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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



on a flat, vertical surface of many square feet, it will remain for over 

 an hour, never relaxing its grip. 



A similar grip is used by this pet snake in constricting prey, a habit 

 best known among boas and pythons. (It is indeed possible for a large 

 boa or python to constrict and even kill a man, but there is no cause 

 for alarm when a tamed python is coiled around one's neck or arm and 

 commences to exert pressure; it is merely securing a firmer hold in 

 order to prevent a fall.) 



Rectilinear movement takes place at leisurely speed. At moderate 

 or high speed a snake displays the typical serpentine movement, as it 

 is called, wherein the body undulates in lateral curves and can thereby 

 brace itself against projecting obstacles in its path. "Without such 

 projections a snake cannot proceed, as may be shown by placing it on 

 a highly polished surface. 



Where the flow of curves is restricted, as in a narrow tunnel, a 

 snake may resort to yet a third locomotion called the concertina move- 

 ment. At intervals along its body are stationary curves that press 

 firmly against the sides of the tunnel acting as anchors toward which 

 and away from which the parts in between can be moved. In this way 



B 



B 



-C 



Figure 1. — Stages, from top to bottom, of concertina movement in a snake moving along 

 a tunnel. A, Body at rest v/here It touches the tunnel walls. B, Body In front mov- 

 ing forward. C, Body behind being drawn up. 



it progresses in steps along the tunnel (fig. 1). Frequently a mixture 

 of both this and rectilineal movement takes place in which the belly 

 shields at the stationary points along the body grip the underlying 

 surface. 



In some snakes, especially certain vipers and rattlesnakes, a curious 

 sideways progression occurs. One North American rattlesnake 

 {Crotcdus cerastes), in fact, is named after this peculiarity. It is 

 called the sidewinder. When side winding a snake proceeds in a direc- 



