cockroach differs from that of the female.

The eyes of some Crustacea are carried upon jointed appendages, but this is never the case in Insects, though the eye-bearing surface may project from the head, as inDiopsisandStylops. ProfessorHuxley48supposes that the head of an Insect may contain six somites, the eyes representing one pair of appendages. The various positions in which the eyes of Arthropoda may be developed weakens the argument drawn from the stalk-eyed Crustacea. Claus and Fritz Müller go so far on the other side as to deny the existence of an eye-segment even in Crustacea.

Before entering upon a full description of the mouth-parts of the Cockroach, which present some technical difficulties, the beginner in Insect anatomy will find it useful to get a few points of nomenclature fixed in his memory. Unfortunately, the terms employed by entomologists are at times neither convenient nor philosophical.

There are three pairs of jaws, disposed behind the labrum, as in thediagram:—

Labrum.1st pair of Jaws(Mandibles).2nd "(Maxillæ).3rd "(Labium, or 2nd pair of Maxillæ).

Labrum.

1st pair of Jaws(Mandibles).

1st pair of Jaws

(Mandibles).

2nd "(Maxillæ).

2nd "

(Maxillæ).

3rd "(Labium, or 2nd pair of Maxillæ).

3rd "

(Labium, or 2nd pair of Maxillæ).

Fig. 19.—Diagram of Cockroach Jaws, in horizontal section.

The mandible is undivided in all, or nearly all, Insects. Each maxilla may consist of

Apalpon the outer side,Agalea(hood),Alacinia(blade), on the inner side.

The galea (hood) of the 3rd pair of jaws is sometimes called theparaglossa.

A tongue-like process may be developed from the front wall of the mouth (epipharynx), or from the back wall (hypopharynxorlingua).49Both epipharynx and hypopharynx project into the mouth, and, in some Diptera, far beyond it.

The tip of the labium is sometimes produced into a long tongue, called theligula(strap).

The mouths of Insects may be classedas:—

Biting.—Orthoptera, Neuroptera, Coleoptera (in some Coleoptera a licking tongue is developed), most Hymenoptera.

Licking and Sucking.—Some Hymenoptera—e.g., Honey Bee.

Sucking.—(a) With lancets—Diptera, Hemiptera. (b) Without lancets—Lepidoptera.

The reference of these to a common plan, and the determination of the constituent parts, is mainly the work of Savigny. Mouth-parts were made the basis of the classification of Insects by Fabricius (1745–1808).

The mandibles of the Cockroach are powerful, single-jointed50jaws, each of which is articulated by a convex “condyle” to the lower end of the epicranial plate, and again by a concave “ginglymus” to the clypeus. The opposable inner edges are armed with strong tooth-like processes of dense chitin, which interlock when the mandibles close; those towards the tip of the mandible are sharp, while others are blunt, as if for crushing. Each mandible can be moved through an angle of about 30°. A flexible chitinous flap extends from its inner border to the labrum. The powerful flexor of the mandible arises within the epicranial vault; its fibres converge to a chitinous tendon, which passes outside the central plate of the tentorium, and at a lower level through a fold on the lower border of the clypeus, being finally inserted near the ginglymus. A short flexor arises from the crus of the tentorium. The extensor muscle arises from the side of the head, passes through the fold formed by the lower end of the gena, and is inserted close to the outer side of the condyle of the mandible.

Fig. 20.—The Jaws, separated.Mn, mandible, seen from behind (to left) and front (to right);Mx' maxilla (first pair);Mx" labium, or second pair of maxillæ. The other letters as before. × 20.

The anterior maxillæ lie behind the mandibles, and like them are unconnected with each other. They retain much more of the primitive structure of a gnathite than the mandibles, in which parts quite distinct in the maxillæ are condensed or suppressed. The constituent pieces are seen in fig.20. There is a two-jointed basal piece, consisting of thecardo(ca) and thestipes(st). The cardo is a transverse plate bent upon itself, and enclosing muscles; it is attached to the outward-directed pedicel of the occipital frame, and carries the vertical stipes. To the side and lower end of the stipes is attached the five-jointed palp (pa), a five-jointed limb used in feeding and in exploration, while the lacinia (la) and galea (ga) are articulated to its extremity. The lacinia is internal and posterior to the galea; it is broad above, but narrows below to a bifid tooth of dense chitin; its inner surface is beset with a cluster of strong setæ. The galea is more flexible, and forms an irregular three-corneredprism with an obliquely truncated end, upon which are many fine hairs. A flexible and nearly transparent flap connects the inner edges of the stipes and cardo, and joins both to the labium. The muscles which move the bases of the maxillæ spring from the crura, central plate, and keel of the tentorium.

