Crustacea.

Fig. 378.—Forms of Cercaria; stages in the development of the Fluke.1. An infusorial embryo; 2. a Trematode embryo having quite recently escaped from the egg; 3. embryo cercaria; 4. fully-formed cercaria, showing alimentary canal and sucker-like head; 5. encysted form of same; 6.Cercaria furcata, with the nervous system and forked tail displayed; 7. in the act of breaking up; 8. tail portion half an hour after division; 9. parasitic worm of another species of Trematoda. (Magnified from 10 to 25 diameters.)

Fig. 378.—Forms of Cercaria; stages in the development of the Fluke.

1. An infusorial embryo; 2. a Trematode embryo having quite recently escaped from the egg; 3. embryo cercaria; 4. fully-formed cercaria, showing alimentary canal and sucker-like head; 5. encysted form of same; 6.Cercaria furcata, with the nervous system and forked tail displayed; 7. in the act of breaking up; 8. tail portion half an hour after division; 9. parasitic worm of another species of Trematoda. (Magnified from 10 to 25 diameters.)

The study of these embryos throws a flood of light upon theobscure history of Cercariæ. After a short period of wandering, their embryos fasten upon the water-snail, and compel it to act as a wet-nurse, and prepare it for a further and higher stage of life. The earliest condition in which I have discovered them concealed about the body of the water-snail is shown at No. 2; in appearance, a simple elongated sac filled with ova or germs, and which in a shorttime develop into the caudate worms already spoken of; their tails gradually attaining to the length of the mature embryos, Nos. 3 and 4, the latter being a full-grownCercaria ephemera.

Diesing described no less than twelve species of Cercariæ, some of the most curious of which live on the puddle-snail, in colonies of thousands. All throw off their tails at the moment of changing into a fluke. On placing someCercaria furcata(Nos. 6 and 7) under the microscope, they were seen to plunge about in frantic attempts to escape from confinement. Suddenly I saw them shed their tails and their bodies divide into two parts, each half swimming about as vigorously as before, quite indifferent as to the severance, and apparently dying from exhaustion. Those represented in Nos. 6 and 7 have a highly-organised nervous system, forming a continuous circuit throughout the body and tail. The mouth is furnished with a sucker and hooklets, which can be projected out some distance, while a digestive apparatus and ventral opening or sucker can be differentiated. The tail is bifurcated and articulated with the body by a sort of ball-and-socket joint, and when broken off, the convexity of one part is seen to accurately fit into the concavity of the other; it lashes about this appendage with considerable dexterity, rarely attaching itself to any of the small aquatic plants.75

There is yet another Filarian worm, a pest to the poultry-yard, the Gape-worm,Sclerostoma syngamus. This parasite is widely distributed, and is invested with special interest, since it produces disease, and kills annually thousands of young chickens, pheasants, partridges, and many of the larger kinds of wild birds. The worms find their way into the windpipe or tracheæ, through the drinking water, while in the embryotic or cercarian stage of existence, and their increase is so rapid, the birds quickly die of suffocation. The female gape-worm often attains to a considerable size, and when full grown resembles the well-known mud-worm of the Thames (Gordius aquaticus). She measures full six-eighths of an inch in length, while the male only measures one-eighth. So insignificantly small is he that the female carries him about tucked into a side pocket. The ova sac occupies a considerable portion of the internal body space, and is always found loaded with eggs in all stages of development, numbering some five hundred or a thousand. In shape these are ovoid.On cutting open the windpipe of chicken and partridges, I have found their tracheæ literally swarming with the gape-worm.76

A remarkable form of the Trematode worm isBilharzia hæmatobraof Cobbold,Distomia hæmatobiumof other authors (Plate IV., No. 102). This genus of fluke, discovered by Dr. Bilharz in the human portal system of blood vessels, gives rise to a very serious state of disease among the Egyptians. So common is the occurrence of this worm, that this physician expressed his belief that half the grown-up population of Egypt suffer from it. Griesinger conjectures that the young of the parasite exist in the waters of the Nile, and in the fish which abound. Dr. Cobbold thinks “it more probable that the larvæ, in the form of cercariæ, rediæ, and sporocysts, will be found in certain gasteropod mollusca proper to the locality.” The anatomy of this fluke is fully described by Küchenmeister in his book on parasites, by Leuckart,77and by Cobbold. The eggs and embryos of Bilharzia are peculiar in possessing the power of altering their forms in both stages of life; and it is more than probable that the embryo form has been mistaken for some extraordinary form of ciliated infusorial animal, its movements being quick and lively. We cannot fail to notice the curious form of the male animal, and, unlike the Filarian previously described, it is he who carries the female about and feeds her. The whip-like appendage seen in the figure is a portion of the body of the female. The disease produced by this parasite is said to be more virulent in the summer months, probably owing to the greater abundance of cercarian larvæ at this period of the year.

