Chapter IV: Arthropoda--Insecta (1)

_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 is named 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.

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 of _ocelli_, 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 in Fig. 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 of lesser 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 body no 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 the _imago_, 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 volume[81] of 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 domestica_ needs 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 computed with tolerable ease, their exceeding minuteness being taken into consideration.

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

_l._ Lenses; _co._ Cones; _pl._ Pigment layer, consisting of rings round the rods; _r.r._ Rods; _a.v^1._ Air vessels between the rods; _m^1._ Membrane on which the rods and air vessels rest; _a.v^2._ Shorter lengths of air vessels which form a layer above the first nerve junction; _n.j^1._ First nerve junction; _m^2._ Membrane on which it stands; A. V., A. V. Large air vessel surrounding the eye; _n.j^2._ Second nerve junction; _a.v^3._ 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 refractive media 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.

A. Vertical section of Eye of _Melolontha vulgans_, Cockchafer; B. A few facets more highly magnified, showing facets and pigment layer.]

In the accompanying Fig. 387, A is a vertical section of the eye of _Melolontha vulgans_, the fan-like arrangement of the facets, together with the transparent pyramidal gathering of the retinal rods proceeding towards the brain; B is a few of the corneal tubes more highly magnified, the darker portion representing the pigment layer of the corneal tubes. In Plate 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 in Fig. 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.

There are very many rows of these minute hairs on each of the 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 is represented in Plate 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.

Fig. 390.--Tongue, Proboscis, and piercing apparatus of Drone-fly (_Eristalis tenax_).]

In the drone-fly (_Eristalis tenax)_, the mouth organs are larger than in the house-fly, and differ in many respects. The tongue is split up for a certain distance, and then again united, as represented in Fig. 390. The labium, mandibles, and maxillæ are converted into well-developed lancet-shaped organs; these both pierce the skin of animals, and form tubes by which their blood may be sucked up. Next to the maxillary palpi a couple of lancets are seen to project out; these again are associated with two other instruments, one resembling in appearance a two-edged sword, and a peculiar one with pincers or cutting teeth at the extremity. It is very peculiar, and resembles an instrument used in surgery for enlarging the wound, and in this case to increase the flow of blood. This remarkable compound piercing apparatus of the drone-fly is of exquisite finish, and must strike the observer with amazement, while it greatly transcends the work of human mechanism. The fleshy tongue itself projects some distance from the apparatus described, and is furnished with setæ or hairs, shorter and fewer in number than those of the house-fly, and while its spiral structure is not so fully developed, its retractor, muscles, and ligaments are even more so.

The further development of the mouth organs must be looked for in other members of the insect tribe, when it will be seen many assume a more or less modified form of structure, that, for example, in Hymenoptera (the bee and wasp), in which insects the mouth and tongue are divided into lobes which are used to extract the nectary (as Linnæus termed it) from the plants on which they feed. The tongue in most species is capable of extension and contraction.

1. Sting of Wasp (_Vespa vulgaris_), with its muscular attachments and palpi for cleansing the apparatus; 2. Sting of Bee.]

In Fig. 391 the under-surface of the wasp’s tongue is shown, together with its two pairs of antennæ, and pair of brushes on either side, for brushing off the gathered pollen and honey from the broad tongue. It is amply provided with muscular structure. The antennæ, or feelers, are as curious in form as they are delicate in structure. Those of the male differ from those of the female.

Both the bee and the wasp are armed with an exceedingly venomous sting, as is well known. This structure takes the form of a well-adapted mechanical contrivance, and is a weapon of offence as well as of defence. The sting consists of two barbed needle-points, of a sufficient length to pierce the flesh to some depth. From the peculiar arrangement of their serrated edges their immediate withdrawal cannot take place, and it is this circumstance, with the drop of poison injected into the open wound, that renders their sting of the most painful and irritating kind. The gland containing the poison is contained in a minute sac situated at the root of the piercing apparatus. In Fig. 392 is shown the sting of the wasp and the bee.

Very many insects are provided with instruments for boring into the bark or solid wood itself. The female Cynip bores into the oak-apple for the purpose of depositing her egg. The larva, when full grown, eats its way out of the nut, and drops to the ground, where it attains the form of the perfect fly (Fig. 393).

