MECHANICS OF SNAKES — LEUTSCHER 305 



In climbing or creeping at a slow pace some snakes are capable of 

 moving forward in a perfectly straight line. A wave action on the 

 lower sides of the body, which moves forward in a series of steps, is 

 caused by successive contractions and relaxations of the muscles 

 underneath. Attached to the ribs, those nniscles operate on the lower 

 body surface, which in most snakes is covered by a series of broad, 

 transverse scales, known as shields. These shields correspond in num- 

 ber to the vertebrae and overlap like roofing slates along their hind 

 edges. The free borders of the shields grip onto any rough surface or 

 irregularity over which the snake happens to be passing, and this action 

 is most ai;)parent along those parts of the body that are in contact with 

 the ground, rock, or branch. This rectilinear movement has been 

 compared with that of an earthworm and has often been described as 

 "a snake walking on its ribs." Actually, one gets a better analogy 

 when one visualizes the rowing of a boat. The oar blade which grips 

 the water corresponds to the ventral shield, the oar is the lever of rib 

 and muscle, and the ground is the fulcrum. Such movement is in 

 common use in ground snakes, such as our native adder, and arboreal 

 snakes such as the Aesculapian snake {Elaphe longissima) of south 

 Europe (pi. 1, fig. 1) . This latter, like many of its relatives, has over- 

 lapping belly shields which bend at a sharp angle along their shorter 

 sides. This forms a sharp keel which gives' an extra grip on the 

 smallest irregularities on bark or rock over which it climbs. 



The feats of climbing displayed by a pet Aesculapian snake in my 

 collection reveal the high degree of specialization achieved by snakes. 

 This specimen will climb over the furniture or crawl along the picture 

 rail in my study, never missing its hold or making a false move. It 

 can cling by its tail or any part of the body to the buttons of one's 

 waistcoat, or hang from a projecting pen fixed into a pocket. When 

 seeking a new perch it will stretch out its slender body in a horizontal 

 plane to a third of its length, unsupported in the air, as it reaches over 

 a gap to secure a fresh hold. The rest of the body, and in particular 

 the prehensile tail, meanwhile retains a secure grip w^hich it refuses 

 to relax until the next foothold is secured. If I attempt to pull it free 

 the grip is tightened. If I stand nearby it may attempt to bridge the 

 gap in order to reach an arm or shoulder. Should I move slightly it 

 draws back, remaining on its former perch. This is strongly reminis- 

 cent of an experienced mountaineer who always makes sure of his 

 next step before he releases a previous foothold. 



Along a tightened rope this snake will hang its body in loops over 

 each side, edging itself along slowly in a remarkable display of coordi- 

 nated muscle action in order to maintain its balance. Its "star turn" 

 is the grip it can maint^iin when hanging onto an electric-light switch 

 set into a wall. Here, on the only projection, less than half an inch, 



