NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 19 



the anterior margins. Each disc has a central muscle attachment. It is 

 not clear that the arrangement of the crochets here has any relation to 

 the boring habits of the larva, since in other boring species the crochet 

 pattern is highly variable. 



The prolegs are clearly very useful structures to the caterpillars as 

 props for the abdomen and for holding on while climbing, but they are 

 not active organs of locomotion. Though usually they are present on 

 abdominal segments III-VI and X, and are rarely more numerous, 

 they are frequently fewer in number, and in leaf miners may be absent 

 entirely. 



A few caterpillars, such as the case makers and the bagworms, walk 

 with the thoracic legs while the body is held erect. To the ordinary 

 free caterpillars, however, both the thoracic and the abdominal legs are 

 of little or no use for progression, and the caterpillars have developed 

 a remarkable motor mechanism for direct movement of the body itself. 

 In addition to crawling and climbing, various caterpillar species have 

 become leaf miners and others bore into the stems or wood of plants 

 or burrow in the ground. Furthermore, most caterpillars are able to 

 twist and turn in all directions, and often, while holding on with the 

 abdominal legs, they raise the anterior part of the body and swing it 

 about in search of new foliage on which to feed. For all these move- 

 ments the caterpillars have a most elaborate body musculature (figs. 

 10, 11), which includes the usual dorsal and ventral longitudinal bands 

 of intersegmental muscles, and a complex outer system of small 

 muscles going in various directions against the body wall. Similar 

 muscle patterns in other species are illustrated by Forbes (1914). 



The common caterpillar does not walk ; as already said it progresses 

 by movements of its body, not by means of its legs. It has therefore 

 evolved a type of motor mechanism that propels it forward while the 

 body is close to the support, and its manner of moving is not that of a 

 worm or a snake. The locomotor movements are best seen on a 

 slender, naked caterpillar, such as a noctuid ; the following observa- 

 tions were made on a species of Heliothis. When the resting caterpillar 

 is about to move, the thoracic legs may first become active and some- 

 what stretch the anterior part of the body, but they do not bring up 

 the heavy abdomen. Forward movement is initiated by lifting the 

 posterior end of the body, curving it downward and forward, so 

 shifting the anal prolegs anteriorly to a new grasp on the support. 

 Immediately then the deflected segments contract, straighten, and 

 produce a hump on the back, while the anal legs maintain their hold, 

 though reversed in position. A wave of successive forward contrac- 



