LEGS OF INSECTS. 299 



several pieces, connected together by articulations of different kinds, 

 which require our notice. The first division of the leg, or that in im- 

 mediate connexion with the thorax, to which it is united by a kind of 

 ball-and-socket joint enclosed in a strong membranous capsule and 

 possessing very various degrees of motion in different insects, is called 

 the hip (coxa) ; and upon this, as upon a centre, the movements of the 

 limb are performed. To the extremity of the coxa a small moveable 

 piece is attached, called the trochanter ; to which succeeds the thigh 

 (femur), which is the thickest and most robust of all the divisions of 

 the limb. The next piece, called the shank (tibia), is occasionally of 

 considerable length, and is connected to the last by a hinge; to its 

 extremity is appended the foot (tarsus), composed of a consecutive series 

 of small segments, varying in number from five to one, the last of which 

 is armed with claws, or other appendages, adapted to different kinds of 

 progression. These divisions of the leg the reader will easily recognize ; 

 they are for the most part united together by articulations so constructed 

 as to allow simply of flexion and extension, which will be best under- 

 stood by inspecting, in some large insect, the junction between the femur 

 and the tibia, or the knee-joint, as we might term it. Upon the upper 

 extremity of the tibia the observer will find on each side a precise semi- 

 circular furrow, behind which is a concentrical but smaller ridge, and 

 still further back a circular depression or fossulet. On examining the 

 corresponding surfaces of the femur, he will detect a ridge accurately 

 corresponding to the above-mentioned furrow ; behind this, a furrow 

 corresponding to the preceding ridge ; and still further back, a minute 

 elevation adapted to the fossulet of the tibia, wherein it is fastened by a 

 minute but very strong ligament. Such ridges and grooves, when fitted 

 into each other, form a joint evidently admitting of a free and hinge - 

 like motion, while, from its structure, dislocation is almost impossible. 



(779.) The above general description of the leg of an insect will 

 prepare us to examine various modifications in outward form and 

 mechanical arrangements by which these simple organs are adapted to 

 progression under a great diversity of circumstances. "When, indeed, 

 we reflect how extensively this class of animals is distributed, and the 

 variety of situations in which insects live, we are led to expect corre- 

 sponding adaptations in the construction of their instruments of loco- 

 motion ; and in this our expectations will not be disappointed. 



(780.) In the generality of terrestrial species, the last segment of 

 the tarsus or foot is provide^ with a pair of strong horny hooks, which 

 are available for many purposes, being used either for creeping upon 

 a moderately rough surface, for climbing, or for clinging to various 

 substances. 



(781.) Such simple hooks, however, would not always serve. In the 

 case of the louse (Pediculus) for example, that is destined to climb 

 slender and polished hairs, such prehensile organs could be of little use. 