There are numbers of species living exclusively upon the leaves of plants, to which they do much damage by the excrescences or galls they form. Each tree seems to be infested by its own species of gall-mite, the so-called _nail-gall_ of the lime being caused by a species named _Phytoptus tibiæ_. These galls take the form of a pointed column, standing erect on the upper side of the leaf. Galls of much the same structure occur in the sycamore, maple, elm, and various fruit trees.

The gnat (_Culex pipiens_) is furnished with a sting curiously constructed (Fig. 394), and enclosed in a perfectly clothed sheath covered throughout by scales or feathers. This is folded up when not in use. The mouth is provided with a complete set of lancets for piercing the flesh; after having inflicted a severe wound, it injects an acid poison through the proboscis. The scales of the gnat vary in structure accordingly as these are found on the wing, the body, or the proboscis. A magnified wing is shown at No. 2, Fig. 394, and a magnified scale from the proboscis at No. 3. In Fig. 405, Nos. 3 and 5, more highly magnified wing and body scales are given. The proboscis is protected on either side by antennæ and feelers.

1. Head of _Culex pipiens_, female Gnat, detached from body; 2. Wing, showing nervature and fringed edges; 3. Scale from Proboscis; 4. Proboscis and Lancets. The reticulated markings on each side of the head show the proportionate space occupied by the eyes.]

The giant-tailed wasp, _Sirax gigas_, is furnished with an even more curious mechanical boring apparatus (Fig. 395) than its congeners. This is a boring ovipositor, skilfully contrived for piercing the bark of trees, in which the insect deposits her eggs, and where the larva, when hatched, will find an ample supply of food to carry it through this stage of existence. The boring tube, it will be seen, is a perfect muscular structure (_c_, _c_, _a_, and _x_); in short, it is an endless form of drill, well known to the mechanic, such as is employed in fine work for drilling holes. The females are of some size, and may be surprised and taken in the act of boring through the bark of the pine tree, for which they have a preference.

Fig. 395.--Boring apparatus of Giant-tailed Wasp (_Sirex gigas_), × 350.]

There is also a species of the broad-bodied saw-fly, _Lyda campestris_. These bore the Scotch fir, and deposit their eggs. The larvæ from these eggs, when hatched out, feed upon the pine-needles, first spinning a fine web to conceal their work of depredation. A better known saw-fly, _Abraxas grossulariata_, plays havoc among our gooseberry trees. The female is provided with a curious mechanical apparatus as an ovipositor, with which she cuts into the thicker under-leaf of the plant. This penetrating and cutting tool consists of a double-saw (Fig. 396) of elaborate construction, which when not in use is kept concealed in a long narrow case situated beneath the abdomen. It is further protected by two horny plates. The saws pass out through a deep groove so arranged that the saws work side by side backwards and forwards, without a possibility of running out of the groove. When the cut is made, the four are drawn together and form a central canal, through which an egg is forced into the leaf. The cutting edges of the saws are provided with about eighteen or twenty teeth; these have sharp points of extreme delicacy, and together make a serrated edge of the exact form given to the finest and best-made surgical saws of the present day. In the summer-time the proceedings of the female insect may be witnessed, and the method of using this curious instrument seen, by the aid of a hand magnifier. These insects are not easily alarmed when busy at work.

