Читать книгу A Text-book of Entomology - A. S. Packard - Страница 39

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has a posterior subdivision or projection, the trochantine; sometimes, as in Mantispa (Fig. 103), the trochantine is obsolete.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.

Fig. 103.—Side view of meso- and metathorax of Mantispa brunnea, showing the upper and lower divisions of the epimerum (s. em′, s. em″, i. em′, i. em″); s. epis, i. epis″, the same of the episternum.

Fig. 104.—Divided (ditrochous) trochanter of an ichneumon: cx, coxa; tr, the two divisions of the trochanter; f, femur.—After Sharp.

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.

Fig. 105.—Foot of honey-bee, with the pulvillus in use: A, under view of foot; t, t, 3d–5th tarsal joints; a n, unguis; f h, tactile hairs; p v, pulvillus; cr, curved rod. B, side view of foot. C, central part of sole; pd, pad; cr, curved rod; pv, pulvillus unopened.—After Cheshire.

Fig. 106.—Modifications of the legs of different bees. A, Apis: a, wax-pincer and outer view of hind leg; b, inner aspect of wax-pincer and leg, with the nine pollen-brushes or rows of hairs; c, compound hairs holding grains of pollen; d, anterior leg, showing antenna-cleaner; e, spur on tibia of middle leg. B, Melipona: f, peculiar group of spines at apex of tibia of hind leg; g, inner aspect of wax-pincer and first tarsal joint. C, Bombus: h, wax-pincer; i, inner view of the same and first tarsal joint, all enlarged.—From Insect Life, U. S. Div. Ent.

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.”

Fig. 107.—End of tibia and tarsal joints of Anophthalmus; c, comb.

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.

Fig. 108.—Transverse section through a tarsal joint of Telephorus, a beetle: ch, cuticula of the upper side; m, its matrix; ch′, the sole; m′, its matrix; h, adhesive hair; h′, tactile hair, supplied with a nerve (n′), and arising from a main nerve (n); n″, ganglion of a tactile hair; t, section of main trachea, from which arises a branch (t′); dr, glands which open into the adhesive hairs, and form the sticky secretion; e, chitinous thickening; s, sinew; b, membrane dividing the hollow space of the tarsal joint into compartments. See p. 111.—After Dewitz.

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.)

Fig. 109.—Cross-section through tarsus of a locust: ch, cuticula of upper side,—ch′, ch″, ch‴, of sole; ch, tubulated layer; ch″, lamellate layer; ch‴, inner projections of ch″. Other lettering as in Fig. 101. See p. 113.—After Dewitz.

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.)

Fig. 110.—Last tarsal joint of Melolontha vulgaris, drawn as if transparent to show the inner mechanism: un, claws; str, extensor plate; s, tendon of the flexor muscle; vb, elastic membrane between the extensor plate and the sliding surface u; krh, process of the ungual joint; emp, extensor spine, and th, its two tactile hairs.—After Ockler, from 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.)

Fig. 111.—Fore tibia of Phanæus carnifex, ♂, showing no trace of the tarsus.

Fig. 112.—Fore leg of the mole-cricket: A, outer, B, inner, aspect; e, ear-slit.—After Sharp.

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).

Fig. 113.—Leg of Litognatha: cx, coxa; f, femur; t, tibia; ep, its epiphysis, and sh, its shield-like process. The tarsus entirely wanting.—After Smith.

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.