“The locomotive machine of insects may be called, to a certain extent, a double set of three feet each, as most insects, and particularly those provided with a broad trunk, are able to balance themselves with one of these two sets of feet, and indeed when walking, as well as when standing still, can move about even better with one set of these feet than with four legs. In the latter case, that is, if one cuts off a pair of legs from an insect, the trunk can balance itself only with extreme difficulty, and there is therefore little prospect that insects will ever become four-footed.

“But if one compels insects to run on three legs, he will thus make the interesting discovery that to make up the deficiency they place the remaining feet and bring them to the ground somewhat differently than when the second set of feet is active. Figs. 124 and 126 may be compared for this purpose. The former shows the footprints of a burying beetle running with all six legs, the latter the track of the same insect, which, however, has at its disposal only the right fore leg, the left middle leg, and the right hind leg. One may plainly see here that the track of the hind leg on the right side (r₃) approaches the track of the middle leg on the left side, and then further, that the right fore leg (r₁) steps out more to the right to make up for the deficiency of the middle leg.

“A similar adaptation of the position of the legs, which is entirely dependent on the choice of the insect, may also be observed there, if one compels insects which are not provided with corresponding adhesive lobes to run away over crooked surfaces. Fig. 123 shows the footprints of a Blaps when running upon a horizontal plane. Fig. 127, on the contrary, shows the tracks of the legs when going diagonally over a gradually inclined surface. Here, also, the insect holds on with his fore and middle legs (r₁, r₂) stretched upward, whereby also the impressions on both sides come to lie farther apart than in the normal mode of walking.

“It will not surprise the reader who is familiar with the gait of crabs, to hear that many insects also understand the laudable art of going backward, wherein the hind legs simply change places with the fore legs.

“The jumping motion of insects may be best studied in grasshoppers. When these insects are preparing for a jump, they stretch out the upper thigh horizontally, clap the tibiæ together, and also retract the foot-segment. After a slight pause for rest, during which they are getting ready for the jump, they then jerk the tibiæ suddenly backward and against the ground with all their strength by means of the extensor muscles.”

The correctness of Graber’s views has been confirmed by Marey by instantaneous photographs (Figs. 128, 129).

Locomotion on smooth surfaces.—How flies and other insects are able to walk up, or run with the body inverted, on hard surfaces has been lately discovered by Dewitz, Dahl, and others. All authors are agreed that this power is due to the presence of the specialized empodium of each tarsus.

Dewitz confirmed the opinion of Blackwell, that a glutinous liquid is exuded from the apices of the tenent hairs which fringe the empodium. By fastening insects feet uppermost on the under side of a covering glass which projects from a glass slide, the hairs which clothe the empodia of the foot of a fly (Musca erythrocephala) may be seen to be tipped with drops of transparent liquid. On the leg being drawn back from the glass, a transparent thread is drawn out, and drops are found to be left on the glass. In cases where these hairs are wanting, as in the Hemiptera, the adhesive fluid exudes directly from pores in the foot. In the beetles (Telephorus dispar) and other insects the tenent hairs on the foot end in sharp points, below which are placed the openings of the canals. The glands, Dewitz states, are chiefly flask-shaped and unicellular, situated in the hypodermis of the chitinous coat; each gland opening into one of the hairs (Fig. 108); they are each invested by a structureless tunica propria, and contain granular protoplasm, a nucleus placed at the inner side, and a vesicle, prolonged into a tube which, traversing the neck of the gland, is attached to the root of the hair; the vesicle receiving the secretion. Each gland is connected with a fine nerve-twig, and secretion is probably voluntary. Among the tenent hairs of the empodium are others which must be supplied with a nerve, forming tactile hairs, as they each proceed from a unicellular ganglion (Fig. 108, n″). The secretion is forced out of the gland by the contraction of the protoplasm, Dewitz having seen the secretion driven out from the internal vesicle into its neck.

In the spherical last tarsal joint of Orthoptera (Fig. 109), which is without these tenent hairs, nearly all the cells of the hypodermis are converted into unicellular glands, each of which sends out a long, fine, chitinous tubule, which is connected with its fellows by very fine hairs and is continuous with the chitinous coat of the foot and opens through it. The sole of the foot is elastic and adapts itself to minute inequalities of surfaces, while the anterior of each tarsal joint is almost entirely occupied by an enlargement of the trachea, which acts on the elastic sole like an air chamber, rendering it tense and at the same time pliant. Dewitz adds that the apparatus situated on the front legs of the male of Stenobothrus sibiricus (Fig. 131) must have the function of causing the legs to adhere closely to the female by the excretion of an adhesive material. The hairs of the anterior tarsi of male Carabi also appear to possess the power of adhesion. In the house-fly the empodia seem to be only called into action when the insect has to walk on vertical smooth surfaces, as at other times they hang loosely down.

