The apodemes.—The thorax is supported within by beam-like processes, or apodemes, which pass inward and also form attachments for the muscles. Those passing up from the sternum form the entothorax of Audouin, and the process of each thoracic segment is called respectively the antefurca, medifurca, and postfurca. In the Orthoptera (Caloptenus and Anabrus), the antefurca is large, thin, flattened, directed forward, and bounds each side of the prothoracic ganglion. In the Coleoptera two plates (Fig. 97, 2.s) arise from the inside of the sternum and “form a collar or leave a circular hole between them for the passage of the nervous cord” (Newport). The medifurca is a pair of flat processes which diverge and bridge the commissure, while the postfurca is situated under the commissure. In beetles (Dyticus) Newport states that it is expanded into two broad plates, to which the muscles of the posterior legs are attached. Graber also notices in the mole cricket between the apodemes of the meso- and metathorax, a flattened spine (Fig. 98, do) with two perforations through which pass the commissures connecting the ganglia. Besides these processes there are large, thin, longitudinal partitions passing down from the tergum (or dorsum), called phragmas; they are most developed in those insects which fly best, i.e. in Coleoptera (Figs. 97–101), Lepidoptera, Diptera, and Hymenoptera, none being developed in the prothorax. (The term phragma has also been applied to a partition formed by the inflexed hinder edge of this segment, and is present only in those insects in which the prothorax is movable.—Century Dictionary.) All these ingrowths may be in general termed apodemes. There are similar structures in Crustacea and also in Limulus; but Sharp restricts this term to minute projections in beetles (Goliathus) situated at the sides of the thorax near the wings. (Insecta, p. 103, Fig. 57.) The internal processes arising from the sternal region have been called endosternites.

The acetabula.—These are the cavities in which the legs are inserted. They are situated on each side of the posterior part of the sternum, in each of the thoracic segments. They are, in general, formed by an approximation of the sternum and epimerum, and sometimes, also, of the episternum, as in Dyticus (Fig. 97, A). This consolidation of parts, says Newport, gives an amazing increase of strength to the segments, and is one of the circumstances which enables the insect to exert an astonishing degree of muscular power.

b. The legs: their structure and functions

The mode of insertion of the legs to the thorax is seen in Figs. 90, 97, 101, and 103. They are articulated to the episternum, epimerum, and sternum, taken together, and consist of five segments. The basal segment or joint is the coxa, situated between the episternum and trochanter. The coxa usually has a posterior subdivision or projection, the trochantine; sometimes, as in Mantispa (Fig. 103), the trochantine is obsolete. We had previously supposed that the trochantine was a separate joint, but now doubt whether it represents a distinct segment of the leg, and regard it as only a subdivision of the coxa. It is attached to the epimerum, and is best developed in Panorpidæ, Trichoptera, and Lepidoptera. In the Thysanura the trochantine is wanting, and in the cockroach it merely forms a subdivision of the coxa, its use being to support the latter. The second segment is the trochanter, a more or less short spherical joint on which the leg proper turns; in the parasitic groups (Ichneumonidæ, etc., Fig. 104) it is usually divided into two pieces, though there are some exceptions. The trochanter is succeeded by the femur, tibia, and tarsus, the latter consisting of from one to five segments, the normal number being five. Tuffen West believed that the pulvillus is the homologue of an additional tarsal joint, “a sixth tarsal joint.” The last tarsal segment ends in a pair of freely movable claws (ungues), which are modified setæ; between the claws is a cushion-like pad or adhesive lobe, called the empodium or pulvillus (Fig. 105, also variously called arolium, palmula, plantula, onychium, its appendage being called paronychium and also pseudonychium). It is cleft or bilobate in many flies, but in Sargus trilobate. All these parts vary greatly in shape and relative size in insects of different groups, especially Trichoptera, Lepidoptera, Diptera, and Hymenoptera. In certain flies (e.g. Leptogaster) the empodium is wanting (Kolbe). By some writers the middle lobe is called the empodium and the two others pulvilli.

