14 INSECT TRANSFORMATION 



built up of three segments, the terminal one carrying a pair 

 of strong claws and a central pad or pulvillus. The number 

 of foot-segments varies in different insects ; in most adult 

 insects there are five (Fig. 6 B), and where fewer are present, 

 it is believed that diminution from the primitive number has 

 taken place. This is indeed quite evident in some cases, 

 where the apparently absent segments may be recognized in 

 a reduced condition. 



ta 



B 



FIG. 6. 



A, Intermediate leg of a locust (Rutidoderes). ex, haunch ; 

 tr, trochanter ; /, thigh ; tb, shin ; ta, foot ; cl, claws ; 

 p, pulvillus. B, tip of shin (tb) and five segmented 

 foot (ta) of cockroach (Periplaneta). x 2j. 



While many insects have the legs of the three pairs closely 

 alike, grasshoppers and their allies are remarkable for the 

 excessive size of the hindermost legs, in which both thigh and 

 shin are greatly lengthened relatively to those of the other 

 two pairs, the thigh being strong and stout at the base, taper- 

 ing gradually towards the tip. This modification confers 

 on these insects their well-known power of leaping. The 

 grasshopper's leap results from the sudden straightening or 

 extension of the hind legs ; when the insect is at rest these 

 limbs are sharply bent or flexed at the knee-joint (Fig. i). 

 The straightening or the bending of the leg is caused by the 

 contraction of alternative sets of muscles known respectively 

 as the extensors and the flexors. The large size of the grass- 