Burmeister observed the use of a glutinous secretion for walking in dipterous larvæ, and Dewitz found that the larva of a Musca used for this purpose a liquid ejected from the mouth. The larvæ of another fly (Leucopis puncticornis) perform their loop-like walk by emitting a fluid from both mouth and anus. A Cecidomyia larva is able to leap by fixing its anterior end by means of an adhesive fluid. The larva of the leaf-beetle, Galeruca, moves by drawing up its hinder end, fixing it thus, and carrying the anterior part of the body forward with its feet until fully extended, when it breaks the glutinous adhesion. The abdominal legs of some saw-fly larvæ have the same power.

Dahl could not detect in the foot of the hornet (Vespa crabro) any space which could be considered as a vacuum.

Simmermacher states that in most cases of climbing beetles the tubular tenent hairs pour out a secretion (Figs. 133, 134), “and it is probable that we have here to do with the phenomena not of actual attachment by, as it were, gluing, but of adhesion; the orifice of the tubes is divided obliquely, and the tubes are, at this point, extremely delicate and flexible, so as to adhere by their lower surface; in this adhesion they are aided by the secreted fluid.” In the case of the Diptera he does not accept the theory by which the movement of the fly along smooth surfaces is ascribed to an alternate fixation and separation, but believes in a process of adhesion, aided by a secretion, as in many Coleoptera. (In the Cerambycidæ there is no secretion, and the tubules are merely sucking organs, like those observed in the male Silphidæ.) “The attaching lobes, closely beset with chitinous hairs, are enabled, in consequence of the pressure of the foot, to completely lie along any smooth surface; this expels the air beneath the lobes, which are then acted on by the pressure of the outer air.” (Journ. Roy. Micr. Soc., 1884, p. 736.) Another writer (Rombouts) thinks this power is due to capillary adhesion.

The action of the pulvillus and claws when at rest or in use by the honey-bee is well shown by Cheshire (Fig. 135, B). In ascending a rough surface, “the points of the claws catch (as at B) and the pulvillus is saved from any contact, but if the surface be smooth, so that the claws get no grip, they slide back and are drawn beneath the foot (as at A), which change of position applies the pulvillus, so that it immediately clings. It is the character of the surface, then, and not the will of the bee, that determines whether claw or pulvillus shall be used in sustaining it. But another contrivance, equally beautiful, remains to be noticed. The pulvillus is carried folded in the middle (as at C, Fig. 105), but opens out when applied to a surface; for it has at its upper part an elastic and curved rod (cr, Figs. 105 and 135), which straightens as the pulvillus is pressed down; C and D, Fig. 135, making this clear. The flattened-out pulvillus thus holds strongly while pulled, by the weight of the bee, along the surface, to which it adheres, but comes up at once if lifted and rolled off from its opposite sides, just as we should pull a wet postage stamp from an envelope. The bee, then, is held securely till it attempts to lift the leg, when it is freed at once; and, by this exquisite yet simple plan, it can fix and release each foot at least twenty times per second.” (Bees and Bee-keeping, p. 127.)

Ockler divides the normal two-clawed foot into three subtypes: (1) with an unpaired median empodium; (2) with two outer lateral adhesive lobes; (3) with two adhesive lobes below the claws; the latter is the chief type and forms either a climbing or a clasping foot. The amount of movement possessed by the claws is limited, and what there is, is effected by means of an elastic membrane and the extensor plate (Fig. 110). The “extensor sole” which is always present in insects with an unpaired median fixing or adhesive organ (empodium) is to be regarded as a modification of the extensor seta. The extensor plate is peculiar to an insect’s foot. Ockler states that the so-called “pressure plate” of Dahl is only a movably articulated, skeletal, supporting plate for the median fixing lobule.

Climbing.—In certain respects the power of climbing supplies the want of wings, and even exists often in house-flies among which there is shown a many-sided motion that is quite unheard of in other groups of insects.