The fore legs are usually directed forward to drag the body along, while the middle and hind legs are directed outward and backward to push the body onwards. While arachnids walk on the tip ends of their feet, myriopods, Thysanura, and all larval insects walk on the ends of the claws, but insects generally, especially the adults, are, so to speak, plantigrade, since they walk on all the tarsal joints. In the aquatic forms the middle and hind tarsi are more or less flattened, oar-like, and edged with setæ. In leaping insects, as the locusts and grasshoppers, and certain chrysomelids, the hind femora are greatly swollen owing to the development of the muscles within. The tibia, besides bearing large, lateral, external spines, occasionally bears at the end one or more spines or spurs called calcaria. The fore tibia also in ants, etc., bear tactile hairs, and chordotonal organs, as well as other isolated sense-organs (Janet), and, in grasshoppers, ears.

In the Carabidæ the legs are provided with combs for cleaning the antennæ (Fig. 107), and in the bees and ants these cleansing organs are more specialized, the pectinated spine (calcar) being opposed by a tarsal comb (Fig. 106, d; for the wax-pincers of bees, see g). In general the insects use their more or less spiny legs for cleansing the head, antennæ, palpi, wings, etc., and the adaptations for that end are the bristles or spinules on the legs, especially the tibiæ.

Osten Sacken states that among Diptera the aerial forms (Bombylidæ, etc.) with their large eyes or holoptic heads, which carry with them the power of hovering or poising, have weak legs, principally fit for alighting. On the other hand, the pedestrian or walking Diptera (Asilidæ, etc.) “use the legs not for alighting only, but for running, and all kinds of other work, seizing their prey, carrying it, climbing, digging, etc.; their legs are provided not only with spines and bristles, but with still other appendages, which may be useful, or only ornamental, as secondary sexual characters.”

Tenent hairs.—Projecting from the lower surface of the empodium are the numerous “tenent hairs,” or holding hairs, which are modified glandular setæ swollen at the end and which give out a minute quantity of a clear adhesive fluid (Figs. 108, 109, 130, 134). In larval insects, and the adults of certain beetles, Coccidæ, Aphidæ, and Collembola, which have no empodium, there are one or more of these tenent hairs present. They enable the insect to adhere to smooth surfaces.

Striking sexual secondary characters appear in the fore legs of the male Hydrophilus, the insect, as Tuffen West observes, walking on the end of the tibia alone and dragging the tarsus after it. The last tarsal joint is enlarged into the form of an irregular hollow shield. The most completely suctorial feet of insects are those of the anterior pair of Dyticus (Fig. 132). The under side of the three basal joints is fused together and enlarged into a single broad and nearly circular shield, which is convex above and fringed with fine branching hairs, and covered beneath with suckers, of which two are exceptionally large; by this apparatus of suckers the male is enabled to adhere to the back of its mate during copulation. The line branching hairs around the edge prevent the water from penetrating and thus destroying the vacuum, “while if the female struggle out of the water, by retaining the fluid for some time around the sucker, they will in like manner under these altered conditions equally tend to preserve the effectual contact.” (Tuffen West.)

In the saw-flies (Uroceridæ and Tenthredinidæ) and other insects, there are small membranous oval cushions (arolia, Figs. 109 and 131) beneath each or nearly each tarsal joint.

The triunguline larvæ of the Meloidæ are so called from apparently having three ungues, but in reality there is only a single claw, with a claw-like bristle on each side.

Why do insects have but six legs?—Embryology shows that the ancestors of insects were polypodous, and the question arises to what cause is due the process of elimination of legs in the ancestors of existing insects, so that at present there are no functional legs on the abdomen, these being invariably restricted (except in caterpillars) to the thorax, and the number never being more than six. It is evident that the number of six legs was fixed by heredity in the Thysanura, before the appearance of winged insects. We had thought that this restriction of legs to the thorax was in part due to the fact that this is the centre of gravity, and also because abdominal legs are not necessary in locomotion, since the fore legs are used in dragging the insect forwards, while the two hinder pairs support and push the body on. Synchronously with this elimination by disuse of the abdominal legs, the body became shortened, and subdivided into three regions. On the other hand, as in caterpillars, with their long bodies, the abdominal legs of the embryo persist; or if it be granted that the prop-legs are secondary structures, then they were developed in larval life to prop up and move the abdominal region.

