258 VERTEBRATE ZOOLOGY 



the genus Crotalus (Fig. 141, C). Of these the Texas rattler is much 

 the largest and that of Canada the smallest. The largest known spec- 

 imens reach a length of seven feet and are stockily proportioned. The 

 bite is serious but seldom fatal. The rattle of the "rattler" is a curi- 

 ous structure, made by leaving the end of the moulted skin attached 

 to the tip of the tail, each moult adding a new ring to the rattle. The 

 ratthng sound, which is more like a shrill hiss, is made by quivering the 

 tail, a movement of excitement or fear rather than a purposeful warn- 

 ing signal. Nevertheless it is a sound that, even when heard for the 

 first time, causes one to "bring up all standing" and watch one's step. 

 Give a rattle-snake half a chance and he will run away without at- 

 tempting to attack. 



Adaptive Radiation Among the Snakes 



Although somewhat limited in their adaptive versatility by the 

 lack of limbs, the snakes show quite a wide range of speciaUzation 

 for the various life zones. The more generalized types are the common 

 ground snakes that have holes in the ground merely as retreats in 

 time of danger or for hibernation. 



A great many arboreal types have been developed, as the structure 

 of the snake is peculiarly well adapted for that type of climbing, for 

 which we have no other name than "serpentine." The Boidm (boa- 

 constrictors) are typical examples of arboreal snakes (Fig. 141, A). 

 These large, rapacious creatures secure their prey by dropping upon 

 it out of trees and crush it to death within their powerful coils. The 

 largest of these snakes are upwards of twenty feet long, about six 

 inches in diameter, and capable of crushing a tiger or a stag. They 

 are unable, however, to eat such large prey, their limit being rabbits 

 and fairly large birds, which they are able to swallow whole without 

 difficulty. There are several types that are more highly specialized 

 for arboreal life than the Boidse. Among these are the members of 

 the family Colubrince, which are characterized by their great length 

 and slenderness, and by the great flexibility of the prehensile tail. 



Another adaptive type is that which is native of the arid regions and 

 which has adopted the burrowing habit to protect itself against the 

 extremes of temperature so characteristic of desert regions. A spe- 

 ciaUzed burrowing type is represented by the genus Typhleps, of 

 which there are about one hundred species. They dig typical burrows 

 in the ground in which they spend much of their time. 