The best climbers are obviously those insects which live on trees and bushes, as, for example, longicorn beetles and grasshoppers. These may be accurately called the monkeys of the insect kind, even if their movements take place less gracefully, and indeed rather stiffly and woodenly. We already know what are the proper climbing organs; that is, the sharp easily movable claws on the foot. With the help of these claws certain insects, May-beetles for example, can hang upon one another like a chain; indeed, bees and ants in this manner bind themselves together into living garlands and bridges. There are still added to the chitinous hooks flaps and balls of a sticky nature, by help of which likewise the insects glue themselves together. To facilitate the spanning of still thicker twigs, the climbing foot of insects has a greater movability even than when it only serves as a sole. (Graber.)

The mode of swimming of insects.—To study the swimming movements of insects, let us examine a Dyticus. It will appear, as Graber states, to be wonderfully adapted to its element.

“The body resembles a boat. There is nowhere a projecting point or a sharp corner which would offer unnecessary resistance to motion; bulging out in the middle and pointed at the end, it cuts through the resistance of the water like a wedge. The movable parts, the oars, seem to be as well fitted for their purpose as the burden to be moved by them. That the hind legs must bear the brunt of this follows from their position exactly in the middle of the body, where it is widest. In other insects also these legs are used for the same purpose as soon as the insects are put in the water. But the swimming legs of water-beetles are oars of quite peculiar construction. They are not turned about in the coxæ, as are other legs, but at the foot-joint. The coxa, namely, has grown entirely together with the thoracic partition. The muscles we have mentioned, exceeding in strength all the soft parts taken together, take hold directly of the large wing-shaped tendons of the upper thigh, and extend and retract the leg in one of the planes lying close to the abdominal partition. The foot forms the oar, however. It is very much lengthened and still more widened, and can be turned and bent in by separate muscles in such a way that in the passive movement, that is, the retraction, the narrow edge is turned to the fore, and therefore to the medium to be dislodged; however, as soon as the active push is to be performed and the leg is extended with greater force, it cuts down through the water with its whole width. These effective oar-blades are still considerably enlarged by the hairs arising on the side of the foot, which spread out at the decisive moment.

“Every one knows that the oar-blades of swimming beetles always go up and down simultaneously and in regular time. On the other hand, as soon as one puts a Dyticus on the dry land, i.e. on an unyielding medium, it uses its hind legs entirely after the manner of other land insects; that is, they are drawn in and extended again alternately, as takes place clearly enough from the footsteps in Fig. 119, A. We learn from this that water insects have not yet, from want of practice, forgotten the mode of walking of land insects.

“The forcing up of the water as a propelling power is added to the repulsion produced by the strong strokes of the oars. If the beetle stood up horizontally in the water, he would be lifted up.

“As the trunk, however, assumes an oblique position when the insect wishes to swim, one can then imagine the driving up of the water as being divided into two forces, one of which drives the body forward in a horizontal direction, while the other, that is, the vertical component, is supplied by the moving of the oars. The swimming insect is thus, as it were, a snake flying in the water.

“The long streamer-like hind legs of many water-bugs, for example Notonecta, approach more nearly our artificial oars. These legs are turned out from the bottom.

“There is no doubt but that the legs of insects, as regards the many-sidedness and exactitude of their locomotive actions, place the similar contrivances of other animals far in the shade. We shall be forced to admire these ingenious levers still more, however, when we take into consideration their energy and strength. That the force with which the locomotive muscles of insects is drawn together is enormous compared with that of vertebrates, we may learn if we try to subdue the rhythmical movements of the thorax of a large butterfly by the pressure of our finger or to open against the insect’s will the closed jumping leg of a grasshopper, or the fossorial shovel of a mole-cricket.”

MacLeay, W. S. On the structure of the tarsus in the tetramerous Coleoptera of the French entomologists. (Trans. Linn. Soc. London, xv, 1825, pp. 63–73.)

Speyer, O. Untersuchung der Beine der Schmetterlinge. (Isis, 1843, pp. 161–207, 243–264.)

Pokorsky Joravko, A. von. Quelques remarques sur le dernier article du tarse des Hyménoptères. (Bull. Soc. imp. Natur. Moscou, 1844, xvii, pp. 140–159. Ref. in Isis, 1848, v, p. 347.)

Rossmassler, E. A. Das Bein der Insekten. (Aus der Heimath, 1860, 3 kap., pp. 327–334, Fig.)

