THE BEETLES 3091 



slender, are strong and well developed. But in certain groups, where the habits 

 and environment of the insects require it, the legs are adapted to various other pur- 

 poses. Beetles that jump usually owe their leaping powers to the greatly thickened 

 femora and straight and relatively long tibiae of the hind-legs. It would, however, 

 be a mistake to suppose that when a beetle has thickened and strongly developed 

 hind-legs it must consequently be able to jump. Some burrowing species, and 

 others that are not very active in their movements, have very thick hind-legs; 

 though, as a rule, it is the front pair of legs which is thickened and otherwise 

 modified to serve as digging organs in those beetles that burrow underground. In 

 aquatic beetles the swimming legs are disposed like oars, and have all their parts 

 broad and flat, while their breadth is further increased by rows of bristles. Either 

 the hind-legs only, which is the rule, or the middle pair also, as in the whirligig 

 beetles (Gyrinidce), may be thus transformed into swimming organs. The coxae 

 or basal joints of the legs vary much in shape and in the mode in which they are 

 inserted in their sockets on the under side of the thorax. Those of each pair are 

 sometimes close together, sometimes widely separated from another; while a longer 

 or shorter distance may intervene between the coxae of the different pairs of legs, 

 and especially between those of the two hinder pairs. Considerable importance 

 attaches to the number of joints in the tarsi or feet. In classifying beetles this 

 number is one of the first things to be noticed. If a beetle has five joints in each 

 of its tarsi, it is placed in that section of the order which is known as the Pentam- 

 era; if it appears to have only four joints in each foot, it belongs to the Tetram- 

 era; and if but three, to the Trimera. When there are five joints in each of the 

 four anterior feet, and only four in the hind-feet, the beetle may be regarded as one 

 of the Heteromera. To these general rules there are a few exceptions which need 

 not be discussed here; but we must point out that although in the Tetramera the 

 tarsi appear to be four jointed, and in the Trimera three jointed, they are really 

 composed of five joints and four respectively. The fourth joint in the one case, 

 and the third in the other, are, however, usually so small as not to be noticed 

 except upon very close examination. The abdomen is never stalked in beetles, but 

 attached to the thorax by a broad base, which is applied against the posterior coxae; 

 exceptionally, however, as in certain mimicking species, its base may be more or 

 less narrowed. It is generally somewhat flattened in shape; and on the upper side 

 eight segments are usually distinguishable, which, so far as protected by the elytra, 

 have a soft and but slightly horny integument. Five or six segments are generally 

 visible on the ventral side, but in certain cases the number may be reduced. The 

 terminal segments are usually retracted within the abdomen, and completely hid- 

 den from view, but in the females of many species they can be exserted in the 

 form of a tubular ovipositor, which enables the insect to lay its eggs deep in the 

 crevices of bark. 



Although beetles do not always exhibit differences in external form by which 

 the sexes may be distinguished, such differences frequently exist, and are some- 

 times of the most pronounced character. As a rule, the male is more slenderly 

 built than the female, and has longer and more fully-developed antennae; his eyes 

 also are often larger, and in the length and shape of the legs, and in the width and 



