22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



tion and expansion of the segments now runs rapidly forward through 

 the length of the body. Each segment contracts from the rear and is 

 then expanded anteriorly by the following contraction of the segment 

 in front. Thus the segments successively extend forward, and finally 

 the head is protruded a short distance. At the same time the dorsal 

 hump runs forward like the crest of a wave along the back, lifting 

 successively the prolegs of segments VI -11 I and carrying them for- 

 ward to a new position ; on reaching the thorax the thoracic legs are 

 carried forward in the same manner. The anal legs then again are 

 brought up for a new hold, and the whole series of events is repeated 

 every few seconds. The caterpillar thus progresses at a uniform rate 

 by short forward thrusts of the body. Some species crawl with less 

 exertion, the anal legs being simply moved forward with little hump- 

 ing of the body. 



The only contribution of the legs to the locomotion of the crawling 

 caterpillar is the successive forward movement of the anal prolegs. 

 These legs are often more strongly musculated than the others. Their 

 reversal of position as the body advances gives them the appearance of 

 pushing forward, but it is the body moving away from them that alters 

 their slant. 



Since the motor force of the caterpillar's movement is the successive 

 contraction of the body segments from behind forward, locomotion is 

 produced evidently by the dorsal and ventral bands of longitudinal 

 muscles of segmental length. There being no muscular mechanism of 

 expansion, contraction has been made the effective agent of forward 

 movement, each contracting segment expanding the segment behind 

 in a forward direction. Other writers, as Barth (1937) and Fiedler 

 (1938), have attributed the caterpillar's movement to contraction of 

 the segments without noting the expansion. The locomotor mechanism 

 of the caterpillar is clearly a substitute for leg action, but in no way 

 resembles that of the earthworm. The legless worm moves forward 

 by a compression of the rear part of the body, which extends the 

 anterior part by internal pressure. Nothing of this sort is evident 

 in the caterpillar. The caterpillar's method of locomotion depends on 

 the soft, flexible nature of the integument, and the successive contrac- 

 tion of the longitudinal muscles in individual segments. In a last 

 analysis, therefore, locomotion is controlled by the action of the 

 nervous system in momentarily stimulating the segmental muscles in 

 order from the rear forward. 



Barth (1937) terms the locomotor movements of the caterpillar 

 "peristaltic." Peristalsis (Gr. peristalticos, clasping or compressing), 

 however, is a propagated series of circular (peri) contractions through 