West, Tuffen. The foot of the fly; its structure and action; elucidated by comparison with the feet of other insects, etc. Part I. (Trans. Linn. Soc. London, xxiii, 1861, pp. 393–421, 1 Pl.)

Sundevall, C. On insektenas extremiteter samt deras hufoud och munddelar. (Kongl. Vetenskaps Akad. Handlingar. iii, Nr. 9, 1861.)

Lindemann, C. Notizen zur Lehre vom ausseren Skelete der Insekten (Gelenke und Muskeln der Füsse). 1 Taf. (Bull. Soc. imp. d. Natur. Moscou, xxxvii, 1864, pp. 426–432.)

Liebe, O. Die Gelenke der Insekten. Chemnitz, 1873. 4º. 1 Taf.

Canestrini, J. Ueber ein sonderbares Organ der Hymenopteren. (Zool. Anzeiger, 1880, pp. 421, 422.)

Dahl, F. Beiträge zur Kenntnis des Baues und der Funktionen der Insektenbeine. (Archiv f. Naturgesch. 1 Jahrg., 1884, pp. 146–193, 3 Taf. Sep., 48 pp. Vorlauf. Mitteil, in Zool. Anz., 1884, pp. 38–41.)

Langer, K. Ueber den Gelenkbau bei den Arthrozoen. Vierter Beitrag zur vergleichenden Anatomie und Mechanik der Gelenke. (Denkschriften der Akad. d. Wissensch. Wien, xviii, Bd. Physikal.-mathem. Classe, pp. 99–140. 3 Taf.)

Graber, Vitus. Ueber die Mechanik des Insektenkörpers. (Biolog. Centralbl., iv, 1884, pp. 560–570.)

—— Die ausseren mechanischen Werkzeuge der Tiere, ii Teil. Wirbellose Tiere, 1886, pp. 175–182, 208–210.

Dewitz, H. Ueber die Fortbewegung der Tiere an senkrechten glatten Flächen vermittelst eines Sekretes. 3 Taf. (Pflüger’s Archiv f. d. ges. Physiologie, xxxiii, 1884, pp. 440–481.)

Ockler, A. Das Krallenglied am Insektenfuss. (Archiv f. Naturgesch., 1890, pp. 221–262, 2 Taf.)

LITERATURE OF LOCOMOTION (WALKING, ETC.)

Carlet, G. Sur le mode de locomotion des chenilles. (Compt, rend. Acad. Paris, 1888, cvii, pp. 131–134. Naturwiss. Rundschau, iii Jahrg., 1888, No. 42, p. 543.)

—— De la marche d’un insecte rendu tetrapode par la suppression d’une paire de pattes. (Ibid., pp. 565, 566.)

—— Sur la locomotion des insectes et des arachnides. (Ibid., 1879, T. 89, pp. 1124, 1125.)

—— Ueber den Gang eines vierfüssig gemachten Insekts. (Naturwiss. Rundschau, viii Jahrg., 1888, pp. 666–667; Compt. rend. 1888, cvii.)

Demoor, J. Recherches sur la marche des insectes et des arachnides. Étude experimentale d’Anatomie et de Physiologie comparées. (Archiv de Biologie, Liège, 1880, 42 pp. 3 Pls.)

—— Ueber das Gehen der Arthropoden mit Berücksichtigung der Schwankungen des Körpers. (Compt. rend. Acad. d. Sc. Paris, 1890, cxi, pp. 839–840.)

Osten-Sacken, C. R. von. Ueber das Betragen des kalifornischen flügellosen Bittacus (apterus McLachl.). (Wiener Ent. Zeit., 1882, pp. 123.)

Dixon, H. H. Preliminary note on the walking of some of the Arthropoda. (Proc. R. Dublin Soc. vii, pp. 574–578, 1892. Also Nature, 1897.)

Also the works of Graber, Marey, Cheshire, etc.

LITERATURE OF WALKING ON SMOOTH SURFACES

Blackwell, J. Remarks on the pulvilli of insects. (Trans. Linn. Soc. London, xvi, 1831, pp. 487–492, 767–770.)

Lowne, B. T. On the so-called suckers of Dytiscus and the pulvilli of insects. (Trans. Roy. Micr. Soc., pp. 267–271, 1871, 1 Pl.)