Fig. 379.—The double parasitic worm (Diplozoum paradoxum).

There are also double parasitic worms, which may be described as a sub-order of Trematoda, differing very much from those previously described. These live on the gills of several species of fresh-water fish, the gudgeon and minnow, for instance. Among them is a most remarkable creature well deserving the name ofDiplozoum paradoxumwhich has been bestowed upon it. It consists of two complete mature similar halves, each possessing every attribute of a perfect animal (a). Each of the pointed front ends has a mouth aperture,and close to it two small sucking discs; while each individual has a separate intestine, consisting of a medium tube and innumerable side-branches. At the hinder end of the body are two suckers sunk in a depression, and protected by four hard buckle-shaped organs. The eggs are elongated, and provided at one end with a fine thread-like appendage (b). In this egg the young (c)—which at the time of hatching is only about one-hundredth of an inch—takes about a fortnight to develop. It is covered with cilia, has two eyes and two suckers; after quitting the egg, the larvæ are very lively and restless intheir movements, gliding about and then swimming off with rapidity. If unable to find the fish into which they are destined to live, they grow feeble and perish, but if successful they grow into the Diporpa (d), which is flattened and lancet-shaped, and bears a small sucking disc on the under surface and a conical excrescence on the back. After living in this state for some weeks, and gaining nourishment by sucking the blood from the fish’s gills, the worms begin to join together in pairs, one specimen seizing the conical excrescence of another by its ventrical sucker; then, by a truly acrobatic feat, the second twists itself to the dorsal excrescence of the first, and in this state an inseparable fusion takes place between the suckers and the excrescences involved in the adhesion.78

In the group Vermes, the more highly-organised Annelida must be included. These, for the most part, live either in fresh or salt water. The Annelids are various, while the Planaria, a genus of Turbellaria, are very common in pools, and resemble minute leeches; their motion is continuous and gliding, and they are always found crawling over the surfaces of aquatic plants and animals, both in fresh and salt water. The body has the flattened sole-like shape of theTrematode entozoa(Fig. 378, No. 9), the mouth is surrounded by a circular sucker; this is applied to the surface of the plant from which the animal draws its nourishment; it is also furnished with a rather long proboscis, which is probably employed for a similar purpose.

Planariæ multiply by eggs, and by spontaneous fissuration in a transverse direction, each segment becoming a perfect animal. Professor Agassiz believes that the infusorial animals, Paramæcium and Kolpoda, are simply planarian larvæ.

Hirudinidæ, the leech tribe, are usually believed to form a link between the Annelida on the one hand, and the Trematoda on the other; their affinities place them closer with the latter than the former. Although deprived of the characteristic setæ of the Annelida, and exhibiting no sectional divisions, they are provided with a sucker-like mouth possessed by Trematoda, but they present no resemblance to them in their reproductive organs. On the other hand, in the arrangement of the nervous system and in their vascular system, the Hirudinidæ resemble Annelida. The head in most of the Annelidais distinctly marked, and furnished with eyes, tentacles, mouth, and teeth, and in some instances with auditory vesicles, containing otolithes. The nervous system consists of a series of ganglia running along the ventral portion of the animal, and communicating with a central mass of brain.

Hirudina medicinalisputs forth a claim for special attention on the ground of services rendered to mankind. The whole of the family live by sucking the blood of other animals; and for this purpose the mouth of the leech is furnished with a number of strong horny teeth, by which they cut through the skin. In the common leech three rows of teeth exist, arranged in a triangular, or rather triradiate form, a structure that accounts for the peculiar appearance of leech bites. The most interesting part of the anatomy of the leech to microscopists is certainly the structure of the mouth (Fig. 380). This is a muscular dilatable orifice, within which three beautiful little semi-circular saws are situated, arranged so that their edges meet in the centre. It is by means of these saws that the leech makes the incisions whence blood is to be procured, an operation which is performed in the following manner. No sooner is the sucker firmly fixed to the skin, than the mouth becomes slightly everted, and the edges of the saws are thus made to press upon the tense skin, a sawing movement being at the same time given to each, whereby it is made gradually to pierce the surface, and cut its way to the capillary blood-vessels beneath.

Fig. 380.—Mouth of Leech.

In Clepsinidæ the body is of a leech-like form, but very much narrowed in front, and the mouth is furnished with a prehensile proboscis. These animals live in fresh water, where they may often be seen creeping over aquatic plants. Their prey is the pond-snail.

