17] LEP1D0PTER0US LARVAE— FRACKER 17 



COMPARATIVE ANATOMY 



Caterpillars may be said to bear setae in all possible positions. Their 

 great number and uniform distribution in some species makes naming 

 and homologizing them seem impossible. A little study of the less 

 "hairy" ones, however, soon shows that the arrangement is quite con- 

 stant and further investigation proves that they have all been derived 

 from the same typical plan. 



The method used in determining this plan was the ordinary one 

 in problems of this kind. The setae of the prothorax, metathorax, and 

 abdomen of the generalized members of both suborders of Lepidoptera 

 were plotted, one segment over the other, as if all were on the same 

 segment. It was then found that the number of setae in this composite 

 picture (Fig. 1) was about fifteen, and that they were in approxi- 

 mately the same position as on the prothorax of the most generalized 

 forms of the order. 



The next problem concerns the relation of the different segments 

 of a larva to each other and to the generalized type. It is clear from 

 the first that the prothorax is least modified throughout the series. We 

 can not assume, however, that the setae of the following segments of a 

 caterpillar may be directly homologized with the type by comparing 

 them with the prothorax of the same individual. Differentiation into 

 the three types of arrangement must have occurred long before the 

 suborders of Lepidoptera were differentiated from each other. The 

 first insect with scaly wings and long maxillae probably arose from a 

 larva possessing a different arrangement of the setae on the abdomen 

 from that on the prothorax. It is possible that all of the thoracic 

 segments were similar, but the abdomen was certainly distinct. In view 

 of that fact, one easily sees that complete reliance on any single 

 modern form is likely to lead to error. An abdominal "type" must be 

 worked out in the way used to establish a hypothetical generalized 

 type. Then a comparison of the thoracic and abdominal types with the 

 general type will bring us as near as comparative anatomy can, to a 

 correct view of the homology between the prothorax and abdomen. 



ONTOGENY 



That the embryological development of an animal has an important 

 bearing on the study of its phylogeny has been recognized ever since 

 the "recapitulation theory" was first advanced. This theory has not, 

 however, shown the way to a complete solution of the problems of 

 ancestry. The development of members of nearly all animal groups 

 and that of the rarest and most peculiar forms has attracted numerous 