West, Tuffen. On certain appendages to the feet of insects subservient to holding or climbing. (Journ. of the Proceed. Linn. Soc. London, Zoölogy, vi, 1862, pp. 26–88.)

Dewitz, H. Ueber die Fortbewegung der Tiere an senkrechten, glatten Flächen vermittelst eines Sekrets. (Pflüger’s Archiv f. d. ges. Physiologie, xxxiii, 1884, pp. 440–481. 3 Taf. Also Zool. Anzeiger, 1884, pp. 400–405.)

—— Wie ist es den Stubenfliegen und anderen Insekten möglich, an senkrechten Glaswanden emporzulaufen. (Sitzungsb. Ges. naturf. Freunde zu Berlin, 1882, pp. 5–7.)

—— Weitere Mitteilungen über den Klettern der Insekten (Ibid., 1882, pp. 109–113).

—— Die Befestigung durch einen klebenden schleim beim springen gegen senkrechte Flächen. (Zool. Anzeiger, 1883, pp. 273, 274.)

—— Ueber die Wirkung der Haftlappchen toter Fliegen. (Ent. Nachr., x Jahrg., 1884, pp. 286, 287.)

—— Weitere Mitteilungen über das Klettern der Insekten an glatten senkrechten Flächen. (Zoolog. Anzeiger, 1885. viii Jahrg., pp. 157–159.)

—— Richtigstellung der behauptungen des Herrn F. Dahl. (Archiv f. mikroskop. Anat., 1885, xxvi, pp. 125–128.)

Rombouts, J. E. Ueber die Fortbewegung der Fliegen an glatten Flächen. (Zool. Anzeiger, 1884, pp. 619–623.)

—— De la faculté qu’out les mouches de se mouvoir sur le verre et sur les autres corps polis. (Archiv Museum Teyler (2), 4 Part, pp. 16. Fig.)

Simmermacher, G. Untersuchungen über Haftapparate an Tarsalgliedern von Insekten. (Zeitschr. f. wissensch. Zool. xl, 1884, pp. 481–556. 3 Taf., 2 Figs. Also Zoolog. Anzeiger, vii Jahrg., 1884, pp. 225–228.)

—— Antwort an Herrn Dr. H. Dewitz. (Ibid., pp. 513–517.)

Dahl, F. Die Fussdrüsen der Insekten. (Archiv f. mikroskop. Anat., 1885, xxv, pp. 236–263. 2 Taf. See also p. 118.)

Emery, C. Fortbewegung von Tieren an senkrechten und überhangenden glatten Flächen. (Biolog. Centralbl., 1884, 4 Bd., pp. 438–443.)

Léon, N. Disposition anatomique des organes de succion chez les Hydrocores et les Géocores. (Bull. Soc. des Medec. et Natur, de Jassy., 1888.)

The insects differ from all other animals except birds in possessing wings, and as we at the outset have claimed, it is evidently owing to them that insects are numerically so superior to any other class of animals, since their power of flight enables them to live in the air out of reach of many of their enemies, the greatest destruction to insect life occurring in the wingless larval and pupal stages.

The presence of wings has exerted a profound influence on the shape and structure of the body, and it is apparently due to their existence that the body is so distinctly triregional, since this feature is least marked in the synapterous insects. The wings are thin, broad leaf-like folds of the integument, attached to the thorax and moved by powerful muscles which occupy the greater part of the thoracic cavity. The two pairs of wings are outgrowths of the middle and hinder part of the thorax, the anterior pair being attached to the mesothoracic and the hinder pair to the metathoracic segment. The larger pair is developed from the middle segment of the thorax. The differentiation of the tergites into scutum, scutellum, etc., is the result of the appearance of wings, because these sclerites are more or less reduced or effaced in wingless insects, such as apterous Orthoptera and moths, ants, etc.

The size of the hinder thoracic segments is closely related to that of the wings they bear. In those Orthoptera which have hind wings larger than those of the fore pair, the metathorax is larger than the mesothorax. In such Neuroptera as have the hind wings nearly or quite as large as the anterior pair, or in the Trichoptera and in the Hepialidæ, the metathorax is nearly as large as the mesothorax, while in Coleoptera the metathorax is as large and often much larger. In the Ephemeridæ, Diptera, and Hymenoptera, which have either only rudimentary (halteres) or small hind wings, the metathorax is correspondingly reduced in size.