Tubicola.—The worms belonging to this series of branchiferous Annelida are all marine, and distinguished by their invariable habit of forming a tube or case, within which the soft parts of the animal can be entirely retracted. This tube is usually attached to stones or other submarine bodies. Externally it is composed of various foreign materials, sand, crystalline bodies, and thedébrisof shells;internally it is lined with a smooth coating of sarcode, sometimes of a harder consistency. The Tubicola generally live in societies, winding their tubes into a mass which often attains a considerable size; only a few are solitary in their habits. They retain their position in their cases by means of tufts of bristles and spines; the latter, in the tubicular Annelids, are usually hooked, so that by applying them to the walls of the case, the animal is enabled to oppose a considerable resistance to any effort made to withdraw it. In the best known family of the order (Sabellia), the branchiæ are placed in the head, and form a circle of plumes, or a tuft of branched organs. The Serpulidæ form irregularly twisted calcareous tubes, and often grow together in large masses, when they secure themselves to shells and similar objects; other species, Terebellidæ, which build their cases of sand and stones, appear to prefer a life of solitude. The best known form isTerebella littoralis.79The curious little spiral shells seen upon the fronds of seaweeds are formed by an animal belonging to the Spirorbis.

Fig. 381.—Serpula with extended tentacles and body protruding from calcareous case.

If the animals be placed in a vessel of sea-water a very pleasing spectacle will soon be witnessed. The top part of the tube is seen to open, and the creature cautiously protrudes a fringe of tentacles; these gradually spread out two beautiful fan-like rows of tentacles, surrounded by cilia of a rich purple or red colour. These serve the double purpose of breathing and feeding organs. When withdrawn from its calcareous case, the soft body is seen to be constructed of a series of rings, with a terminal prehensile foot by which it attaches itself.

Many Annelids are without tubes or cells of any kind, simply burying their bodies in the sand near tidal mark. The Arenicola, lob-worm, is a well-known specimen of the class; its body is sotransparent that the circulating fluids can be distinctly seen under a moderate magnifying power. Two kinds of fluids flow through the vessels, one nearly colourless, the other red; the vessels through which the latter circulate are described as blood-vessels.

Not very much interest attaches to the developmental stage of the Annelida. They issue forth from ova, and the embryo so closely resemble ciliated polypes, that competent observers have mistaken them for animals belonging to a lower class; a few hours’ careful watching is sufficient to dispel a belief of the kind, when the embryonic, globular, or shapeless mass is seen to assume a form of segmentation, and soon the various internal organs become more and more developed, eye spots appear, and the young animal arrives at the adult stage of its existence.

The crustaceans comprise a large assemblage of Arthropods, presenting great diversity of structure. Some of the parasitic species have become so simplified in organisation that they appear to present no relationship with the higher members of the class, yet it is certain that all the species, whether terrestrial or aquatic, belong to the same stock, and may have had origin in the same fundamental plan of structure. Essentially, the body consists of a large number of segments, to each of which is attached a pair of two-branched appendages; the external branch is termed the exopodite and the internal the endopodite. Five segments at the front end of the body unite to form a head, the appendages of the first two being situated in front of the mouth, and performing the office of feelers or antennæ, while those of the remaining three segments are transformed into jaws, the first pair of jaws being the mandibles and the following two pairs the maxillæ. The rest of the appendages are variously modified and to some are attached respiratory organs in the form of gills. Crustaceans are broadly divided from Centipedes, Millipedes, Insects, &c., by the presence of two pairs instead of one pair of antennæ, and by the possession of branchial and not tubular (tracheal) respiratory organs. Arachnida and some other species are again widely separated. The majority of the young on leaving the egg are quiteunlike the parent, and only acquires their definite form after undergoing a series of changes. The earliest stage, which has been called the Nauplius, already referred to in connection with the barnacle, is a minute body showing no trace of segmentation, and provided with a single eye, and three pairs of swimming appendages, which become the two pairs of antennæ and the mandibles of the adult. This stage is by no means of invariable occurrence, but is chiefly characteristic of the lowest members, the Entomostraca, and is rare in the higher, Malacostraca. The typical crustaceans are shrimps, crayfish, &c., so familiarly described by Huxley. The zoæa stage of the crab, a minute transparent creature, which undergoes several changes, swims about flapping its long jointed abdomen, like some of the Entomostraca, and the shrimp in particular. The larva of crayfish, the so-called glass-crab, is very peculiar and interesting. The sessile-eyed series, in which the compound eyes are never mounted on a movable stalk, and to which the Isopoda belong, exhibits great diversity of structure as well as of habits and habitat. Some live in fresh water, most are marine, while others live on land and take to a parasitic life.

