Rectilinear Locomotion in Snakes

Abstract

IN his recent paper dealing with mechanism of locomotion in Gray (1946: 101) recognizes four main types of terrestrial progression among serpents. Three of these, termed (1) serpentine, (2) concertina, and (3) sidewinding, respectively, were described on basis of observations made on European grass snake (Natrix natrix). The fourth type, somewhat inaptly called caterpillar movement by Mosauer (1932: 585), is referred to as rectilinear movement by Gray. It was not observed in Natrix, but Gray notes that type of locomotion depends on movements on part of and ventral scales. Despite Mosauer's observation that the are not employed in sense of 'rib walking' theory, notion appears to be prevalent that rectilinear movement, in which snake progresses with its body oriented along a straight line, is dependent upon movements of It was formerly a textbook cliche that snakes crawl on their ribs and sometimes snake was even compared to a centipede, with implication that correspond in function to legs of latter. Not having observed rectilinear movement in grass snake, Gray may have been misled by some of accounts of snake locomotion that preceded Mosauer's. At any rate his impression is a false one. Rectilinear movement is quite impossible for such as Coluber, which have skin on venter firmly attached, but it is characteristic of Bitis, Crotalus, and other thick-bodied venomous when they are prowling. The sidewinder (Crotalus cerastes) is a notable exception, although even this specialized rattler occasionally resorts to rectilinear movement in its natural habitat (as may be seen from tracks) and commonly does so when moving along edge of a cage under captive conditions. The ventral scales are raised in some sections of body and carried forward, sometimes dragging over loose sand, while those in other portions of body are resting on substratum. The ventrals carried forward are then anchored in sand, and snake's body is drawn forward by muscular action to rest on these sections of body, while alternate series of ventral scales are, in turn, slightly raised and carried anteriorly. Thus, by moving alternate sections of venter forward, and by means of muscular action drawing body over ventral scales resting on substratum, snake is able to draw its body forward in a straight line. Rectilinear movement is dependent upon muscles connecting to integument, and on elasticity-of skin, ventral scutes of which serve as a sort of telescoping tread, segments of which are connected by cutaneous muscles, as well as by connective tissue. But there is no active movement of ribs. The muscles controlling successive waves in ventral scutes are attached to ribs, but themselves remain stationary with reference to vertebrae. This can be ascertained by cutting a window in skin of live snake just above