The wings morphologically, as their development shows, are simple sac-like outgrowths of the integument, i.e. of the free hinder edge of the tergal plates, their place of origin being apparently above the upper edge of the epimera or pleural sclerites. Calvert^[24] however, regards the upper lamina of the wing as tergal, and the lower, pleural.

The wings in most insects are attached to the thorax by a membrane containing several little plates of chitin called by Audouin articulatory epidemes.

The wings, then, are simple, very thin chitinous lamellate expansions of the integument, which are supported and strengthened by an internal framework of hollow chitinous tubes.

The veins.—The so-called “veins” or “nervures,” which are situated between the upper and under layers of the wing are so disposed as to give the greatest lightness and strength to the wings. Hagen has shown that in the freshly formed wings these two layers can be separated, when it can be seen that the veins pass through each layer.

These veins are in reality quite complex, consisting of a minute central trachea enclosed within a larger tube which at the instant the insect emerges from the nymph, or pupa, as the case may be, is filled with blood (Fig. 136). Since these tubes at first contain blood, which has been observed to circulate through them, and since the heart can be most easily injected through them, they may more properly be called veins than nervures. The shape and venation of the wings afford excellent ordinal as well as family and generic characters, while they also enable the systematist to exactly locate the spots and other markings of the wings. The spaces enclosed by the veins and their cross-branches are called cells, and their shape often affords valuable generic and specific characters.

The structure of a complete vein is described by Spuler. In a cross-section of a noctuid moth (Triphæna pronuba, Fig. 136) the chitinous walls are seen to consist of two layers, an outer (U) and inner (c), the latter of which takes a stain and lies next to the hypodermis (hy). In the cavity of the vein is the trachea (tr), which shows more or less distinctly the so-called spiral thread; within the cavity are also Semper’s “rib” (r) and blood-corpuscles (bc), which proves that the blood circulates in the veins of the completely formed wing, though this does not apply to all Lepidoptera with hard mature wings. We have been able to observe the same structure in sections of the wing of Zygæna.

A cross-section of a vein of Pieris brassicæ shows that the large trachea is first formed, and that it extends along the track between the protoplasmic threads connecting the two hypodermal layers.

The main tracheæ throw off on both sides a number of secondary branches showing at their end a cell with an intracellular tracheal structure; these accessory tracheæ afterwards branch out. The accessory or transverse tracheæ often disappear, though in some moths they remain permanently. Fig. 137 tr₂ represents these secondary veins in the edge of the fore wing of Laverna vanella, arising from a main trachea (tr) passing through vein I (v), two of the twigs extending to the centre, showing that the latter has no homology with a true vein. Only rarely and in strongly developed thick folds are the transverse tracheæ provided with a chitinous thickening, as for example in Cossus ligniperda. Since from such accessory tracheæ the transverse veins in lepidopterous wings are developed, we can recognize in them the homologies of the net-veins in reticulated venations. There is no sharply defined difference between reticulated and non-reticulated venations; no genetic difference exists between the two kinds of venation, since there occur true Blattidæ both with and without a reticulated venation (Spuler).

In the fore wings of Odonata, Psocina, Mantispidæ, and most Hymenoptera is an usually opaque colored area between the costal edge and the median vein, called the pterostigma.

In shape the wings are either triangular or linear oval, and at the front edge the main veins are closer together than elsewhere, thus strengthening the wings and affording the greatest resistance to the air in making the downward stroke during flight. It is noticeable that when the veins are in part aborted from partial disuse of the wings, they disappear first from the hinder and middle edge, those on the costal region persisting. This is seen in the wings of Embiidæ (Oligotoma), Cynipidæ, Proctotrupidæ, Chalcids, ants, etc.

The front edge of the wing is called the costal, its termination in the outer angle of the wing is called the apex; the outer edge (termen) is situated between the apex and the inner or anal angle, between which and the base of the wing is the inner or internal edge.

While in Orthoptera, dragon-flies, Termitidæ, and Neuroptera the wings are not attached to each other, in many Lepidoptera they are loosely connected by the loop and frenulum, or in Hymenoptera by a series of strong hooks. These hooks are arranged, says Newport, “in a slightly twisted or spiral direction along the margin of the wing, so as to resemble a screw, and when the wings are expanded attach themselves to a little fold on the posterior margin of the anterior wing, along which they play very freely when the wings are in motion, slipping to and fro like the rings on the rod of a window curtain.”