Before bringing my remarks on proboscides of insects to a conclusion, attention must be given to that of the honey bee (_Apis mellifica_), and its curious accessories. The mouth of bees exhibits a combination of the suctorial and the masticatory form of oral apparatus. Thus the labial, or upper lip, and the mandibles, or large pair of jaws, are well developed, while the maxillæ, or lesser pair, are elongated to form a tubular organ, through which, together with the tongue, the flower juices, “honey-dew,” may be sucked up. The labium, lower lip, is also rather prolonged, and the palpi, or organs of touch, with which it is endowed form a useful protective apparatus. The mandibles are employed by bees in the construction of their abodes, while the suctorial portion of the mouth is devoted to the reception of nourishment and to prehension. The sting of the bee, already noticed, is in fact an ovipositor, the female alone being provided with this weapon as an egg-depositing organ, although better known as an _aculeus_ or sting; but it forms no part of the oral apparatus (as shown in Fig. 397). The proboscis itself will be seen to be curiously divided; the divisions are elegant and regular, beset with numerous setæ or hairs. The two horny outside lancets are spear-shaped and partially set with short hairs; at the base of each is a hinge articulation; this permits of considerable motion in several directions, and is much used by the busy insect for forcing open the more internal parts of flowers, thus facilitating the introduction of the proboscis. The two shorter feelers are closely connected with the proboscis, and terminate in three-jointed articulations. The structure of the proboscis is so arranged that it can be enlarged at the base, and thus made to contain a greater quantity of the collected honey-dew; at the same time it is in this cavity the nectar appears to be converted into pure honey. The proboscis tapers off to a little nipple-like extremity, and at its base is seen two shorter and stronger mandibles, from between which is protruded a long and narrow lance-like tongue, the whole being most curiously connected by a series of strong muscles and ligaments. The basal or first joint of the hind leg in the neuter or working bee is developed into an enlarged form of pocket, used by the insect for conveying the pollen of flowers and the propolis to the hive. Indeed, both the tibia and the first joint of the tarsus are broadened out into plates, but the two sides of the plates are differently furnished. On one side is a thick coating of hairs, those on the tarsus taking the form of a brush, evidently used for brushing out the pollen, as these special developments are not found on the hind legs of the drones or of the queen.

1. Honey bee’s tongue; 2. Leg of worker bee. (The small circles show the objects about the natural size.)]

1. Foot and leg of Ophion; 2. Foot and leg of Flesh-fly; 3. Foot and leg of Drone-fly, with pad or sucker appendage.]

The wax used in the formation of cells is a secretion that exudes through certain portions of the body of the bee, since it is found in little pouches situated on the under part of the body, but it is not brought home ready for use. The walls of the cells are strengthened when completed by a kind of varnish, already referred to as the propolis, collected from the buds of poplar and lime trees, and this is spread over the walls of the cell by that wonderful pair of broad spatulæ, represented in the drawing.

Many interesting variations will be found in the legs and feet of flies, as well as in those of other orders of insects (Lepidoptera). One or two typical forms are represented in Plate VI., and in Fig. 398.

Fig. 399.--Sucker on the leg of Water-beetle. (The dot in the circle represents the object natural size.)]

The tarsus, or foot of the fly (Fig. 398), consists of a deeply bifid, membranous structure, _pulvillus_; anterior to its attachment to the fifth tarsal joint, or the upper surface, are seated two claws, or “tarsal ungues”; these are freely movable in every direction. These ungues differ greatly in their outline, size, and relative development to the tarsi, and to the bodies of the insects possessing them, and in their covering; most are naked over their entire surface, having however a hexagonal network at their bases, which indicates a rudimentary condition of minute scale-like hairs, such as are common on some part of the integument of all insects. Flexor and extensor muscles are attached to both ungues and flaps; the flaps are either corrugated or arranged on the ridge and furrow plan, in other cases they are perfectly smooth on their free surface, while others are covered with minute scale-like hairs. The thickness of the divided membrane on the blow-fly does not exceed the 1/2000th of an inch at the margin; they somewhat increase in thickness towards the point of attachment. Projecting from the flap are organs which have been termed “hairs,” “hair-like appendages,” “trumpet-shaped hairs.” These are doubtless the immediate agents in holding on to a smooth surface, as that of glass, and are termed “tenent-hairs,” in allusion to their office. The under surface of left forefoot of _Musca vomitoria_ is shown with tenent-hairs (Plate VI., No. 140); _a_ and _b_ are more magnified hairs, _a_ from below, _b_ from the side. No. 142 is the left forefoot of _Amara communis_, showing the under surface and form of tenent appendages, one of which is seen more magnified at _a_; No. 143, under surface of left forefoot, _Ephydra riparia_. This fly is met with in immense numbers on the surface water in salt marshes. It does not possess the power of climbing glass; this is explained by the structure of the tenent-hairs; the central tactile organ is also very peculiar, the whole acting as a float, one to each foot, to enable the fly to rest on the surface of the water; _a_ is one of the external hairs, No. 135, under surface of left forefoot of _Cassida viridis_ (tortoise-beetle), showing the bifurcate tenent appendages, one of which is given at _a_ more magnified. These, in ground beetles, are met with only in males, and are used for sexual purposes. The delicacy of the structure of these hairs in the fly and the elastic membranous expansion of the foot are marvellous. When the fly is climbing, a minute quantity of some glutinous fluid is exuded, so that the tubular nature of the tenent-hairs hardly admits of a doubt.

