The Tetrapoda: Amphibia, Reptiles, Birds 609 



propeltis, which feed largely on other snakes or mice depending on 

 the species; the garter snakes, TJiamnophis, which feed largely on in- 

 sects ; the green snake, Opheodrys, also an insect feeder ; and the water 

 snakes, Matrix, which feed chiefly on fish and other cold-blooded verte- 

 brates. Most of these snakes can be regarded as useful forms due 

 to their eating of insects and small rodents. Some such as the black 

 snake and bull snake are particularly valuable species. 



While most species of snakes in this country are harmless, there 

 are some poisonous forms. The members of the genus Micrurus, the 

 coral snakes, are colorfully ringed with yellow and red and are about 3 

 feet long. Species occur in the southeastern United States and in 

 the Southwest. They have an extremely potent venom which has a very 

 pronounced neurotoxic effect. Their grooved fangs are in a fixed posi- 

 tion in the front of the mouth. Inasmuch as they are secretive in their 

 habits, they are seldom encountered. In general, they are inoffensive 

 in their habits, and seldom bite, thus should not be considered as a 

 dangerous snake. As their poison is extremely potent, however, it is 

 wise to treat them with caution. 



The most famous group of poisonous snakes in the United States 

 are the pit vipers. These include the copperhead, Agkistrodon mokasen; 

 the water moccasin, A. piscivorus; and the rattlesnakes of the genera 

 Sistrurus and Crotalus. The rattlesnakes are distinguished from other 

 snakes by their possession of a series of horny segments which rattle 

 when vibrated, producing a characteristic buzzing sound. Each time 

 the snake sheds, a new segment is produced on the rattle. As they shed 

 several times a year and often lose rattles, the number of segments is 

 no indication of the age of the snake. The members of the genus 6"^- 

 trurus are small, swamp-loving snakes that are relatively inoffensive. 

 By contrast, the members of the genus Crotalus (Fig. 202,/) are large, 

 active forms whose bite may at times be fatal. Some members of this 

 genus are the eastern timber rattler, C. horridns, the Texas diamond- 

 back, C. atrox, the Florida diamondback, C. adamanteus, and the side- 

 winder, C. cerastes. 



The paired fangs are hollow and can be retracted. They function 

 in a manner similar to hypodermic needles to deliver the venom 

 which is produced by large paired glands in the upper part of the 

 head. The name, "pit viper," is derived from the paired facial pits 

 which are located on either side of the head. These are lined with a 

 sensory epithelium which is very sensitive to temperature changes ; ap- 



