6. Clothing. The hairs on the legs of insects, though at first sight they may seem unimportant, in many cases are of great use to them, both in their ordinary avocations and motions: but as most of these were sufficiently noticed when I treated of the sexes of insects[1996], I shall not here repeat my observations, but confine myself to cases not then adverted to. Some insects have all their legs very hairy, as many spiders, the diamond beetle (Entimus imperialis), or at least a species very near it and common in Brazil[1997], &c.: in others they are nearly naked, as in the stag-beetle. In the Crepuscular Lepidoptera (Sphinx L.) and some of the Nocturnal ones (Bombyx L.) the thighs are much more hairy than the rest of the legs: and in Lucanus, Geotrupes, and many other Lamellicorns, &c., the anterior ones have a yellow or golden spot at their base, composed of decumbent hairs, which prevent them from suffering by the violent friction to which they are exposed in burrowing. In most Petalocerous beetles the tibiæ are set with scattered bristles, and sometimes the thighs. The Tiger beetles (Cicindela) are similarly circumstanced: but the bristles, which are white, are generally arranged in rows. In Dytiscus, Hydrophilus, &c., the four posterior tarsi; and in Notonecta the posterior pair, and also the tibiæ—are fringed on each side with a dense series of hairs, which structure assists them in swimming[1998]. The tarsi, especially the anterior pair, in a certain family of Lamia F. (L. papulosa, &c.[1999]), are similarly fringed, only the hairs curl inwards; and the hand in Sphex and Ammophila, but not in Pelopæus and Chlorion, is fringed externally with long bristles.
7. Composition. With regard to their composition, both arms and legs generally consist of five pieces, which Entomologists have denominated—the coxa or hip—the trochanter—the femur or thigh—the tibia or shank—and the tarsus or foot. Where the structure and use of the fore-leg is different from that of the four hind-legs, I propose calling these pieces by names corresponding with those which anatomists have appropriated to the arm in the higher vertebrate animals: thus, as you will see in the table, I call the whole fore-leg the brachium or arm; and the coxa becomes the clavicula or collar-bone; the trochanter, the scapula or shoulder-blade; the femur, the humerus or shoulder; the tibia, the cubitus or arm; the tarsus, the manus or hand. But let me not lead you to suppose that the pieces, either in the arms or legs of insects, which are there named after certain others in vertebrate animals, precisely correspond with them—by no means—since that is a very doubtful point; and some of them, as the trochanter, clearly do not. Many gentlemen skilled in anatomy, as I have before observed[2000], have thought that what is regarded as the coxa in insects really represents the femur: but there are considerable difficulties in the way of this supposition, several of which I then stated. I shall not however enter further into the subject, and take the above names; since this application of them is so general and so well understood, except with regard to the fore-leg, under certain circumstances, as I find them. I shall now consider them in the order in which I have named them.
a. Coxa or Clavicula[2001]. The coxa is the joint that connects the leg with the trunk of the insect. With regard to their shape, the most general form of the four anterior is more or less that of a truncated cone: in the Staphylinidæ, however, they tend to a pyramidal or four-sided figure; as do the whole six in the Trichoptera: in numbers of the weevils and capricorns they are subglobose; in the Lamellicorns they are mostly oblong, and not prominent: the posterior pair in the Coleoptera are generally flat and placed in a transverse position, and more or less oblong and quadrangular: in Elater, &c., they are cuneiform: in Haliplus Latr. they are dilated, and cover the thigh[2002]: in Buprestis, Copris, &c., they have a cavity that partly receives it: the corresponding part, the clavicle, in the arm of Gryllotalpa, is very large and remarkable; viewed underneath it is triangular, and trifid where the trochanter articulates with it: in that of Megachile Willughbiella the clavicle is armed with a spine[2003]. As to their proportions, the most general law seems to be, that the anterior pair shall be the shortest and smallest, and the posterior the longest and largest. In some instances, as in Buprestis, the two anterior pair are nearly equal; in others (Mantis, Eurhinus K.), the anterior are the longest, in the former as long as the thigh, and the four posterior the shortest: in the Trichoptera, Lepidoptera, &c., all are nearly equal; in Mantis the two posterior, and in Phengodes the intermediate pair are the largest; but in Necrophorus they are the smallest:—though almost universally without articulations, in Galeodes the clavicle consists of two and the coxa of three[2004].
b. Trochanter or Scapula[2005]. This is the second joint of the leg: and if the coxa is regarded as the analogue of the thigh in vertebrate animals, this should seem to represent the patella or rotula, vulgarly called the knee-pan. Latreille and Dr. Virey consider this articulation as merely a joint of the coxa[2006]; but if closely examined, especially in Coleopterous insects, you will find it so fixed to the thigh as scarcely to have separate motion from it, and in many cases it seems to be merely its fulcrum; but I am not aware that any instance occurs in which it has not motion separate from that of the former joint.
