94 INTRODUCTION 



the presence of an enemy, at the same time widen- 

 ing or inflating the region behind the head. 



Most snakes can climb, and in this case the ribs 

 and ventral shields are of great assistance. The Tree- 

 snakes, usually characterized by a very slender, some- 

 times compressed, body, or by a prehensile tail, are 

 specially adapted for twining themselves round 

 branches, and in several of them the presence of a keel 

 on each side of the ventral and subcaudal shields, 

 accompanied by a notch corresponding to the keel, 

 affords an additional help for climbing on vertical un- 

 even surfaces, such as the trunks of trees. This con- 

 dition of the ventral shields has a bearing on the 

 extraordinary mode of locomotion with which some 

 Tree-snakes [Chrysopelea, and probably also Dendro- 

 phis) have long been credited by the Malays. We allude 

 to the so-called Flying-snakes, remarkable for their 

 habit of shooting down from trees and descending to 

 the ground at an oblique angle, the body being kept 

 rigid the whole time of the " flight." It has been 

 observed in Chrysopelea that the ventral surface 

 between the lateral keels, which may be compared 

 to hinges, can be drawn in and become deeply 

 concave, whilst at the same time a slight dorso- 

 ventral flattening of the body takes place. During 

 this muscular contraction the snake is like a piece of 

 bamboo bisected longitudinally, and is buoyed up in 

 such a way as to explain its parachute-like descent. 



All snakes are able to swim, and the more aquatic 