The constancy of the number of six legs is explained by Dahl as being in relation to their function as climbing organs. One leg, he says, will almost always be perpendicular to the plane when the animal is moving up a vertical surface; and, on the other hand, we know that three is the smallest number with which stable equilibrium is possible; an insect must therefore have twice this number, and the great numerical superiority of the class may be associated with this mechanical advantage. (This numerical superiority of insects, however, seems to us to be rather due to the acquisition of wings, as we have already stated on pages 2 and 120.)

Loss of limbs by disuse.—Not only are one or both claws of a single pair, or those of all the feet atrophied by disuse, but this process of reduction may extend to the entire limb.

In a few insects one of the claws of each foot is atrophied, as in the feet of the Pediculidæ, of many Mallophaga, all of the Coccidæ, in Bittacus, Hybusa (Orthoptera), several beetles of the family Pselaphidæ, and a weevil (Brachybamus). Hoplia, etc., bear but a single claw on the hind feet, while the allied Gymnoloma has only a single claw on all the feet. Cybister has in general a single immovable claw on the hind feet, but Cybister scutellaris has, according to Sharp, on the same feet an outer small and movable claw. In the water bugs, Belostoma, etc., the fore feet end in a single claw, while in others (Corisa) both claws are wanting on the fore feet. Corisa also has no claws on the hind feet; Notonecta has two claws on the anterior four feet, but none on the hind pair. In Diplonychus, however, there are two small claws present. (Kolbe.)

Among the Scarabæidæ, the individuals of both sexes of the fossorial genus Ateuchus (A. sacer) and eight other genera, among them Deltochilum gibbosum of the United States, have no tarsi on the anterior feet in either sex. The American genera Phanæus (Fig. 111), Gromphas, and Streblopus have no tarsal joints in the male, but they are present in the female, though much reduced in size, and also wanting, Kolbe states, in many species of Phanæus. The peculiar genus Stenosternus not only lacks the anterior feet, but also those of the second and third pair of legs are each reduced to a vestige in the shape of a simple, spur-like, clawless joint. The ungual joint is wanting in the weevil Anoplus, and becomes small and not easily seen in four other genera.

Ryder states that the evidence that the absence of fore tarsi in Ateuchus is due to the inheritance of their loss by mutilation is uncertain. Dr. Horn suggests that cases like Ateuchus and Deltochilum, etc., “might be used as an evidence of the persistence of a character gradually acquired through repeated mutilation, that is, a loss of the tarsus by the digging which these insects perform.” On the other hand, the numerous species of Phanæus do quite as much digging, and the anterior tarsi of the male only are wanting. “It is true,” he adds, “that many females are seen which have lost their anterior tarsi by digging; have, in fact, worn them off; but in recently developed specimens the front tarsi are always absent in the males and present in the females. If repeated mutilation has resulted in the entire disappearance of the tarsi in one fossorial insect, it is reasonable to infer that the same results should follow in a related insect in both sexes, if at all, and not in the male only. It is evident that some other cause than inherited mutilation must be sought for to explain the loss of the tarsi in these insects.” (Proc. Amer. Phil. Soc., Philadelphia, 1889, pp. 529, 542.)

The loss of tarsi may be due to disuse rather than to the inheritance of mutilations. Judging by the enlarged fore tibiæ, which seem admirably adapted for digging, it would appear as if tarsi, even more or less reduced, would be in the way, and thus would be useless to the beetles in digging. Careful observations on the habits of these beetles might throw light on this point. It may be added that the fore tarsi in the more fossorial Carabidæ, such as Clivina and Scarites, as well as those of the larva of Cicada and those of the mole crickets (Fig. 112), are more or less reduced; there is a hypertrophy of the tibiæ and their spines. The shape of the tibia in these insects, which are flattened with several broad triangular spines, bears a strong resemblance to the nails or claws of the fossorial limbs of those mammals which dig in hard soil, such as the armadillo, manis, aardvark, and Echidna. The principle of modification by disuse is well illustrated in the following cases.