Fig. 382.—Male Gnathia, enlarged.

This genus contains Gnathia, in which the male and female are so dissimilar, that they are frequently referred to as members of two families. In the adult male the mandibles are powerful and prominent, and the head is large, squared, and as wide as the thorax. In the female, on the contrary, the head is curiously small and triangular, without visible mandibles, and the thorax is much dilated. The creatures are about one-sixth of an inch long, and of a greyish colour, and the destruction they bring about is due to their habit of boring into timber below water mark.Fig. 382represents an enlarged view of the male Gnathia. These crustaceans are vegetarians, and feed on wood. Other members of the group, known as fish lice, are much larger in size, and chiefly infest the cetacea, and bear inaddition two large eyes. By means of their powerful fore feet the Cymothordæ attach themselves to both marine and fresh-water fish, showing a preference for the inside of the mouth of their host.

Fig. 383.—1. Cypris; 2. Cyclops; 3.Branchipus grubei.

The bar-footed group Copepoda are free living, and the thorax bears four or five swimming feet; the abdomen is without appendages. The best known fresh-water form is Cyclops, the structure of which serves as a type of the order. The body is, as is well known to microscopists, broad in front and tapering behind, being thus, when viewed swimming, pear-shaped in outline. The dorsal elements of the head are fused to form a carapace, which bears a single eye, from which circumstance it derives its name. The eggs are carried by the female in a couple of ova-sacs attached to the last segment of the thorax, and so prolific are these creatures that a female will produce over four thousand million young. The young when hatched is an oval Nauplius, which after two or three moults acquires the adult state. In the family of the Apodidæ we have an equally well-known crustacean, the Branchipus. In the Branchipodidæ the body is also elongated, but there are no appendages to the abdomen, which consists of nine segments, while there are eleven pairs of thoracic appendages. The head shield is not developed backwards, and the large separated eyes are supported on distinct stalks. In the male the second antennæ are converted into claspers. These crustaceans swim upside down (Fig. 383).

Cladocera(Daphniadæof Dr. Baird).—The water-flea (Daphnia pulex) may be taken as the best known example of the order. The body of this little active animal is narrowed in front, and at the posterior end, where the carapace is deeply notched, is the tip of the abdomen bearing the pair of rigid barbed setæ from which the genus takes its name. At the front of the head is a large compound eye and two pairs of branched plumed appendages, antennæ. The firstpair of these are small and simple. The jaws consist of the mandibles and the first pair of maxillæ, the second pair of maxillæ being obsolete in the adult. The thorax comprises five segments, each bearing a pair of leaf-like swimming limbs. The abdomen consists of three segments, and is destitute of limbs. The males are usually smaller than the females, and much rarer, being rarely met with before the end of summer.

Eggs are laid both in summer and winter, and are passed into a brood-pouch, separating the upper surface of the thorax from the backward extension of the carapace. Here the summer eggs hatch, but the winter set are enclosed in a kind of capsule developed from the carapace. This capsule, termed theephippium, is cast off with the next moult of the mother’s integument (a process necessary for the gradual growth of the crustacean), and falling to the bottom of the water, gives exit to the embryos, which hatch in its interior, and the young born from these “ephippial” eggs produce young, which in their turn become mothers. It appears, then, the winter eggs are enclosed in capsules of more than usual hardness to enable them to withstand any degree of cold that might otherwise prove fatal to the parent. Dr. Baird found, on examining ponds that had been again filled up by rain after remaining two months dry, numerous specimens of Daphnia andCyclops quadricornisin all stages of growth.80

