NO. 6 CATERPILLAR AND BUTTERFLY — SNODGRASS 5 



development the larval form, whatever it may be, precedes the adult. 

 The larva is the direct development from the egg, but it carries the 

 factors of adult development suppressed temporarily until the larva 

 has completed its growth, which then allow the adult development to 

 proceed. 



It may be disputed whether the holometabolous insects represent 

 one or several lines of descent. There is no modern larva that might 

 plausibly be selected as of the type from which the others may have 

 been evolved. Yet all holometabolous larvae have one feature in 

 common, which is the internal development of the wings. Just what 

 essential survival value the endopterous condition may have had is 

 difficult to see, since the young of numerous other insects seem to get 

 along very well with external wing pads. However, whatever may 

 have been the form of the primary holometabolous larva, or why it 

 became endopterous, it seems highly probable that external wingless- 

 ness was a condition favorable for many potential habitats, and thus 

 led to the great diversification of modern larval structure in adaptation 

 to various ways of living. The association of the endopterous condi- 

 tion of the larva with holometabolism is probably because internal 

 wing rudiments could become fully developed external wings only 

 in a pupal stage. 



Of all the holometabolous larvae, the lepidopterous caterpillar is 

 structurally one of the most standardized. Though caterpillars differ 

 in size and details of structure, they never depart from the funda- 

 mental caterpillar organization. By way of contrast consider the 

 difference among the Diptera between a mosquito larva and the 

 maggot of a muscoid fly, or in the Hymenoptera the contrast between 

 a sawfly larva and the larva of a wasp or bee. Some function of the 

 caterpillar has demanded a basic uniformity of structure in all species. 



In conformity with the principle already discussed that the adult 

 insect is responsible for the structure of the larva, we must look to 

 the moth or butterfly to find the reason for the caterpillar. 



EVOLUTION OF THE ADULT 



The moths and butterflies are named Lepidoptera because of their 

 scaly wings, but their scales do not make them what they are, any 

 more than do the bright colors that many of them wear. The mosquito 

 has scales on its wings, and some moths have clear wings. The typical 

 lepidopteron is distinguished from all other insects by the possession 

 of a tubular, nonpiercing, maxillary proboscis, coiled beneath the 

 head when not in use (fig. 3E). The other mouth parts are much 



