NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 23 



a tube, such as the oesophagus, that drives the contents of the tube 

 through the lumen. The caterpillar type of movement, on the other 

 hand, is a successive lengthwise contraction and expansion of the body 

 segments that gives a forward motion to the tube itself, in this case 

 the caterpillar. Barth, however, makes a detailed analysis of the 

 muscles involved in the progressive movements of the caterpillar. 

 He points out, furthermore, that the preservation of internal pressure, 

 or turgor, is an important factor in maintaining the action of the 

 muscles. This function he ascribes to the outer system of body-wall 

 muscles, which counteracts a dilation of the soft integument of the 

 caterpillar that would otherwise reduce the turgor. The somatic 

 musculature, according to Barth, is thus composed of locomotor 

 muscles and turgor-preserving muscles. 



Caterpillars of the Geometridae, the loopers, or measuring worms, 

 have adopted a method of locomotion by which they can progress at 

 a much greater speed than the ordinary caterpillar. In these cater- 

 pillars (fig. 12 A) the prolegs of only segments VI and X are func- 

 tional organs, the others have been eliminated or greatly reduced in 

 size. Instead of making a short forward step with the anal legs and 

 then extending the body a corresponding distance, the geometrids, 

 while holding to the support with the thoracic legs, bring the prolegs 

 of segment VI clear up to the thorax (C), thus throwing the first five 

 abdominal segments into a dorsal loop. Then, retaining their grip by 

 the prolegs, they straighten the body forward and upward, and finally 

 bring it down on the support ready to repeat the action. By this 

 method, with each forward step, the caterpillar advances a distance 

 (a) equal to the length of the first five abdominal segments. The rate 

 of progression can be varied; the caterpillar may go along at a lei- 

 surely pace but when in a hurry it loops and straightens in rapid 

 succession. According to Barth (1937) the geometrids are not able to 

 crawl in the manner of other caterpillars. In their movements there 

 is no visible contraction and expansion of the body segments. 



If the prime function of the caterpillar is feeding and the storage 

 of food reserves in its body as fuel for the pupa or to supplement 

 the sugar diet of the adult, it may be accredited with having fully met 

 its obligations. Almost its entire organization is a reconstructive adap- 

 tation for feeding. This is seen in the structure of the mandibles, the 

 long, soft, flexible body, the shortness of the thoracic legs, the presence 

 of prolegs on the abdomen, the great size of the stomach, the abun- 

 dance of fat tissue for food conservation, the extraordinary develop- 

 ment of the muscular system, the body mechanism of locomotion, and 

 the correlated action of the nervous system. All these features are 



