d. Tibia or Cubitus[2024]. The tibia or shank is the fourth joint of the leg, which according to the hypothesis lately alluded to is the analogue, in the anterior leg of the carpus or carpal bones, and in the four posterior ones of the tarsus or tarsal bones of vertebrate animals. This may be called the most conspicuous of the articulations of the leg; for though it is generally more slender and often shorter than the thigh, it falls more under the eye of the observer, that joint being more or less concealed by the body: it consists in general of a single joint; but in the Araneidæ and Phalangidæ it has an accessory one, often incrassated at its base, which I have named the Epicnemis[2025].
With respect to the articulation of the tibia with the thigh—we may observe that in general it is by means of three processes or condyles, two lateral and one intermediate, of the head of the former joint[2026]: the lateral ones are usually received by a cavity or sinus of the gonytheca of the thigh[2027]; and upon these the tibia turns, with a semirotatory motion up and down as upon a pair of pivots: at the same time the mola or head of the latter joint, which has often a flexure so as to form an elbow with the rest of it, inosculates in the gonytheca, and is also suspended by ligament to the orifice through which the muscles, nerves, and bronchiæ are transmitted: so that in fact the articulation, strictly speaking, belongs exclusively to none of the kinds observable in vertebrate animals, but partakes of several, and may properly be denominated a mixed articulation,—a term applicable in numerous instances also to the other articulations of the legs of insects. In the different Orders some variations in this respect take place,—I will notice some of the most remarkable. In no Coleopterous insects is the structure more distinctly visible than in the larger Lamellicorns. In Copris bucephalus, for instance, if you divide the thigh longitudinally, you will find on each side, at the head, that it is furnished with a nearly hemispherical protuberance, perforated in the centre for the transmission of muscles, and surrounded externally by a ridge, leaving a semicircular cavity between them[2028]: if you next examine the tibia, after having extracted it, you will find on each side, at the base, a cavity corresponding with the protuberance of the thigh which it receives, having likewise a central orifice, and surrounded by a semicircular ridge corresponding with the cavity in the thigh in which it acts: below this ridge another cavity, forming a small segment of a circle, receives the ridge of the thigh[2029]. You will observe that the ridge of the tibia represents the lateral condyle lately noticed: in the Dynastidæ this is more prominent, and often forms a smaller segment of a circle. In these also the protuberance of the thigh is more minute, and its ridge is received by a cavity of the tibia nearly semicircular[2030]; in Geotrupes Latr. the articulation is not very different, though on a reduced scale; in Calandra Palmarum the lateral condyles of the tibiæ are flatter and broader[2031]; and the articulation not being quite so complex, this joint is kept steady by an intermediate process observable in the gonytheca[2032]. From the above description it appears that the dislocation of the tibia is effectually prevented in the Lamellicorns by the protuberance and ridge of the thigh working in their corresponding cavities, while the condyle of that part turns with a rotatory motion in the cavity of the thigh. In the Orthoptera Order the tibia is suspended by a ligament, in the gonytheca the lateral condyles, which are very prominent, working in a sinus of that part[2033]. The subsequent Orders exhibit no very striking variations from these types of articulation, I shall therefore not detain you longer upon this head.
With regard to the proportions and magnitude of the joint we are considering,—the most general law is, that the anterior pair should be shorter and more slender than the intermediate; and the intermediate than the posterior; and that all the tibiæ should be shorter and more slender than the thighs, and longer and thicker than the tarsi. Various exceptions, however, to this rule in all these cases might be produced; but I shall only observe that in all those insects in which the fore-legs are calculated for digging or seizing their prey, as in the Petalocerous beetles, the Gryllotalpa, Mantis, &c., this joint of the leg is usually much enlarged and more conspicuous than the others.
