8 



INTRODUCTION. 



while the raesonotum carries the elytra, and the raetanotam the 

 membranous wings. Each of tbe sterna is made up of three parts : 

 the central (or sternum proper), the episternum, and the epimeron. 

 The whole of these parts are seldom visible in any one insect, some 

 of them being often more or less hidden by the epipleurce or re- 

 flexed sides of the elytra. An insect has no internal skeleton 

 proper, but the structure of the tentorium is more or less repeated 

 in the segments of the thorax and in all these the extensions 

 must be regarded as really ingrowths of the external skeleton. 

 These are of three kinds : dorsal or phragfnata, lateral or 

 apodemes, and ventral or apophyses ; the latter term is somewhat 

 unfortunate, as it is also applied to the appendages of the apical 

 abdominal segments of the ColltriNjE, etc. The phragmata have 

 evidently to do with the muscles of the wings, as there are none 

 in the prothorax, while the apodemes and apophyses probably 

 support the muscles of the legs. 



The legs are six in number and are extremely variable in size 

 and shape, according to the purposes for which they are adapted. 

 In very active species, such as the Cictndelid^;, they are very long 

 and slender (sometimes extraordinarily so), while in the case of 

 the fossorial beetles they are, as might be expected, short, broad, 

 and very hard ; in the Dytiscidje the hind pair are formed for 

 swimming, and in the HalticiDjE, with their strongly thickened 

 femora, for jumping ; occasionally, as in Sagra, the hind femora 

 are very strongly thickened, though the insects have no jumping 

 power ; in many of the Oueculionii)^; the legs are especially 

 adapted for clinging, while in numerous cases they are strongly 

 retractile and fit closely to the body, enabling the insect to escape, 

 without attracting notice, as long as it keeps motionless and feigns 

 death. The legs are joined to the body by the coace, which fit into 

 cavities called the coxal cavities or acetabida and form a more or 

 less perfect ball and socket joint. These cavities are formed by 

 two sterna, or are situated entirely within the prosternum. In 

 the first case they are said to be open behind, and, in the second, 

 to be closed behind : this is a very important point in classification, 

 and the species with the anterior coxal cavities closed probably 

 belong to more perfectly developed forms. The portion of the leg 

 next the coxa is called the femur, and to the base of this is some- 

 times joined a small and somewhat variable piece called the tro- 

 chanter ; in some genera this is almost or quite absent, in others 

 it is strongly developed. On the outer side of the anterior and 

 middle coxae a small piece, not connected with the legs, is some- 

 times present: this is called the trochantin or paracoxa. Next to 

 the femur comes the tibia, and next to the tibia the tarsus, which 

 is never composed of more than five joints, and very rarely, if ever, 

 has less than two. The number of these joints has formed the 

 basis of several of the classifications of Coleoptera, and is still held 

 to be of considerable weight ; but it gives rise to many difficulties, 

 and it would perhaps be best to follow Latreille's rule (Gen. Crustac. 

 et Insect, i, p. 172), quoted by Lacordaire ((xen. Col. i, p. xiii) : 



