EXTERNAL STRUCTURE OF INSECTS. 35 



thorax as follows : the first segment bears the first pair 

 of legs ; the second segment bears the second pair of 

 legs and the first pair of wings ; the third segment bears 

 the third pair of legs and the second pair of wings. 



The legs of insects are six in number. These are 

 adapted to several modes of progression, as walking, 

 leaping, burrowing; while they serve other purposes also, 

 the fore legs being prehensile in some species, the hind 

 legs adapted to carry burdens in others, and so on. 

 The principal parts of the leg are, the Fi s- & - 

 coxa; trochanter; femur; tibia, and 

 tarsus (see fig. 9). The coxa (a) is 

 a large and flat joint hinged to the 

 body. It is very conspicuous in the 

 large water-beetle, Dyticus. The tro- 

 chanter (6) is the next, and a very 

 small joint. In the hind leg ef some 

 insects, e.g. saw-flies, it is formed of Middle le of pyti- 



cus margirialis. 



two pieces instead of one. The next a coxa 

 joint is the femur (c) or thigh the & trochanter. 

 large and usually thick joint which <% tibia!"' 

 stands out horizontally from the in- e - tarsus, 

 sect's body. Next is the tibia (d), or shank, usually of 

 about the same length as the femur, but thinner ; and 

 lastly the tarsus (e), which is composed of a series of 

 joints, terminating in a clawed foot. The joints in the 

 tarsus in different insects are from one to five in number. 

 It is in the fore and hind legs that we find the most 

 striking variations of development for special ends ; the 

 following figures are intended for comparison with 

 figure 9, in order to give the reader an idea of 

 the manner in which the parts are modified. Fig. 9 

 shows an ordinary form of insect's leg. Fig. 10 



