Fig. 59.—Foot of Pelopaeus, a fossorial wasp: A, tarsus entire; B, terminal joint, upper side; C, under side. a, claw; b, base of pressure-plate; c, extension-plate; d, extension-sole; e, pad; f, lobe of pad retracted.
The legs of the young Insect are usually more simple than those of the adult, and in caterpillars they are short appendages, and only imperfectly jointed. If a young larva, with feet, of a beetle, such as Crioceris asparagi be examined, it may be seen that the leg is formed by protuberance of the integument, which becomes divided into parts by simple creases; an observation suggesting that the more highly developed jointed leg is formed in a similar manner. This appears to be really the case, for the actual continuity of the limb at the chief joint—the knee—can be demonstrated in many Insects by splitting the outer integument longitudinally and then pulling the pieces a little apart; while in other cases even this is not necessary, the knee along its inner face being membranous to a considerable extent, and the membrane continuous from femur to tibia.
Turning to the wings, we remark that there may be one or two pairs of these appendages. When there is but one pair it is nearly always mesothoracic, when there are two pairs one is invariably mesothoracic, the other metathoracic. The situation of the wing is always at the edge of the notum, but the attachment varies in other respects. It may be limited to a small spot, and this is usually the case with the anterior wing; or the attachment may extend for a considerable distance along the edge of the notum, a condition which frequently occurs, especially in the case of the posterior wings. The actual connexion of the wings with the thorax takes place by means of strong horny lines in them which come into very close relation with the little pieces in the thorax which we have already described, and which were styled by Audouin articulatory epidemes. There is extreme variety in the size, form, texture, and clothing of the wings, but there is so much resemblance in general characters amongst the members of each one of the Orders, that it is usually possible for an expert, seeing only a wing, to say with certainty what Order of Insects its possessor belonged to. We shall allude to these characters in treating of the Orders of Insects.
Each wing consists of two layers, an upper and a lower, and between them there may be tracheae and other structures, especially obvious when the wings are newly developed. It has been shown by Hagen that the two layers can be separated when the wings are recently formed, and it is then seen that each layer is traversed by lines of harder matter, the nervures. These ribs are frequently called wing-veins, or nerves, but as they have no relation to the anatomical structures bearing those names, it is better to make use of the term nervures. The strength, number, form and inter-relations of these nervures vary exceedingly; they are thus most important aids in the classification of Insects. Hence various efforts have been made to establish a system of nomenclature that shall be uniform throughout the different Orders, but at present success has not attended these efforts, and it is probable that no real homology exists between the nervures of the different Orders of Insects. We shall not therefore discuss the question here. We may, however, mention that German savants have recently distinguished two forms of nervures which they consider essentially distinct, viz. convex and concave. These, to some extent, alternate with one another, but a fork given off by a convex one is not considered to be a concave one. The terms convex and concave are not happily chosen; they do not refer to the shape of the nervures, but appear to have been suggested by the fact that the surface of the wing being somewhat undulating the convex veins more usually run along the ridges, the concave veins along the depressions. The convex are the more important of the two, being the stronger, and more closely connected with the articulation of the wing.
The wings, broadly speaking, may be said to be three-margined: the margin that is anterior when the wings are extended is called the costa, and the edge that is then most distant from the body is the outer margin, while the limit that lies along the body when the wings are closed is the inner margin.
The only great Order of Insects provided with a single pair of wings is the Diptera, and in these the metathorax possesses, instead of wings, a pair of little capitate bodies called halteres or poisers. In the abnormal Strepsiptera, where a large pair of wings is placed on the metathorax, there are on the mesothorax some small appendages that are considered to represent the anterior wings. In the great Order Coleoptera, or beetles, the anterior wings are replaced by a pair of horny sheaths that close together over the back of the Insect, concealing the hind-wings, so that the beetle looks like a wingless Insect: in other four-winged Insects it is usually the front wings that are most useful in flight, but the elytra, as these parts are called in Coleoptera, take no active part in flight, and it has been recently suggested by Hoffbauer[[25]] that they are not the homologues of the front wings, but of the tegulae (see Fig. 56), of other Insects. In the Orthoptera the front wings also differ in consistence from the other pair over which they lie in repose, and are called tegmina. There are many Insects in which the wings exist in a more or less rudimentary or vestigial condition, though they are never used for purposes of flight.
The abdomen, or hind body, is the least modified part of the body, though some of the numerous rings of which it is composed may be extremely altered from the usual simple form. Such change takes place at its two extremities, but usually to a much greater extent at the distal extremity than at the base. This latter part is attached to the thorax, and it is a curious fact that in many Insects the base of the abdomen is so closely connected with the thorax that it has all the appearance of being a portion of this latter division of the body; indeed it is sometimes difficult to trace the real division between the two parts. In such cases a further differentiation may occur, and the part of the abdomen that on its anterior aspect is intimately attached to the thorax may on its posterior aspect be very slightly connected with the rest of the abdomen. Under such circumstances it is difficult at first sight to recognise the real state of the case. When a segment is thus transferred from the abdomen to the metathorax, the part is called a median segment. The most remarkable median segment exists in those Hymenoptera which have a stalked abdomen, but a similar though less perfect condition exists in many Insects. When such a union occurs, it is usually most complete on the dorsal surface, and the first ventral plate may almost totally disappear: such an alteration may involve a certain amount of change in the sclerites of the next segment, so that the morphological determination of the parts at the back of the thorax and front of the abdomen is by no means a simple matter. A highly modified hind-body exists in the higher ants, Myrmicidae. In Fig. 60 we contrast the simple abdomen of Japyx with the highly modified state of the same part in an ant.
Fig. 60.—Simple abdomen of Japyx (A) contrasted with the highly modified one of an ant, Cryptocerus (B). The segments are numbered from before backwards.