“At the root of the pulvillus, or its under surface, is a process, which in some instances is short and thick, in others long and curved, and tapering to its extremity (Scatophaga), setose (Empis), plumose (Hippoboscidæ), or, in one remarkable example (Ephydra), closely resembling in its appearance the very rudimentary pulvillus with which it is associated. Just at the base of the fifth tarsal joint, on its under surface, there is present, in Eristalis, a pair of short, very slightly curved hairs, which point almost directly downwards.”[82]

Tenent-hairs are usually present in some modification or other. It is really difficult to name a beetle which has not some form of them; the only one I yet know that seems to me really to possess nothing of the kind is a species of Helops, living on sandy heaths. I suppose the dense cushion of hairs on the tarsi to be for the protection, simply, of the joints to which they are attached. I have detected them on the tarsal joints of species of Ephydra, and on the first basal tarsal joint of the drone of the hive-bee. A very rudimentary form of tenent-hairs is present on the under surface of some of the tree-bugs (Pentatomidæ), which have in addition a large, deeply-cleft organ at the extremity of the tarsus; this appears to be a true sucker.

When walking on a rough surface, the foot represents that of a Coleopterous insect without any tenent appendages. The ungues are always attached to the last joint of an insect’s tarsus. They are not attached to the fifth tarsal joint of a Dipterous insect, neither are they attached to the fifth tarsal joint of a Hymenopterous insect, but to the terminal sucker, which again, in this great order, is a sixth tarsal joint, membranous, flexible, elastic in the highest degree, retractile to almost its fullest extent within the fifth tarsal joint--a joint modified to an extraordinary degree for special purposes.

In plantula of Lucanus, with its pair of minute claws, the ungues are hairs modified for special purposes; and they have the structure of true hairs. The sustentacula of Epeira, the analogous structures on the entire under surface of the last tarsal joints in Pholcus, the condition of the parts in the hind limbs of Notonecta, in both its mature and earlier conditions, as well as in Sarcoptes, Psoroptes, and some other Acari, all may be cited in proof of this fact. The various orders of insects have, for the most part, each their own type of foot. Thus there is the Coleopterous type, the Hymenopterous type, the Dipterous type, the Homopterous type, &c.; each so very distinctive, that in critical instances they will sometimes serve at once to show to which order an insect should be referred. Thus, amongst all the Diptera, I have as yet met with but one subdivision which presents an exception to the structure described. This exception is furnished by the Tipulidæ, which have the Hymenopterous foot. With hardly an exception, then, I believe the form of foot described will be found universal among the Diptera.

It may be desirable to add a few words on the best plan of conducting observations on the feet of insects. Their action should be studied by placing the insect under the influence of chloroform. It is of advantage to carefully preserve the parts examined, and for this purpose Deane’s medium or glycerine jelly suits very well; some of the more delicate preparations, however, can only be kept unchanged in a solution of chloride of zinc. The plan of soaking in caustic potash, crushing, washing, putting into spirits of wine and then into turpentine, and lastly into Canada balsam, is perfectly useless, excepting in rare instances where points connected with the structure of the integument have to be made out. Of course, the parts should be viewed from above, from below, and in profile, in order to gain exact ideas of their relations. The binocular microscope diminishes the difficulties which formerly had to be encountered, as by its aid many parts may be clearly viewed without preparation of any kind.

1. Antenna of the Silkworm-moth; 2. Tongue of Butterfly; 3. A portion of tongue highly magnified, showing its muscular fibre; 4. Tracheæ of silkworm; 5. Foot of silkworm. (The small circles enclose each object somewhat near the natural size.)]