We learn also from his investigations that the Daphnia have many enemies. “The larva of theCorethra plumicornis, known to microscopical observers as the skeleton larva, is exceedingly rapacious of Daphnia. Pritchard says they are the choice food of a species of Nais; and Dr. Parnell states that the Lochleven trout owes its superior sweetness and richness of flavour to its food, which consists of small shell-fish and Entomostraca.” These crustaceans abound in fresh and salt water. Artemiæ are formed exclusively in salt water, in salt marshes, and in water highly charged with salt. Myriads of these Entomostraca are found in the salterns at Lymington, in the open tanks or reservoirs where the brine is deposited previous to boiling. A pint of the fluid contains about a quarter of a pound of salt, and this concentrated solution destroys most other marine animals. During the fine days in summer Artemiæ may be observedin immense numbers near the surface of the water, and, as they are frequently of a lively red colour, the water appears tinged with the same hue. The movements of this little animal are peculiar. It swims about on its back, and by means of its tail, its feet being at the same time in constant motion. They are both oviparous and ovoviviparous, according to the season of the year. At certain periods they only lay eggs, while during the hot summer months they produce their young alive. In about fifteen days the eggs are expelled in numbers varying from 50 to 150. As is the case with many of the Entomostraca, the young present a very different appearance from the adult animals; and they are so exactly like the young ofChirocephalus, that with difficulty are they distinguishable one from the other. The ova of other species are furnished with thick capsules, and imbedded in a dark opaque substance, presenting a minutely cellular appearance, and occupying the interspace between the body of the animal and the back of the shell; this is called the ephippium. The shell is often beautifully transparent, sometimes spotted with pigment; it consists of a substance known as chitine, impregnated with a variable amount of calcium carbonate, which produces a copious effervescence on the addition of a small quantity of a strong acid to the water in which the shell is immersed. When boiled, Artemiæ turn red as their congeners, lobsters. Their shells may be said to consist of two valves united at the back, resembling the bivalve shell of a mussel, or simply folded at the back to appear like a bivalve, but are really not so; or they may consist of a number of rings or segments. The body of Cypris presents a reticulated appearance, somewhat resembling cell structure. Entomostraca should be narcotised and prepared for examination under the microscope as directed by Mr. Rousselet at pages 345, 346.

Insects’ Eggs, etc.Tuffen West, del.Edmund Evans.Plate VI.

Insects’ Eggs, etc.

Tuffen West, del.Edmund Evans.

Plate VI.

Distinctive Characters of Insects.—The term Insect, although originally and according to the meaning of the word correctly employed in a wide sense to embrace all those articulate creatures in which the body is externally divided into a number of segments, including, of course, flies, butterflies, beetles, bugs, spiders, scorpions, crabs, shrimps, &c., is now by common consent used in a much more restricted sense to apply only to such of these animals as have six walking legs. Insects belong to a class of Arthropoda, and are distinguished by having the head, chest, and abdomen distinctly marked out and separable; by having not more than three pairs of legs in the adult state; by having the legs borne by the thoracic segments only; by having usually two pairs of wings; by the possession of tracheæ, or air-tubes, as respiratory organs; and by being provided with a single pair of antennæ, or feelers. The insect class is one exhibiting uniformity of type and structure. Extreme variations are no doubt seen within certain limits, but these variations are sharply marked off from the groups we have been previously considering. The examination of insects may be pursued according to a defined order, and it will be found that no class of animals will afford the microscopist a more wonderful field of observation and a greater variety of interesting objects than that of the insect tribes.

In the insect, as in the crustacean, the hard parts of the body form an outer and protecting covering, and also serve for the attachment of muscles. The casing, however, in insects is purely of a chitinous, or horny nature, and has in its composition only a trace of calcium carbonate. Each somite, or joint of the body, is usually composed of six pieces; the upper, or dorsal half of each segment isnamed the tergum, the lower half the sternum, the side pieces pleura, the sternum being further sub-divided into epimeral and espisternal pieces. The body as a whole consists of some twenty segments, of which five or six form the head, the thorax of three joints, while the abdomen may number from nine to eleven. The head segments are united to form apparently a single mass, and the appendages of this region are modified for sensory purposes, and also serve as cutting and masticatory organs. The appendages of the head, examined in order, will be found to consist of eyes, antennæ, or feelers, and organs of the mouth. The antennæ of insects rarely exceed two in number, but these present great variations in form and size. In their simplest form they exist as straight jointed filaments, but in many insects they are forked, in others club-shaped, while in others they mimic forms of vegetation, and for the most part are extremely interesting objects for the microscope.

Fig. 384.—Vertical section of cornea of Eye of Fly.

The principal use of these antennæ is that of organs of touch, but it is quite probable that they may subserve other functions, as of taste or even hearing. The eyes of insects consist of either a pair ofocelli, or of a great number, when they are termed compound eyes, formed of an aggregation of external hexagonal facets and lenses, and nerve filaments, all of which have a distinct connection with the mass of ganglia recognised as the brain, as will be seen inFig. 384, a section of the eye of a fly. The number of facets varies very greatly in these compound eyes; ants, for example, have fifty facets, flies two thousand or more, and butterflies as many. Dr. Hooke counted seven thousand, and Leuwenhoeck as many as twelve thousand in the eye of a dragon fly. The eyes of some insects are supported on short stalks or pedicles, but these are never movable, as, for example, in Stalk-eyed crustaceans.