In many butterflies the fore legs are small and shortened, and of little use, and held pressed against the breast. In the Lycænidæ the fore tarsi are without claws; in Erycinidæ and Libytheidæ the fore legs of the males are shortened, but completely developed in the females, while in the Nymphalidæ the fore legs in both sexes are shortened, consisting in the males of one or two joints, the claws being absent in the females. Among moths loss of the fore tarsi is less frequent. J. B. Smith^[21] notices the lack of the fore tarsi in the male of a deltoid, Litognatha nubilifasciata (Fig. 113), while the hind feet of Hepialus hectus are shortened. In an aphid (Mastopoda pteridis, Esl.) all the tarsi are reduced to a single vestigial joint (Fig. 114).

Entirely legless adult insects are rare, and the loss is clearly seen to be an adaptation due to disuse; such are the females of the Psychidæ, the females of several genera of Coccidæ (Mytilaspis, etc.), and the females of the Stylopidæ.

Apodous larval insects are common, and the loss of legs is plainly seen to be a secondary adaptive feature, since there are annectant forms with one or two pairs of thoracic legs. All dipterous and siphonapterous larvæ, those of all the Hymenoptera except the saw-flies, a few lepidopterous larvæ, some coleopterous, as those of the Rhyncophora, Buprestidæ, Eucnemidæ, and other families, and many Cerambycidæ are without any legs. In Eupsalis minuta, belonging to the Brenthidæ, the thoracic legs are minute.

The legs of larvæ end in a single claw, upon the tips of which the insect stands in walking.

c. Locomotion (walking, climbing, and swimming)

Mechanics of walking.—To Graber we owe the best exposition of the mechanics of walking in insects.

“The first segment of the insect leg,” he says, “upon which the weight of the body rests first of all, is the coxa. Its method of articulation is very different from that of the other joints. The enarthrosis affords the most extensive play, particularly in the Hymenoptera and Diptera.”

In the former the development of their social conditions is very closely connected with the freest possible use of the legs, which serve as hands. In the beetles, however, which are very compactly built, there exists a solid articulation whereby the entire hip rests in a tent-like excavation of the thorax, and can only be turned round a single axis, as may be seen in Fig. 115, where c represents the imaginary revolving axis and d the coxa. In the case we are supposing, therefore, only a backward and forward movement of the coxa is possible, the extent of the play of which depends on the size of the coxal pan, as well as certain groin or bar-like structures which limit further rotation. In the very dissimilar arrangement which draws in the fore, middle, and hind legs toward the body it is self-evident that their extent of action is also different. This arrangement seems to be most yielding on the fore legs, where the hips, to confine ourselves to the stag-beetles, can be turned backward and forward 60° from the middle or normal position, and therefore describe on the whole a curve of 120°. The angle of turning on the middle leg hardly exceeds a legitimate limit, yet a forward as well as a backward rotation takes place. The former is entirely wanting in the hind hips; they can only be moved backward.

The number and strength of the muscles on which the rotation of the hips depends, correspond with these varying movements of the individual legs. Thus, according to Straus Durckheim, the fore coxa of many beetles possesses five separate muscles and four forward and one backward roll; the middle coxa a like number of muscles but only two forward rolls, while the hind hips succeed in accomplishing each of the motions named with a single muscle.

One can best see how these muscles undertake their work, and above all how they are situated, if he lays bare the prothorax of the stag beetle (Fig. 116). Here may be seen first the thick muscle which turns to the front the rotating axis in its cylindrical pan, and thus helps to extend the leg, while two other tendons, which take the opposite direction, are fitted for reflex movements.

In Fig. 115 the muscles mentioned above, and their modes of working, may be distinguished by the arrows a and b.

In order to simplify matters, we will imagine the second component part of the normal insect leg, i.e. the trochanter (Figs. 116, 117, r), as grown together with the third lever, i.e. the femur, as the movement of both parts mostly takes place uniformly.

The pulling of the small trochanter muscle works against the weight of the body when this is carried over on to the trochanter by means of the coxa, as seen at the arrow e in Fig. 115. It may be designated as the femoral lever.

The plane of direction in which the femur, as seen by the rotation just mentioned, is moved, exactly coincides in insects with that of the tibia and the foot, while all can be simultaneously raised or dropped, or, as the case may be, stretched out or retracted. Therein, therefore, lies an essential difference from the fully developed extremities of vertebrates among which, even on the lever arms which are stationary at the end, an extensive turning is possible.