As to its figure and shape—most commonly the tibia grows thicker from the base to the apex, as in the majority of Coleoptera, Hymenoptera, &c.; in the Orthoptera, Neuroptera, &c., it is generally equally thick every where. Another peculiarity relating to this head observable in it, is its tendency to a trigonal figure: this, however, though very general, is not universal;—thus, in some Orthoptera, as Pterophylla K., its horizontal section is quadrangular; in others, as Locusta Leach and many other insects, it is nearly a circle; in some scorpions it is almost a hexagon. The superficial shape also of this joint in numerous instances is more or less triangular, but it sometimes recedes from this form:—thus, in Callichroma latipes it is a segment of a circle; in some Empides it is clavate; in Onitis Sphinx, dolabriform; in the Orthoptera, Neuroptera, &c., it is usually linear; in some Lygæi it is angular[2034]: but the most remarkable tibiæ in this respect are those of such species of this last genus as have the posterior ones winged or foliaceous, so that they resemble the leaf of some plant—the tibia being the rachis, and the wing (which in some species is veined) representing the leaf itself. This structure is exemplified in Lygæus compressipes, phyllopus, foliaceus, &c.[2035] Under this head I must say a few words upon the flexure of this joint, which in some cases merits notice. I have before mentioned its bend at the knee[2036] or base: the apex also is sometimes incurved—in the anterior one of the male of Macropus longimanus so as almost to form a hook[2037]: in Lygæus Pharaonis the posterior pair are flexuose[2038]; in Bruchus Bactris, Leucospis, and several species of Chalcis, these tibiæ curve so as to adapt themselves to the bend of the thigh when folded. The notch on the inside of the anterior pair, in a large majority of Carabus L., armed above by a spur[2039], a structure which probably assists them in seizing and detaining their prey, may also here be introduced: in the generality it is a little removed from the apex of the joint in question; but in Pamborus it is very near to it, and in Cychrus, Carabus, &c., it becomes obsolete. I may mention here also a singular character which distinguishes the cubit of both sexes of Gryllus campestris, domesticus, &c. At the base there is an aperture which passes through the joint—anteriorly it is oval, and posteriorly elliptical and much larger, and on both sides is closed by a tense membrane.
The most striking peculiarities as to the clothing of his joint have been chiefly noticed under the sexual characters of insects[2040], but some appear not to be of that description. In Sphæridium Leach, while the thighs and tarsi are naked, the posterior tibiæ are remarkably beset with stiff bristles; in Empis pennipes they are thickly fringed on both sides; in Scarabæus McL. only externally, and in Dytiscus serricornis internally; in Necydalis barpipes K. this fringe is longer at the apex; and in Saperda hirtipes Ol. the same tibiæ at that part are adorned with a large brush, like that observable in the antennæ of some Lamiæ[2041].
I must next call your attention to the teeth, spines, and spurs with which the tibiæ of insects are sometimes armed. With regard to teeth, you have doubtless often observed those that distinguish the cubitus of the arm of most Lamellicorn beetles: these vary in number from one, as in Trox suberosus, to seven, as in Geotrupes autumnalis; but the most universal number is three: in some species of Geotrupes, as G. stercorarius, &c., the third tooth from the apex, and those that follow it, may be called double. These teeth, in their cubit or anterior shank, doubtless assist these insects in burrowing. The four posterior tibiæ in this tribe are also distinguished by a kind of teeth which occupy their whole diameter, and resemble so many steps. I have before noticed the remarkable cubit of the Gryllotalpa, and likewise that of Scarites, Pasimachus, &c., in which some of the teeth are prolonged into spines[2042], which are the next description of tibial arms that I mentioned. Spines are of two kinds—those which are merely processes of the crust of the tibia, and those that are implanted in it, and seem to have a gomphosis or perhaps an amphiarthrosis articulation[2043]. An instance of the first kind may be seen in the hind-legs of some grasshoppers[2044] (Locusta Leach), the Rutelidæ, &c. though in others they are implanted:—of the second, in the cubitus of the Mantidæ, and of all the tibiæ of the dragon-flies (Libellulina McL.)[2045];—and of both kinds in the hind-legs of Acrida K., those which arm the upper angles of the tibiæ being processes, and those of the lower being implanted. The term spine I think ought to be restricted to the first kind; the second ought rather to be denominated spurs (calcaria), and may perhaps be regarded as in some degree synonymous with those most important appendages of the joint in question, that are implanted in or near their apex, which have been hitherto distinguished by this last denomination, and which I am next to consider. But though I have not altered a term generally adopted, I must here express my opinion that they ought rather to be considered as minute toes or fingers, and that the denomination best agreeing with their functions, as accessories to the main toe, would be digituli: this is proved particularly by a character peculiar to those of many species of the genus Cimbex amongst the saw-flies, in which these organs are furnished with a sucker or pulvillus (as they are also in Œnas a kind of blister beetle), as well as the joints of the tarsi[2046]; which makes it evident that they are