220 



EVOLUTION AND GENETICS 



practically useless for that purpose. In addition, the size of 

 insects is so small in relation to objects about them that a single 

 powerful leap is often a guarantee of safety, since it may carry 

 them readily into concealment. The suddenness of this type of 

 locomotion is its most valuable feature. 



The structural modification of a saltatory insect leg includes 

 great enlargement of the femur, which contains large extensor 



■^imi& 



il^--; 



■ !^.,,.-r-^:. 



iA'«g^^;^«?,i>iirc/j^y*-*;-'ii^ 



Fig. 129. — The Rock Wallaby Petrogale xanlhopus. (From Parker and Has- 



well, after Vogt and Specht.) 



muscles (Fig. 128B). Elongation is common, but not essential, 

 for among the beetles and the fleas are included very powerful 

 jumpers with hind legs as short as is normally the case in other 

 insects. The Orthoptera include more familiar jumping insects, 

 however, and in this order the long hind legs of the grasshoppers 

 and crickets are familiar to everyone. In these legs another 

 specialization is evident. The foot is not a dependable support, 

 but heavy spines are developed at the tip of the tibia, analogous 