The muscles which move the tibia, and indirectly the femur, also consist of an extensor muscle which is situated in the upper side of the femur (Fig. 116, s, Fig. 115, f), and of a flexor (Fig. 116, b, Fig. 115, g), which lies under the former.

The stilt-like spines on the point (Figs. 115 and 118, L₃n) on which this segment is directly supported are important parts of the tibia. (Graber.)

Considering the respective positions of the individual levers of the leg and the nature of the materials of which they are made, the legs of insects may be likened, as Graber states, to elastic bows, which, when pressed down together from above, their own indwelling elasticity is able to raise again and thus keep the body upright.

This is very plainly shown in certain stilt-legged bark-beetles, in which, as in a rubber doll, as soon as the body is pressed down on the ground, the organs of motion extend again without the intervention of muscles; indeed this experiment succeeds even with dead, but not yet wholly stiff, insects.

Graber then turns to the analysis of the movements of insect legs when in motion, and the mode of walking of these insects in general. This subject had been but slightly investigated until Graber made a series of observations and experiments, of which we can give only the most important results.

The locomotion of insects is an extremely complicated subject.

Let us consider, Graber says, first, a running or carabid beetle, when walking merely with the fore and hind legs. The former will be bent forward and the latter backward.

“Let us begin with the left fore leg (Fig. 118, L₁). Let the same be extended and fixed on the ground by means of its sharp claws and its pointed heel. Now what happens when the tibial flexors draw together? As the foot, and therefore the tibia also, have a firm position, then the contraction of the muscles named must cause the femur to approach the tibia, whereby the whole body is drawn along with it. This individual act of motion may be well studied in grasshoppers when they are climbing on a twig by stretching out their long fore leg directly forward, and then drawing up the body through the shortening of the tibial flexors until the middle leg also reaches the branch.

“But while the fore legs advance the body by drawing the free lever to the fixed leg-segment, the hind legs do this in exactly the opposite way. The hind leg, namely, seeks to stretch out the tibia, and thus to increase the angle of the knee (R₃), thereby giving a push on the ground, by means of which the body is shoved forward a bit.

“Though it might be supposed that the feet would remain stationary during the extension or retraction of the limbs, this never occurs in actual walking. Not merely the upper, but also the lower, thigh is either drawn in or stretched out, as the case may be. The latter then describes a straight line with its point during this scraping or scratching motion (Fig. 115, no), which is obviously the chord to that quadrant which would be drawn by the tibia or foot in a yielding medium, as water, for instance. But even this motion results extremely rarely, and never in actual walking. If we fix our eye anew upon the fore leg at the very moment when it is again retracted, after the resultant ‘fixing,’ we shall then observe that the hip also is simultaneously turned backward in a definite angle. The tibia would describe the arc nq (Fig. 115) by means of the latter alone.

“This plane, in conjunction with the rectilinear ‘movement’ (no) obtained by the retraction of the tibia, produces a path (nr), and this is what is actually described by a painted foot upon a properly prepared surface, as a sheet of paper;^[22] supposing, however, that the body in the meantime is not moved forward by other forces. In the last case, and this indeed always takes place in running, the trunk is moved a bit forward, together with the leg which is just describing its curve with a rapidity corresponding to the momentum obtained; the result of this is that the curve of the foot from its beginning (n) to its end (a) bends round close to itself, just as a man who, when on board a ship in motion, walks across it diagonally, and yet on the whole moves forward, because his line of march, uniting with that of the ship, results in a change of position in space.

“The case is the same in the middle and hind legs, which must make a double course also, yet in such a way that the straight line is drawn, not during the retraction, but during the extension; during which, however, quite as in the fore leg, the members mentioned (R₃) gradually approach the body.

“When the legs have reached the maximum of their retraction, or of their extension, as the case may be, and therefore the end of their active course for that time, then begins the opposite or backward movement; that is, the fore legs are again extended, while their levers draw the remaining legs together again.

“At the same time, as we may see by the uniting leg, the limb is either a little raised, that there may be no unnecessary friction, or it remains during the passive step also, with its means of locomotion in slight contact with the ground.