applied by the animal to surfaces, and assist it in walking or climbing; and in general it may be observed that in most insects their principal use is connected with these motions, and with burrowing. This circumstance tends to prove that the generality of insects (for all have not these organs) have really a didactyle or tridactyle hand or foot; and the hypothesis so often alluded to—that the cubitus or tibia, &c., is really analogous to the carpus or tarsus in vertebrate animals[2047]—seems to receive no small confirmation from it; since, if the spurs be really analogous to fingers or toes, the part they articulate with cannot be the tibia, &c. Though the parts in question did not escape the notice of Reaumur, Linné, De Geer, Latreille, &c., yet they have not been employed in the determination of tribes, genera, &c., except by the author last named, but perhaps adopted from Bonelli[2048], in the subgenera Zabrus and Pelorus: in many instances, however, they afford excellent subsidiary characters, sometimes common to a whole Order, and at others distinguishing its various subdivisions. With regard to their number—I have noticed many variations which I will now state to you, first observing that I shall express them by three figures, the first representing the number of spurs on the anterior leg, the second that of those on the intermediate, and the third on the posterior; and where there are spurs, as in the Trichoptera and Lepidoptera, on the middle as well as at the end of the tibia, I shall express it by one figure over another, the upper one representing the number of the middle spurs. If you make an examination yourself, it will be proper to remind you that these little organs are extremely liable to be broken off, but the socket in which they were planted is usually very visible. The most natural number is represented by 2:2:2; this you will find very prevalent in the Coleoptera Order, as in the Predaceous and numerous other beetles: in the Orthoptera and Hemiptera Orders, however, I have not discovered an instance of it; but in all the rest it more or less occurs: next to this number—tibiæ with obsolete or no spurs seem most prevalent, particularly in the Hemiptera; not a single instance of an insect furnished with them occurring to me in the Heteropterous section; and it is doubtful whether there are any in the Homopterous.—Having stated the most universal structure in this respect, I will next consider the Orders seriatim. Amongst the Coleoptera though the numbers 2:2:2 are most frequent in occurrence, yet there are numerous exceptions. Thus, in the Lamellicorns, 1:1:1 represents the calcaria of one tribe of the Scarabæidæ McL. formed of the genus Scarabæus McL.; 1:2:1 represents those of another tribe of that family, including the subgenera Ateuchus, Copris, Phanæus, &c.; 1:2:2 again forms the character in this respect of Aphodius and the great majority of the Lamellicorns; while 2:2:2 is confined in this section to Æsalus F. and Melolontha chrysomeloides Schranck (Psephus McL. MS.). In the other tribes of Coleoptera other numbers occur. Thus, 0:1:1 characterizes Hylœcetus; 0:1:2 Mordella; 0:2:2 Macropus; 1⁄1:2:2 Harpalus, and all those Carabi L., except Zabrus, that have a notch in their anterior tibiæ; ½:2:2 Zabrus. In the Orthoptera Order it is not easy to distinguish the real spurs from the implanted spines that frequently arm the legs: these in Blatta are extremely numerous, even at the apex of the tibiæ; but I cannot distinguish any that can be regarded as true analogues of the former: the most natural number of spurs in this Order is represented by 0:0:4; this you will see in all the Locusts; in Acrida, Conocephala, Pterophylla; and in Truxalis, Pneumora, &c.; in Phasma there are none. In Mantis, if the terminal process of the cubitus is excluded, it will be 0:2:2; in Gryllotalpa, admitting the terminal teeth of that part[2049] as analogues of spurs, the number is 4:4:4; in Tridactylus Latr. 0:0:5[2050]; in Gryllus Latr. 3:3:5; in Gryllus monstrosus, 4:4:6. In the whole Hemiptera Order I have discovered no instance of an insect furnished with the real spurs: for though in Tettigonia F., Cercopis, &c., there are implanted spines in the posterior tibia, and several at the apex, there are none of them clearly analogous to real spurs. In the Lepidoptera the most general arrangement appears to be 1⁄0:2:2/2; and next to this, 1⁄0:2:2. In this Order most commonly there is no spur at the end of the cubit, but one resembling a thumb[2051] arms its middle; in Pieris, &c., this thumb is not present, so that the number is 0:2:2; in Agarista Leach, Erebus, &c., you will find 1⁄0:2:4, the posterior calcaria being all terminal; and in Attacus Atlas, all these organs are obsolete except the thumb. In the Neuroptera the most general arrangement is 2:2:2; but in the Libellulina, although the legs are very spinose, there are no spurs. In the Trichoptera K., in Phryganea rhombica and affinities, the number of them is expressed by ½:½:½; and in those with long antennæ, P. atra, &c., by 2⁄2:2⁄2:2⁄2. In the Hymenoptera the number 1:2:2 is most prevalent; and next to this, as in Apis L., 1:1:2. In the Ichneumones minuti L. the spurs are 1:1:1; in Atta Latreille, a kind of ant[2052], 1:0:0. In the Diptera it is often difficult to distinguish the spurs from the spines; but the number most universal is, I think, 2:2:2; in Tipula it is 1:2:2; in the Tabanidæ 0:2:0; and in Culex, Limonia, &c., there are none. Amongst the insects with more than six legs, most commonly the tibiæ have no spurs; but in the Araneidæ each is armed with two, a circumstance which also distinguishes the corresponding joint of the pedipalpi.