The organs of the mouth in insects present a striking homology or similarity in their fundamental structure. Two chief types of mouth are found. The biting or masticatory, as in beetles, includes a labium or upper lip, a pair of mandibles or lower jaws, a pair oflesser jaws or maxillæ, which bear one or two pairs of palpi, and a lower lip or labium, also with palpi. This latter and primitive condition of the labium is seen in Orthropterous insects and some Neuroptera. Other structures occurring in those of the mouth are the ligula, this being sometimes divided, as in bees, into three lobes, of which the two outer are the paraglossæ and the middle process the lingua or tongue. There is a second form of mouth, termed the suctorial. This is seen in Lepidoptera (butterflies), and is adapted for extracting the pollen and juices of flowers, and in which the palpi are greatly developed, and form two hairy pads or cushions, between which the proboscis is coiled up when at rest. Thus we find in the Lepidoptera the same fundamental condition of mouth as in some Coleoptera. In Hymenoptera (wasps and bees), a variety of mouth is found which presents a combination of the masticatory with the suctorial types. The labium and mandibles exist as in the beetle, the maxillæ being developed to form long sheaths protecting the labium, which now takes the form of a tongue. In Hemiptera (bugs and their allies), the mandibles and maxillæ exist as sharp lancets, while the labium forms a protective sheath. In the Diptera (flies, gnats, &c.), the labium undergoes a great development, and forms a very prominent tongue, the other parts of the mouth being developed simply as sheaths to the labium. See Figs. 389 and 390.

The thorax or chest of insects consists of three segments, named from before backwards: the prothorax, mesothorax, and metathorax. The first bears the anterior pair of legs; the mesothorax, the second pair of legs and the first pair of wings; and the metathorax, the third pair of legs and second pair of wings. The last joints of the leg constitute the tarsus or foot-claws. The nervures of the wings are in reality hollow tubes, and are extensions of the spiracles, or respiratory apertures.

The muscles of insects lie concealed beneath the integument; they are not gathered into distinct bundles as in the higher animals, although they exhibit in many cases a striated or striped structure. This is well seen in some of the beetle tribe, the water-beetle in particular. In certain larvæ the muscles are exceedingly complicated. Lyonnet found in the larva of the goat-moth, two hundred and twenty-eight muscles in the head alone, and in the whole bodyno less than three thousand nine hundred and ninety-three. The muscular power of insects is, relatively to the size of the body, very great. The flea, for instance, leaps two hundred times its own height. There are beetles weighing a few grammes that will escape from a pressure of from twenty to thirty ounces.

Professor Schäfer infers that the structure of the wing-muscles of insects furnishes the key to the comprehension of the more intricate muscular structure of vertebrates. The sarcode element, however, is not made up of a bundle of rods, but of a continuous sarcous element, readily made out by staining with hæmatoxylin. This substance is then seen to be pierced by minute tubular canals, and the longitudinal striation of muscle is due to this canalisation. The whole is connected and enclosed by a membrane of extreme delicacy.

The digestive system of insects varies with their habits and food. In Stylops, bee-parasites, and in young bees living on fluids, the intestine ends in a blind sac. There are three coats of structure throughout the digestive system. The œsophagus or gullet is provided with a crop in flies, bees, and butterflies; a true analogue of the gizzard in birds. There is in some respects a curious likeness between the conformation of the digestive organs of birds and that of insects. No true liver, but salivary glands in the mouth have been made out; the heart lies dorsally, and consists of a pulsating sac divided into compartments, and the fluid flows through it towards the head, whence it circulates freely to other parts of the body. Each trachea is an elastic tube formed of two delicate membranes, between which the spiral filament is coiled up, and is of sufficient density to prevent the collapse of the tube by the movements of the body. These tracheæ are distributed throughout the muscular tissue and the whole of the body. Thus the insect, like the bird, may be said to breathe in every part of the body, and is in this way rendered light and buoyant for flight. The air is admitted to the tracheæ by apertures termed spiracles, which the insect can close at will, and these are distributed to the number of eleven on each side of the body. The nervous system consists of a chain of ganglia or nerve-knots, which unite towards the head to form a single cord, as seen in the section made through the spider (Fig. 409).