At the base of the hind wings of Trichoptera and in the lepidopterous Micropteryx there is an angular fold (jugum) at the base of each wing (Fig. 138); that of the anterior wings is retained in Eriocephala and Hepialidæ.

In the wings of Orthoptera as well as other insects, the fore wings, especially, are divided into three well-marked areas, the costal, median, and internal; of these the median area is the largest, and in grasshoppers and crickets is more or less modified to form the musical apparatus, consisting of the drum-like resonant area, with the file or bow.

The squamæ.—In the calyptrate Muscidæ, a large scale-like membranous broad orbicular whitish process is situated beneath the base of the wing, above the halter; (Fig. 94, 10 sq.) it is either small or wanting in the acalyptrate muscids. Kirby and Spence state that when the insect is at rest the two divisions of this double lobe are folded over each other, but are extended during flight. Their exact use is unknown. Kolbe, following other German authors, considers the term squama as applicable to the whole structure, restricting the term alula to the other lobe-like division.

More recently (1890 and 1897) Osten-Sacken recommends “squamæ; in the plural, as a designation for both of these organs taken together; squama, in the singular, would mean the posterior squama alone, and antisquama the anterior squama alone;” the strip of membrane running in some cases between them, or connecting the squama with the scutellum, should be called the post-alar membrane. By a mistake Loew, and others following him, used the word tegula for squama, but this term should be restricted to the sclerite of the mesothorax previously so designated (Fig. 90, A, t). The squama or its two subdivisions has also by various authors been termed alula, calypta, squamula, lobulus, axillary lobe, aileron, cuilleron, schuppen, and scale. (Berlin Ent. Zeitschrift, xli, 1896, pp. 285–288, 328, 338.)

The halteres.—In the Diptera the hind wings are modified to form the halteres or balancers, which are present in all the species, even in Nycteribia, but are absent in Braula.

Meinert finds structures in the Lepidoptera which he considers as the homologues of the halteres of Diptera. “In the Noctuidæ,” he remarks, “I find arising from the fourth thoracic segment (segment médiaire), but covered by hair, an organ like the halter of Diptera.” (Ent. Tidskrift., i, 1880, p. 168.) He gives no details.

In the Stylopidæ, on the contrary, the fore wings are reduced to little narrow pads, while the hind wings are of great size.

The thyridium is a whitish spot marking a break in the cubital vein of the fore wing of Trichoptera; these minute thyridia occur in the fore wings of the saw-flies; there is also an intercostal thyridium on the costal part of the wings of Dermaptera.

The fore wings of Orthoptera are thicker than the hinder ones, and serve to protect the hind-body when the wings are folded; they are sometimes called tegmina. It is noteworthy, that, according to Scudder, in all the paleozoic cockroaches the fore wings (tegmina) were as distinctly veined as the hinder pair, “and could not in any sense be called coriaceous.” (Pretertiary Insects of N. A., p. 39.) Scudder also observes that in the paleozoic insects as a rule the fore and hind wings were similar in shape and venation, “heterogeneity making its appearance in mesozoic times.” In the heteropterous Hemiptera, also, the basal half of the fore wings is thick and coriaceous or parchment-like, and also protects the body when they are folded; these wings are called hemelytra. In the Dermaptera the small short fore wings are thickened and elytriform.

The elytra.—This thickening of the fore wings is carried out to its fullest extent in the fore wings of beetles, where they form the sheaths, shards, or elytra, under which the hind wings are folded. The indexed costal edge is called the epipleurum, being wide in the Tenebrionidæ. During flight “the elytra are opened so as to form an angle with the body and admit of the free play of the wings” (Kirby and Spence). In the running beetles (Carabidæ), also in the weevils and in many Ptinidae, the hind wings are wanting, through disuse, and often the elytra are firmly united, forming a single hard shell or case. The firmness of the elytra is due both to the thickness of the chitinous deposit and to the presence of minute chitinous rods or pillars connecting the upper and lower chitinous surfaces.

Hoffbauer finds that in the elytra of beetles of different families the venation characteristic of the hind wings is wanting, the main tracheæ being irregular or arranged in closely parallel longitudinal lines, and nerve-fibres pass along near them, sense-organs being also present. The fat-bodies in the cavity of the elytra, which is lined with a matrix layer, besides nerves, tracheæ, and blood, contain secretory vesicles filled with uric-acid concretions such as occur in the fat-body of Lampyris. There are also a great many glands varying much in structure and position, such occurring also in the pronotum (Fig. 139).

