l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



for their food, because the female moth or butterfly usually deposits 

 her eggs on the proper food plant for the larva. 



Since the usual labial salivary glands of other insects are converted 

 into the silk glands of the caterpillar, many caterpillars at least have 

 long tubular mandibular glands (fig. 8 C, mdGld). The secretion of 

 these glands might be supposed to have a salivary function, but from 

 analyses that have been made Uvarov (1928) suggests that the secre- 

 tion may be of an excretory nature. 



The cylindrical body of the caterpillar accommodates the large ali- 

 mentary canal, but the abdomen is too heavy to be held up by the 

 thorax in the manner of adult insects. The reduction of the thoracic 

 legs, therefore, allows the abdomen to follow in line with the thorax, 

 and also brings the mouth parts into close contact with the feeding 

 surface. The prolegs support the heavy abdomen, and serve for 

 holding on tight when the caterpillar is climbing or feeding. 



The prolegs occupy the position of the abdominal limb vestiges of 

 the embryo, and for this reason some writers have regarded them as 

 appendages serially homologous with the thoracic legs. In structure 

 and musculature, however, the prolegs have nothing in common with 

 the thoracic legs, and they are not moved in the manner of legs as 

 organs of locomotion. Hinton (1955) has thoroughly discussed the 

 whole subject of larval prolegs, not only in the Lepidoptera but in the 

 other orders as well, and gives cogent reasons for believing that in all 

 cases these legs are secondary adaptive structures of the larva having 

 no relation to former abdominal appendages represented by vestiges 

 in the embryo. 



A typical caterpillar proleg (fig. 9 A, G, J) is a short, thick, hollow 

 outgrowth of the body wall ending with a flattened foot lobe known 

 as the planta, which is armed with small hooks, or crochets. Muscles 

 from the body wall are attached on the base of the leg (figs. 9E, 

 10 A), and in some cases within the leg; but the principal proleg 

 muscles are a group of long fibers from the lateral body wall inserted 

 on or near the center of the planta (fig. 9E). These muscles are 

 plantar retractors. Inasmuch as the Onychophora and Tardigrada 

 have a leg musculature similar to that of the caterpillar's prolegs, 

 Pryor (1951) points out that this type of musculature simply meets 

 the need of a similar mechanism in legs of a similar structure, and can 

 have no phylogenetic significance. 



The crochets are arranged in various patterns on the planta in 

 different caterpillar species. Fracker (1930) and Peterson (1948) 

 have made comparative studies of the crochets from a taxonomic 

 standpoint without correlating their arrangement with the habits of 