Moths and butterflies supply the microscopist with some of the most beautiful objects for examination. What can be more wonderful in its adaptation than the antenna of the moth (represented in Fig. 400, No. 1), with a thin, finger-like extremity almost supplying the insect with a perfect and useful hand, moved throughout its extent by a muscular apparatus of the most exquisite construction. The tongue of butterfly (No. 2) is evidently made for the purpose of dipping into the interior of flowers and extracting the juices; this act is assisted by a series of fine muscles. An enlarged view of a portion is given at No. 3; see Plate VI., Nos. 132 and 133, antennæ of Vapour Moth.

Fig. 401.--Breathing aperture or spiracle of silkworm. (In the circle it is shown about the natural size.)]

Fig. 402.--Magnified portions of the trachea of the Hydrophilus, showing spiral tubes.]

The inconceivably delicate structure of the maxillæ or tongues (for there are two) of the butterfly, rolled up like the trunk of an elephant, and capable, like it, of every variety of movement, has been carefully examined and described by Mr. Newport. “Each maxilla is convex on its outer surface, but concave on its inner; so that when the two are united they form a tube, _haustellium_, by their union, through which fluids may be drawn into the mouth. The inner or concave surface, which forms the tube, is lined with a very smooth membrane, and extends throughout the whole length of the organ; while that of each maxilla is hollow in its interior, apparently forming a tube ‘in itself,’ but this is not so; the mistake has arisen from the existence of large tracheæ, or breathing tubes, in the interior of the proboscis. In some species the extremity of the haustellium is studded externally with a number of minute papillæ, or fringes--as in _Vanessa atalanta_--in which they become small elongated barrel-shaped bodies, terminated by smaller papillæ at their extremities. On alighting on a flower, the insect makes a powerful expiratory effort, by which the air is expelled from the interior air-tubes, and from those with which they are connected in the head and body; and at the moment of applying its proboscis to the food, it makes an inspiratory effort, by which the central canal in the proboscis is dilated, and the food ascends it at the same instant to supply the vacuum produced; and thus it passes into the mouth and stomach, the constant ascent of the fluid being assisted by the action of the muscles of the proboscis, which continues during the whole time that the insect is feeding. By this combined agency of the acts of respiration and the muscles of the proboscis we are also enabled to understand the manner in which the humming-bird sphynx extracts in an instant the honey from a flower while hovering over it, without alighting; and which it certainly would be unable to do were the ascent of the fluid entirely dependent upon the action of the muscles of the organ.”

The trachæal or respiratory system of insects varies, or rather is found to exist in modified forms to suit their varied conditions of life. While in the larval stage the breathing apertures are seen to recur at intervals on each side of the abdomen (as that of the silkworm, Fig. 401), thus ensuring a continuous supply of air to the circulating fluids throughout the whole body. These spiracles are usually nine or ten in number, and consist of a membranous ring of an oval form. The air-tubes are exquisitely composed of two thin membranes, between which a delicate elastic thread or spiral fibre is interposed, forming a cylindrical opening and keeping the tube in a distended condition, thus mechanically preserving the sides from collapse or pressure in their passage through the air, which otherwise might occasion suffocation. Fig. 402 represents the double spiral arrangement of a portion of a trachea of Hydrophilus, which ensures both elasticity and strength.

There are other points of interest confined to the water-beetle tribe, among the more striking of which is the foreleg of the _Dytiscus marginalis_. Here the first three joints of the tarsus are expanded into a broad surface, and fringed throughout with curved hairs. From the surface of these spring a number of short hairs, with cup-like discs at their extremities, one of which is seen highly magnified in Plate VI., No. 142. These are so cup-like in form that they have been hitherto described as “suckers,” but it is believed they are simply a special apparatus for the development of the hairs seen on the leg and foot of the beetle. Another curious example occurs in the Gyrinus, or whirligig-beetle. The front pair of legs are of the ordinary kind, but the under pair are furnished with expanding paddles. The trochanter, femur, and tibia, are flat plates of a triangular shape, pointed at their outer angles, from which the apex springs. But the tarsus is jointed on the inner angle of the furthermost end of the tibia, and each of its four joints expands into a flat paddle blade. In the accompanying Fig. 403 one paddle is seen expanded, the other closed.