The reproduction of all insects takes place by ova, and they are diæceous—that is, have two distinct sexes. In some few instances, as that of Aphides, or plant-lice, we have the peculiar phenomenon of parthenogenesis, the process of reproduction being performed by imperfect wingless females. These bring forth living young ones, which begin to feed the moment they are born, and constitute a viviparous brood; in other cases females lay eggs, and the process proceeds in the ordinary way, and nearly all the year round. The former is provided with a lancet-like beak for piercing and sucking the juices of the leaf, and a pair of curious honey-tubes. Insects generally undergo a transformation or metamorphosis in passing from the egg to the adult stage. While within the egg the body may be seen to become segmented, and in the course of time—in such insects as flies, bees, beetles, and butterflies—issue forth from the egg as larvæ, or caterpillars. This worm-like creature makes for itself an investing case or cocoon, in which it passes into the pupa stage of its existence. Within the pupa case a wonderful transformation takes place; the larval body being literally broken down by the process of histolysis, while its elements are rebuilt and transformed into that of theimago, or perfect insect. In grasshoppers, crickets, dragon-flies, bugs, &c., the metamorphosis is incomplete (hemimetabolic). Some few lower insect forms (lice, spring-tails, &c.) undergo no change of the kind, and in no way differ from the adult except in size. These are termed ametabolic insects. Others again, as the cockchafer and gold beetle, pass three years in the larval stage. Development in all cases is arrested or retarded by cold. Reaumur kept a butterfly pupa for two years in an ice-house, and it exhibited no tendency towards a change until removed to a warm temperature.

From the short natural history of insect life I have endeavoured to sketch out, it will have been surmised that insects offer a wide field of research, and an almost endless number of objects of interest for the microscope. The variety of material is great, and the structure and adaptation of means to an end is of the most fascinating kind. Most cabinets abound in preparations gathered together with some care and mounted with all the skill at the command of the collector, affording, as a rule, as endless an amount of pleasure to the tyro as to the more practised entomologist.It may be surmised, then, that to enter fully into a description of the several parts of insect structure would require a volume81of very large bulk, and occupy months and years. I will, therefore, take some points of interest in the structural characteristics of insects, and take them in the order in which they have already been brought to notice. The head, eyes, and other appendages of these insects we are more or less acquainted with.

Fig. 385.—A tangential or side section of Eye of Fly, with palp or pads protruded.

We will take for examination a typical member of Muscidæ, a family embracing a large and varied assortment of species, among which the house-fly and the blow-fly are the best known forms.Musca domesticaneeds no description. An interesting part of the house-fly to the microscopist is the wonderful component parts of the head. On examination we find a couple of protuberances, more or less prominent, and situated symmetrically one on each side. Their outline at the base is for the most part oval, elliptical, circular, or truncated; while their curved surfaces are spherical, spheroidal, or pyriform. These horny, round, and naked parts are the corneæ of the compound eye of the fly, and they are appropriately so termed, from the analogy they bear to the larger transparent tunics in the higher classes of animals. They differ, however, from the latter, as when viewed by the microscope they display a large number of hexagonal facets, which constitute the medium for the admission of light to several hundred simple eyes. Under an ordinary lens, and by reflected light, the entire surface of one cornea presents a beautiful reticulation, like very fine wire gauze, with minute papilla, or at least a slight elevation, in the centre of each mesh. These are resolved, however, by the aid of a compound microscope, and with a power of from 80 to 100 diameters, into an almost incredible number (when compared with the space they occupy) of minute, regular, geometrical hexagons, well defined, and capable of being computedwith tolerable ease, their exceeding minuteness being taken into consideration.

Fig. 386represents a vertical section of the eye, showing the hexagonal faceted arrangement of cylindrical tubes.

Fig. 386.—Section of Eye of Fly.l.Lenses;co.Cones;pl.Pigment layer, consisting of rings round the rods;r.r.Rods;a.v1.Air vessels between the rods;m1.Membrane on which the rods and air vessels rest;a.v2.Shorter lengths of air vessels which form a layer above the first nerve junction;n.j1.First nerve junction;m2.Membrane on which it stands;A. V.,A. V.Large air vessel surrounding the eye;n.j2.Second nerve junction;a.v3.Air vessels;op. n.Optic nerve;b.n.Brain substance. (Magnified × 160.)

Fig. 386.—Section of Eye of Fly.

l.Lenses;co.Cones;pl.Pigment layer, consisting of rings round the rods;r.r.Rods;a.v1.Air vessels between the rods;m1.Membrane on which the rods and air vessels rest;a.v2.Shorter lengths of air vessels which form a layer above the first nerve junction;n.j1.First nerve junction;m2.Membrane on which it stands;A. V.,A. V.Large air vessel surrounding the eye;n.j2.Second nerve junction;a.v3.Air vessels;op. n.Optic nerve;b.n.Brain substance. (Magnified × 160.)

In this section it appears to be questionable whether the normal shape of the lenses is not round, assuming the hexagonal shape during the process of growth in consequence of their agglomeration. The corneal surface can be peeled off, and if carefully flattened out and mounted it will be seen that each lens is not a simple lens, but a double-convex compound one, composed of two plano-convex lenses of different densities or refracting power joined together.