As to its articulation with the coxa,—in the Coleoptera it appears to be of a mixed kind; for it inosculates in that joint, is suspended by ligament to its orifice, and its protuberances are received by corresponding cavities in it; and its cavities receive protuberances, which belongs to a ginglymous articulation. I have observed two variations in this Order, in one of which the motion of the thigh and trochanter is only in two directions, and in the other it is nearly versatile or rotatory. The Lamellicorns afford an example of the first, and the Rhyncophorous beetles or weevils of the second. If you extract from the coxa the thigh with the trochanter of the larger species of Dynastes McL., you will find that the head of the latter is divided into two obtuse incurving lobes or condyles: that on the inner side being the smallest and shortest, and constricted just below its apex: and that under this is a shallow or glenoid cavity, terminating posteriorly in a lubricous flat curvilinear ridge. If you next examine the trochanter in articulation with the coxa, you will perceive that the head of the former inosculates in it, that the lower condyle is received by a sinus of the coxa, which also has a lubricous very shallow cavity corresponding with the ridge, in which it turns; and in the head of the coxa, on the lower side, is an external condyle, which is received by a sinus common to both, of the head of the thigh and of the exterior side of the trochanter[2007], in which it likewise turns: this last condyle has also an internal protuberance, which appears to ginglymate with a cavity of the trochanter: from this structure the leg is limited chiefly to a motion up and down upon two pivots, or to fold and extend itself. You will find an articulation very near this, but on a smaller scale, in the stag-beetle. In the other kind of articulation, which admits of freer motion, the head of the trochanter is prolonged, and the process terminates in a short interior condyle, which appears to work in a corresponding cavity of the interior of the coxa; and the base of the process is encompassed by a ridge with a cavity behind it, which is received by another of the lower part of that piece, and admits a corresponding ridge—a structure that allows a rotatory motion. In the hind-legs of this tribe the motion is chiefly limited to folding and extending; in Carabus, &c., also the head of the trochanter is nearly hemispherical, and the articulation approaches ball and socket. In most of the other Orders, the Hymenoptera excepted, there is little or no inosculation, the trochanter being simply suspended by ligament to the coxa as well as to the thigh; its connection with the latter is similar in Coleoptera; but in Cicindela, &c., it inosculates in it. The part we are considering varies in its position with respect to the thigh: in the hind-legs of Carabus, &c., it forms a lateral fulcrum on the inner side of that part, and does not intervene between its base and the coxa; the muscles from the latter entering the former, not at the bottom of the base, but at its side: but in the four anterior legs it forms their base, as it does in all the legs in Apion, and in all the Orders except the Coleoptera, cutting them entirely off from contact with the coxa: in the Lamellicorns they cut off part of the base obliquely, but so as to permit their coming in contact with the condyle of the coxa, as before mentioned. In the Ichneumonidæ and some other Hymenoptera the trochanter appears to consist of two joints particularly visible in the posterior legs[2008].
As to size in general,—the part in question is smaller than the coxa; but in Notonecta it is larger, and in the dog-tick (Ixodes Ricinus) longer than that joint. It exhibits few variations in its shape or appendages worthy of particular notice. In general, in the Coleoptera it is triangular or trigonal; but in Carabus L., in the hind-leg it is oblong or rather kidney-shaped; in that of Necrophorus[2009] it terminates in one or two teeth or spines, varying in length in the different species: in the other Orders it is not remarkable in this respect.