“The curve of two steps, as inscribed by the end of the tibia of the left fore leg of a stag-beetle, affords an instructive summary of the conditions of which we have been speaking (Fig. 121, B). We see two curves. The thick one (ab), directed toward the axis of the body, corresponds to the effective act of a single walking function, which brings the body a bit forward; the thinner, on the other hand, or we might say the hair line (bc), which, however, is but rarely made quite clearly, is produced by the ineffectual backward movement, by which the insect again approaches its working posture (c). It is at first placed at some distance from the body, in order that (like c also) it may draw near to the body again; but in such a way, naturally, that it coincides with the starting-point of the following active curve (cd). It is evident that even the passive curve is not the imprint of the movement accomplished exclusively by the leg, for this latter, while struggling to reach its resting-place, is really involuntarily carried forward with the rest of the body.

“The scroll-like lines drawn by the swimming beetle (Dyticus), with the large, sharp points of its hind tibia, are also very instructive (Fig. 119, A).

“The diversions and modifications in the course of the active step, as furnished by the moving factor of the remaining legs, are already clearly illustrated by the curves shown by the joints of the hind tibia of a May-beetle (Fig. 120) and a stag-beetle (Fig. 121, c). The actual faint line in this case does not run from the front toward the back, as would correspond to the active leg-motion, but either directly inward (Fig. 121, cb), or even somewhat to the front. In the May-beetles, and even more in the running garden-beetle, the curves of the hind legs present themselves as screw-like lines (Fig. 122, l₃), while the scrawling of the remaining members (l₁, l₂) is much simpler.

“Inasmuch as we now have a cursory knowledge of the movements made by each individual leg for itself,—movements, however, which plainly occur very differently according to the structure of these appendages,—the question now is of the combined play, the total effect of all the legs taken together, and therefore of the walk and measure of the united work of the foot.

“In opposition to the caterpillars and many other crawling animals which extend their legs in pairs and really swing them by the worm-like mode of contraction of the dermo-muscular tube, the legs of fully grown insects are moved in the contrary direction and in no sense in pairs, but alternately—or, more strictly speaking, in a diagonal direction.

“For an examination of the gait of insects, we choose, for obvious reasons, those which have very long legs and which at the same time are slow walkers.

“Insects may be called ‘double-three-footed,’ from the manner in which they alternately place their legs. There are always three legs set in motion at the same time, or nearly so, while in the meantime the remaining legs support the body, after which they change places.

“To be more exact, it is usually thus: At first (Fig. 118) the left fore leg (L₁) steps out, then follows the right middle leg (R₂), and the left hind leg (L₃). Then while the left fore leg begins to retract and thus make the backward movement, the right fore leg is extended, whereupon the left middle leg and the right hind leg are raised in the same order as the first three feet.”

Graber^[23] painted the feet of beetles and let them run over paper, and goes on to say:

“Let us first pursue the tracks of the Blaps, for example (Fig. 123). Let the insect begin its motion. The left fore leg stands at a, the right middle leg at β, and the left hind leg at c. The corresponding number of the other set of three feet at α, b, γ. At the first step the three feet first mentioned advance to a′β′c′, the second set on the other hand to α′b′γ′. Thereby the tracks made by the successive steps fall quite, or almost quite, on each other, as appear also in the tracks of a burying beetle (Fig. 124).

“As the fore legs are directed forward and the hind legs backward, while the middle legs are placed obliquely, the reason of the more marked impressions of the latter is evident.

“The highest testimony to the precise exactitude and accuracy of the walking mechanism of insects is furnished by the fact that in most insects, and particularly in those most fleet of foot, which, whether they are running away or chasing their prey, must be able to rely entirely upon their means of locomotion;—the fact, we say, that whether they desire to move slowly or more quickly, the distances of the steps, measured by the length as well as by the cross-direction, hardly differ a hair’s breadth from one another, and this is also the case when the tarsi are cut off and the insects are obliged to run on the points of their heels (tibiæ).

“Thence, inasmuch as the trunk of insects is carried by two legs and by one on each side alternately, it may surely be concluded a priori that when walking it is inclined now to the right and now to the left, and that the track, too, which is left behind by a precise point of the leg, can in no wise be a straight line; and in reality this is not the case.

“A plainly marked regular curve, which approaches a sinuous line, as seen in Fig. 125, is often obtained by painting many insects, for example Trichodes, Meloë, etc., which, when running, either bring the end of their hind body near to the ground or into contact with it.