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 {109}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