These little organs inosculate each in an appropriate socket of the end, or in many cases of the middle of the tibia; and that part of their head or base that is received by it, is often constricted for the purpose: from hence it follows that they are capable of some degree of motion, but in some insects, as those on the four posterior legs of Scarabæus sacer and its more immediate affinities, and those at the end of the cubit of Gryllotalpa, they are immoveable, and appear almost processes of the joint to which they belong. They are commonly sharp, of a subtriquetrous figure, with the lower side flat: where there are two, the outer one is usually the longest; and in general the spurs on the hind legs are longer than those on the four anterior: but there are exceptions—thus, in Acanthopus Latr. the intermediate spurs are the longest; and in Cicindela the anterior are longer than the former; in Blaps mortisaga those on all the legs are nearly equal in length. They vary sometimes in shape—those on the middle of the cubit of many Lepidoptera, which may be regarded as a kind of thumb[2053], are of a lanceolate shape; in Meloe the external posterior one is flat and obtuse; in Œnas Latr. it is obconical, concave at the extremity, and apparently furnished with a sucker; in Ateuchus smaragdulus the anterior, and in Copris Carolina the posterior is forked and emarginate; in Sirex the former is hooked and winged; in Lamprima it is triangular and dilated; in Aphodius analis it is dolabriform; in Dynastes retusus and Juvencus the spurs are bent like a bow. In many Hymenoptera, as the Sphecidæ, they are pectinated[2054], with a series of minute parallel spines—a structure which assists the animal in burrowing[2055]; in Acanthopus Latr. they are armed with little teeth or spines[2056]; in the hive bee the spur of the cubit is furnished with a membranous appendage which I have called the velum[2057]; and in a subgenus related to Saropoda Latr. (Ctenoplectra K. MS.), the interior spur of the posterior leg is crescent-shaped, fixed transversely, and fitted on the inner side with a membrane, the edge of which is finally pectinated.
e. Tarsus or Manus[2058]. This is the last portion of the leg, usually supposed to be analogous to the hand or foot of vertebrate animals; but, according to the hypothesis so often alluded to, rather the representative of their jointed finger or toe. In treating of this part I shall consider its articulation with the tibia, and of its joints inter se; the number of those joints; their proportion and shape; their parts and appendages.
I seem to have observed three kinds of tarsal articulation. The first is a species of enarthrosis or ball and socket, the joints terminating in a globular head, perforated indeed for the transmission of muscles, &c., and which is received by a corresponding cavity of the tibia or preceding joint, as may be seen in many weevils (Curculio L.[2059]). This admits of some rotatory motion.—The second is a mixed articulation between enarthrosis and ginglymus, when at the base of the ball a deep transverse channel receives a corresponding ridge of the tibiæ or preceding joint: this may be found in Rutela and probably many other Lamellicorn beetles; and something very similar in the Predaceous ones.—The third kind is where there is little or no inosculation, and the joints are scarcely more than suspended: this takes place in the Orthoptera, Neuroptera, &c.; but in Blatta and the hind legs of Mantis there is some approach to the foregoing kinds.