Meinert considers the elytra of Coleoptera to be the homologues of the tegulæ of Lepidoptera and of Hymenoptera. He also calls attention to the alula observed in Dyticus, situated at the base of the elytra, but which is totally covered by the latter. The alulæ of these beetles he regards as the homologues of the anterior wings of Hymenoptera and Diptera. No details are given in support of these views. (Ent. Tidskrift, i, 1880, p. 168.)

Hoffbauer (1892) also has suggested that the elytra are not the homologues of the fore wings of other insects, but of the tegulæ.

Kolbe describes the alula of Dyticus as a delicate, membranous lobe at the base of the elytra, but not visible when they are closed: its fringed edge in Dyticus is bordered by a thickening forming a tube which contains a fluid. The alula is united with the inner basal portion and articulation of the wing-cover, forming a continuation of them. Dufour considered that the humming noise made by these beetles is produced by the alulets.

Hoffbauer finds no structural resemblances in the alulæ of Dyticus to the elytra. He does not find “the least trace of veins.” They are more like appendages of the elytra. Lacordaire considered that their function is to prevent the disarticulation of the elytra, but Hoffbauer thinks that they serve as contrivances to retain the air which the beetle carries down with it under the surface, since he almost always found a bubble of air concealed under it; besides, their folded and fringed edge seems especially fitted for taking in and retaining air. Hoffbauer then describes the tegulæ of the hornet and finds them to be, not as Cholodkowsky states, hard, solid, chitinous plates, but hollow. They are inserted immediately over the base or insertion of the fore wings, being articulated by a hinge-joint, the upper lamella extending into a cavity of the side of the mesothorax, and connected by a hinge-like, articulating membrane with the lower projection of the bag or cavity. The lower lamella becomes thinner towards the place of insertion, is slightly folded, and merges without any articulation into the thin, thoracic wall at a point situated over the insertion of the fore wing. The tegulæ also differ from the wings in having no muscles to move them, the actual movements being of a passive nature, and due to the upward and downward strokes of the wings.

Comstock adopts Meinert’s view that the elytra are not true fore wings, but gives no reasons. (Manual, p. 495.)

Dr. Sharp,^[25] however, after examining Dyticus and Cybister, affirms that this structure is only a part of the elytron, to which it is extensively attached, and that it corresponds with the angle at the base of the wing seen in so many insects that fold their front wings against the body. He does not think that the alula affords any support to the view that the elytra of beetles correspond with the tegulæ of Hymenoptera rather than with the fore wings.

That the elytra are modified paraptera (tegulæ) is negatived by the fact that the latter have no muscles, and that the elytra contain tracheæ whose irregular arrangement may be part of the modified degenerate structure of the elytra. Kolbe finds evidences of veins. The question may also be settled by an examination of the structure of the pupal wings. A study of a series of sections of both pairs of wings of the pupa of Doryphora and of a Clytus convinces us that the elytra are the homologues of the fore wings of other insects.

e. Development and mode of origin of the wings

Embryonic development of the wings.—The wings of insects are essentially simple dorsal outgrowths of the integument, being evaginations of the hypodermis. They begin to form in the embryo before hatching, first appearing as folds, buds, or evaginations, of the hypodermis, which lie in pouches, called peripodal cavities. They are not visible externally until rather late in larval life, after the insect, such as a grasshopper, has moulted twice or more times; while in holometabolous insects they are not seen externally until the pupa state is attained.

The subject of their origin is in a less satisfactory state than desirable from the fact that at the outset the development of the wings of the most generalized insects, such as Orthoptera, Termes, etc., was not first examined, that of the most highly modified of any insects, i.e. the Muscidæ, having actually been first studied.

In the course of his embryological studies on the Muscidæ (Musca comitoria and Sarcophaga carnaria) Weismann (1864) in examining the larvæ of these flies just before pupation, found that the wings, as well as the legs and mouth-appendages, developed from microscopic masses of indifferent cells, which he called “imaginal discs.” From the six imaginal discs or buds in the lower part of the thorax arise the legs, while from four dorsal discs, two in the meso- and two in the metathoracic segment, arise the fore and hind wings (Fig. 141.) These imaginal buds, as we prefer to call these germs, usually appear at the close of embryonic life, being found in freshly hatched larvæ.