1. Leg of Gyrinus, Whirligig, paddle shown expanded. 2. Paddle closed up.]

These paddles are adapted with much precision to ensure the most effectual application of the propelling power; as the beetle strikes out in the act of swimming, the membranous expansion described enables it to move about with great rapidity; upon the legs being drawn back towards the body, the membrane closes up, and thus offers no resistance to the water. The eyes are not the least curious part of the merry little beetle: the upper section is fitted for seeing in the air, and is adapted to the upper or superior part of the head; the lower portion, for seeing under the water, being placed at a lower angle, a thin division only separating the two.

1. Scale of _Morpho menelaus_; 2. Large scale of _Polyommatus argiolus_, azure blue; 3. _Hipparchia janira argiolus_; 4. _Pontia brassica_; 5. _Podura plumbea_; 6. Small scale of azure blue.]

_Wings of Insects._--These exhibit variety of form and structure, as well as of beauty of colouring. At an early period the orders of insects were mainly founded upon these interesting appendages. The Orthoptera were the straight wings; the Neuroptera the nerved; the Trichoptera the hairy wings; the Coleoptera the cased or sheathed wings; the Diptera the two wings; the Hymenoptera the married wings; and the Lepidoptera the scaled wings. A number of wings are small and membranous, and may be mounted dry for examination under the microscope. Others are better seen mounted in benzol-balsam. The elytra, iridescent wing cases of the diamond, and other beetles, as well as the wings of the more highly coloured butterflies, make pretty objects mounted dry for opaque illumination by the Lieberkühn or reflector. The thicker horny cases of other members of the beetle tribe require long soaking, as described in a former chapter.

The wings of moths and butterflies are covered with scales or feathers, carefully overlapping each other, as tiles are made to cover the tops of houses. The iridescent variety of colouring on insects’ wings arises from the peculiar wavy arrangement of the scales. Figs. 404 and 405 are magnified representations of a few of them. No. 1, a scale of the _Morpho menelaus_, taken from the side of the wing, is of a pale-blue colour; it measures about 1/120th of an inch in length, and exhibits a series of longitudinal striæ or lines, between which are disposed cross-lines or other striæ, giving it very much an appearance of brick-work (better seen in Fig. 405, No. 1).

1. Portion of scale of _Morpho menelaus;_ 2. Portion of large scale of _Podura plumbea;_ 3. Scale from the wing of Gnat, its two layers being represented; 4. Portion of a large scale of _Lepisma Saccharina;_ 5. Body scale of Gnat, magnified 650 diameters.]

_Polyommatus argiolus_, azure-blue (Fig. 404, Nos. 2 and 6), are large and small scales taken from the under-side of the wing of this beautiful blue butterfly; the small scale is covered with a series of spots, and exhibits both longitudinal and transverse striæ, these should be clearly defined, and the spots separated by a quarter-inch object-glass. No. 3, _Hipparchia janira_, is a scale from the meadow-brown butterfly: on this brown spots, having an irregular shape with longitudinal striæ, are seen. No. 4, _Pontia brassica_, cabbage butterfly, was at one time taken to be an excellent criterion of the penetration and definition of an object glass. It is seen to have a free extremity or brush-like appendage. With a fairly good power, the longitudinal markings appear like rows of small beads. Chevalier selected for his test object the scale of the _Pontia brassica_. Mohl and Schacht extolled _Hipparchia janira_ as a good test of penetration in an objective of moderate angular aperture. Amici’s test object is _Navicula rhomboides_, the display of the lines forming the test.

The _Tinea vestianella_, clothes-moth, is furnished with unique scales. Small and destructive as this moth is, it suffers much from a parasitic mite, and from which it is unable to free itself.