On the posterior surface of the head, below the occipital foramen, we find a long vertical flap, the labium, which extends downwards to the opening of the mouth. It represents a second pair of maxillæ, fused together in their basal half, but retaining elsewhere sufficient resemblance to the less modified anterior pair to permit of the identification of their component parts. The upper edge is applied to the occipital frame, but is neither continuous with that structure nor articulated thereto. By stripping off the labium upwards it may be seen that it is really continuous with the chitinous integument of the neck. The broad shield-like base is incompletely divided by a transverse hinge into an upper and larger piece, thesubmentum, and a distal piece, thementum. To the mentum are appended representatives of the galeæ (here namedparaglossæ) and laciniæ, while a three-jointed palp with an additional basal joint (distinguished as thepalpiger) completes the resemblance to the maxillæ of the first pair.51In front of the labium, and lying in the cavity of the mouth is a chitinous fold of the oral integument, thelingua, which lies like a tongue in the floor of the mouth. The common duct of the salivary glands enters the lingua, and opens on its hinder surface. The lingua is supported by the chitinous skeleton represented in the figures of the salivary glands. (Chap.vii.,infra.)

The epipharynx, which is a prominent part in Coleoptera and Diptera, is not recognisable in Orthoptera.

We must now shortly consider the functions of the parts just described. The antennæ have long been regarded as sense-organs, and even the casual observer can hardly fail to remark that they are habitually used by the Insect to gain informationconcerning its immediate surroundings. Long antennæ, such as those of the Cockroach, are certainly organs of touch, but it has been much disputed whether they may not also be the seat of some special sense, and if so, what that sense may be. Several authors have found reason to suppose that in the Insect-antenna resides the sense of hearing, but no evidence worth the name is forthcoming in favour of this view. Much better support can be found for the belief that the antenna is an olfactory organ,52and some experiments which seem conclusive on this point will be cited in a later chapter.

In the Cockroach the mandibles and maxillæ are the only important instruments of mastication. The labium is indirectly concerned as completing the mouth behind and supporting the lingua, which is possibly of importance in the ordinary operations of feeding.Plateau53has carefully described the mode of mastication as observed in a Carabus, and his account seems to hold good of biting Insects in general. The mandibles and maxillæ act, as he tells us, alternately, one set closing as the others part. The maxillæ actually push the morsel into the buccal cavity. When the mandibles separate, the head is slightly advanced, so that the whole action has some superficial resemblance to that of a grazing quadruped.

The palps of the maxillæ and labium have been variously regarded as sensory and masticatory instruments. Not a few authors believe that they are useful in both ways. The question has lately been investigated experimentally by Plateau,54who finds that removal of both maxillary and labial palps does not interfere either with mastication or the choice of food. He observes that in the various Coleoptera and Orthoptera submitted to experiment the palps are passive while food is being passed into the mouth.

Plateau’s experiments are conclusive as to the subordinate value of the palps in feeding. The observation of live Cockroaches has satisfied us that the palps are constantly used when the Insect is active, whether feeding or not, to explore the surface upon which it moves. We have seen no ground for attributing to the palps special powers of perceiving odours or flavours, nor have we observed that they aid directly in filling the mouth with food.

It is worthy of note that Leydig has described and figured in the larva ofHydroporus(?), and Hauser inDytiscus,Carabus, &c., a peculiar organ, apparently sensory, which is lodged in the maxillary and labial palps. It consists of whitish spots, sometimes visible to the naked eye, characterised by unusual thinness of the chitinous cuticle and by the aggregation beneath it of a crowd of extremely minute sensory rods. Of this organ no satisfactory explanation has yet been given.55

The jaws of the Cockroach form an excellent standard of comparison for those of other Insects, and we shall attempt to illustrate the chief variations by referring them to this type.56Mouth-parts are so extensively used in the classification of Insects that every entomologist ought to have a rational as well as a technical knowledge of their comparative structure. No part of Insect anatomy affords more striking examples of adaptive modification. In form, size, and mode of application the jaws vary extremely. It would be hard to find feeding-organs more unlike, at first sight, than the stylets of a Gnat and the proboscis of a Moth, yet the study of a few well-selected types will satisfy the observer that both are capable of derivation from a common plan. Nor is this common plan at all vague. It is accurately pictured in the jaws of the Cockroach and other Orthoptera. These correspond so entirely with the primitive arrangement, inferred by a process of abstraction fromthe most dissimilar Insects, as to furnish a strong argument for the descent of all higher Insects from forms not unlike Orthoptera in the structure of their mouth-parts.

Back to Index Next