Experiments made on the eyes of insects, and also of crustaceæ, show that in the insect a real and reversed image of external bodies is formed in each ommatidium; it coincides with the internal face of the crystalline cone in immediate contact with the retina. Although small, the retinal image is distinct and subtends an angle of nearly forty-five. In the same way in the crustacean, the crystalline lens forms on the retinula a reversed image, but the refractivemedia have a longer focus, and the retinal membrane is not connected with the lens, the interval being filled up by a substance analogous to the vitreous of vertebrates. In both cases it would appear that light does not act directly on the rods; these latter can only receive impressions through the intermediary retinal cells. The retinal images of arthropods, as might have been surmised, are much less perfect than those of the higher orders; on the other hand, their eyes seem to be better adapted for seeing objects in relief and the movements of bodies. The shyness of butterflies and moths is certainly an inherited instinct as a protection against danger from their many enemies.

Fig. 387.A.Vertical section of Eye ofMelolontha vulgans, Cockchafer;B.A few facets more highly magnified, showing facets and pigment layer.

Fig. 387.

A.Vertical section of Eye ofMelolontha vulgans, Cockchafer;B.A few facets more highly magnified, showing facets and pigment layer.

In the accompanyingFig. 387,Ais a vertical section of the eye ofMelolontha vulgans, the fan-like arrangement of the facets, together with the transparent pyramidal gathering of the retinal rods proceeding towards the brain;Bis a few of the corneal tubes more highly magnified, the darker portion representing the pigment layer of the corneal tubes. InPlate VI., No. 133, the under surface of the head and mouth of the “Tsetse” fly,Glossina morsitans, is shown. The proboscis of this fly is long and prominent, and the antennæ are peculiar, inasmuch as the third segment is long, and produced almost as far as the flagellum, which is furnished with barbed hairs along its outer surface only. Although this fly barely equals the blow-fly in size, it is one of the greatest pests to the domestic cattle of Equatorial Africa. The palpi, although arising from two roots, are seen joined together when the fly is at rest, but when in the act of piercing or sucking they divide and the sheath is thrown directly upwards. The palpi are furnished on their convex sides with long and sharply-pointed dark-brown setæ or hairs, while the inner concave sides, which are brought into contact with the proboscis, are perfectly smooth and fleshy. Three circular openings seem to indicate the tubular nature of what in the house-fly is a fleshy, expanded, and highly-developed muscular proboscis(seen inFig. 388,Musca domestica). The proboscis (labium) forms the chief part of the organ, dilates into wonderful muscular lips, and enables the insect to employ the tongue as a prehensile organ. The lips are covered with rows of minute setæ, directed a little backwards and arranged rather closely together.

Fig. 388.—Proboscis of House-fly,Musca domestica. (The small circle indicates the object about the natural size.)

There are very many rows of these minute hairs on each ofthe lips, and from being arranged in a similar direction are employed by the insect in scraping or tearing delicate surfaces. These hairs are tests for the best of high powers. It is by means of these that it teases human beings in the heat of summer, when it alights on the hand or face, to sip the perspiration as it exudes from the skin. The fluid ascends the proboscis, partly by a sucking action, assisted by the muscles of the lips themselves, which are of a spiral form, arranged around a highly elastic, tendinous, and ligamentous structure, with other retractile additions for rapidity and facility of motion.

Fig. 389.—Spiral structure of Tongue of House-fly, from a micro-photograph made with a Zeiss 16 mm. and apochromatic projection eye-piece × 150.

The beautiful form of the spiral structure of the tongue should be viewed under a high magnifying power, when it will be seen that no continuing spiral structure really exists; each ring, apparently detached, does not extend quite round; their action is that of sucking tubes. Fluids are evidently drawn up through the entire fissure caused by the opening between the ends of the whole series of rings. It may well be pronounced a marvellous structure. The mounting of the tongue must be done with a considerable amount of care to show this structure, imperfectly represented in my woodcut.

These insects are of some service in the economy of nature, by their consumption of decaying animal matter, found about in quantities ordinarily imperceptible to most people, and that would not be removed by ordinary means during hot weather. It was asserted by Linnæus that three flies would consume a dead horse as quickly as a lion. This was, of course, said with reference to the offspring of such three flies; and it is quite possible the assertion may be correct, since the young begin to eat as soon as hatched, and a female blow-fly will produce twenty thousand living larvæ (one of which isrepresented inPlate VI., No. 141). In twenty-four hours, each will have increased in weight two hundred times, in five days it attains to its full size, and changes into the pupa, and then to the perfect insect.

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