The Podura scale (Fig. 405), with its delicate transparent membrane and curiously inserted “notes of admiration,” as they were called, was long believed to be an excellent test object for the highest powers of the microscope, but I believe it is no longer regarded in that light: indeed, most insect scales have declined in the value and estimation of the skilled microscopist. This is in part due to the improvements made in the objective. The high-angled glasses have cleared up obscure points in the structural characters of the minuter forms of life, and the scales of insects are no longer found to be difficult test objects for the modern objective of a Zeiss or a Powell to resolve. Nevertheless, the scale of the Podura belonging to the order Thysanura, a curious little insect commonly known by the name of springtail, usually found living in most obscure places, and too small to attract attention, is not likely to be entirely thrust aside. The springtails (Collembola) are furnished on the under-side of the first abdominal segment with a curious tube or sucker, from the orifice of which glandular process a secreted viscid matter is protruded; they are remarkable also from the fact that in most of them no trace of a tracheal system has yet been discovered. The eyes when present are in the form of simple or grouped ocelli, the antennæ number six joints, and the abdomen has but six segments, often only three. The forked tail is a curious process turned forward and attached to one of the tender segments and held in position under the body; when released it springs back and bounds up to a very considerable height. Fig. 406 represents _Podura villosa_. There are several species, one of which (_P. aquatica_) is found floating in patches on pools of water on bright summer days.

_Lepisma saccharina_ belongs to the same genus as Podura. This minute springtail derives its name from having been discovered in old sugar-casks. It has a spindle-shaped body covered with silvery scales, long used as test objects. The sides of the abdomen are furnished with a series of appendages with long bristle-like setæ, or hairs, at their extremities. The head is concealed under a prothorax, the antennæ are long, and the maxillary palpi are either five or seven-jointed, and very conspicuous, to enable them to cut the dry wood on which they principally feed. The scales must be mounted under thin cover-glasses; oblique illumination shows up some portions to advantage, while central light from an achromatic condenser and a wide-angled objective renders their markings more distinct. Portion of a scale more highly magnified is shown in Fig. 405.

_Eggs of Insects_ (Plate VI., Nos. 124-139).--In form, colour, and variety of design, the eggs of insects are more surprisingly varied than those of the feathered tribes; but as from extreme smallness they escape observation, an acquaintance with their structure is not so familiar as it might be. Although the eggs of the bird tribe differ much in their external characteristics, they closely resemble each other while yet a part of the ovarian ova, and prior to their detachment from the ovary. At one period of their formation all eggs consist of three similar parts:--1st. The internal nucleated cell, or germinal vesicle, with its macula; 2nd. The vitellus, or yolk-substance; and 3rd. The vesicular envelope, or vitelline membrane. The germinal vesicle is the first produced, then the yolk substance, which gradually envelops it, and the vitelline membrane, the latest formed, incloses the whole. The chemical constituents of the egg are the same in all cases, albumen, fatty matters, and a proportion of a substance precipitable by water. The production of the chorion, or shell membrane, does not take place till the ovum has attained nearly its full size, and it then appears to proceed, in part at least, from the consolidation over the whole surface of one or more layers of an albuminous fluid secreted from the wall of the oviduct.

The embryo cell is so directly connected with the germinal vesicle that at a certain period it disappears altogether, and is absorbed into the germinal yolk, or rather becomes the nucleus of the embryo, when a greater degree of compactness is observed in the yolk, and all that remains of the germinal vesicle is one or more highly refracting fat globules and albuminoid bodies. Towards the end of the period of incubation, the head of the young caterpillar is said to lie towards the dot or opening in the lid, termed the micropyle,[83] from its resemblance to a small gate, or opening through which the larva emerges forth as a butterfly.

The germinal vesicle is comparatively large and well-marked while the egg is yet in the ova-sac. By preparing sections after Dr. Halifax’s method,[84] we find that the germinal vesicle in the bee’s egg is not situated immediately near or even below the so-called micropyle, but rather more to the side of the egg; just in the position which the head of the embryo is subsequently found to occupy at maturity.

The egg membrane, or envelope, of all the Lepidoptera is composed of three separate and distinct layers: an external slightly raised coat, tough and hard in its character, a middle one of united cells, and a fine transparent vitelline lining membrane, perfectly smooth and homogeneous in structure, imparting solidity, and giving a fine iridescent hue to the surface. The germinal vesicle is of a proportionately large size for the egg, and its macula is at first single, then multiple. In the egg of the silkworm the outer membrane is comprised of an inner reticulated membrane of non-nucleated cells, in the outer layer the cells are arranged in an irregular circular form, also non-nucleated, with minute interstitial setæ or hairs projecting outward.