c. Femur or Humerus[2010]. The femur or thigh is the third, and usually the largest and most conspicuous joint of the leg. In the hypothesis before alluded to[2011] it is considered as the analogue of the tibia of vertebrate animals. With regard to the articulation of this part with the trochanter, it has been sufficiently explained under that head, and that with the tibia I shall treat of when I come to that joint. As to the size of the thighs, and their relative proportions to each other and to the remaining joints of the leg, the most general law is, that the anterior pair shall be the shortest and smallest, and the posterior the longest and largest. With respect to the remaining articulations, most commonly the thigh is longer and larger than the tibia, and the tibia than the tarsus. But there are numerous exceptions to both these rules. With respect to the first, we may begin by observing that the increase of the magnitude of the thigh, from the anterior to the posterior pair, is usually gradual: but in many jumping insects, and likewise many that do not jump, the posterior pair are suddenly and disproportionally thicker than the rest[2012]. Again, in many insects the anterior pair are the longest and thickest, as in Macropus longimanus, Bibio, Nabis, &c.: in others, the intermediate exceed the rest in magnitude, as in Onitis Aygulus, cupreus; Sicus flavipes, &c.; in many Lamellicorns all the thighs are incrassated and nearly equal in size: but in some, as Ryssonotus nebulosus McL.[2013], the intermediate pair are rather smaller than the rest. With respect to the second rule—in some, as in the male of Macropus longimanus, the anterior tibia, though more slender, is longer than the thigh; in Hololepta maxillosa it is longer and more dilated; in Lamia marmorata, or one related to it from Brazil, the intermediate pair are longer; in Ateuchus gibbus and others of that tribe the posterior thighs are smaller than the tibiæ: and, to mention no more; in Callichroma latipes the posterior tibia is wider than the part last named. Again, the tarsi are as long as either tibia or thigh in many of the larger Dynastidæ, as Megasoma Actæon, &c.; longer than either in Melolontha subspinosa F.; and in Tiphia, Scolia and affinities, often as long, or longer than both together.
As to shape,—the thigh, especially in the fore-leg, varies considerably: most generally it is flat, linear, and a little thicker where it is united to the tibia, on the outer side convex, and concave next the body; but in many it is gradually thicker from the base to the apex: in some Cerambyces (C. thoracicus) it is clavate; in others of this genus and Molorchus they may be called capitate; in Pterostichus they are rather lanceolate; in Onitis Sphinx the humerus is triangular, and the intermediate thigh rhomboidal; in Bruchus Bactris it is bent like a bow; and in some Brazilian Halticæ it is nearly semicircular. The humerus in Phasma is attenuated at the base; in Empusa gongyloides it is at first ovato-lanceolate, and terminates below in a kind of footstalk[2014]; in Phasma flabelliforme it is dolabriform[2015]; in Mantis often semioval or semielliptical, and thickest at the inner edge, which affords space for two rows of spines with which it is planted. In Phyllium siccifolium all the thighs are furnished on both sides with a foliaceous appendage nearly from base to apex[2016]: in a species of Empusa (E. macroptera), the four posterior ones are so distinguished only on their posterior side[2017]: others of this last genus, as E. gongyloides, have an alary appendage on both sides at the apex of these thighs[2018]; and another family, as E. pauperata, have only one on the posterior side[2019]. The thighs of no insect are more remarkable for their elegant shape,—tapering gradually from the base to the apex, where they swell again into a kind of knee,—than the posterior ones of the locusts (Locusta Leach); each side of these thighs is strengthened with three longitudinal nearly parallel ridges, and the upper and under sides are adorned by a double series, in some coalescing as they approach the tibia, of oblique quadrangular elevations resembling scales[2020].
I shall next say a few words upon the spines and other processes which arm the thigh. Those moveable ones of Mantis which help to form a fearful instrument of destruction, have just been mentioned, and similar ones, but less conspicuous, arm the intermediate thighs of Sicus flavipes: other appendages of this kind are for a less destructive purpose—to keep the tibia when folded in its place. This seems to be the use of the serratures and spine that arm the thigh of Bruchus Bactris, or the Hymenopterous genera Leucospis, Chalcis, &c.; in Onitis Aygulus a short filiform horn arms the humerus, and a longer crooked one that of many species of Scaurus[2021]. In many Stenocori the thighs terminate in two spines, and in Gonyleptes K. the posterior ones are armed internally with very strong ones; with which, as the legs converge at their knee[2022], they may probably detain their prey. The knee-pan (Gonytheca) of the thigh, or the cavity at its end, which receives the head of the tibia, is very conspicuous in the weevils; but in no insects more than in Locusta[2023], in which tribe it deserves your particular attention.