42 ILLINOIS BIOLOGICAL MONOGRAPHS [42 



variation in these forms," but the characters are present and their ob- 

 scurity merely challenges us to closer search. Another demand of eco- 

 nomic workers is an analytical key to the families. Dyar's few tables 

 are usually not adapted to the identification of individual specimens, for 

 he uses characters which, in the later stages of many species, are com- 

 pletely obscured by secondary setae or entirely lost. The best key thus 

 far published is that by Forbes (1910), but it omits nearly all the 

 Microlepidoptera as well as the more obscure families of the larger moths 

 and butterflies. There are species he had not seen which trace to fami- 

 lies in which they do not belong, but such a fault is one a worker on 

 immature insects is unable to avoid. 



In several respects a systematic outline of larvae differs from a 

 classification of a group of imagines. It is clear that while a difference 

 in larval characters indicates a phylogenetic divergence just as surely 

 as does a variation in adult structure, similarity in the adults of a group 

 of genera does not necessarily indicate the presence of common charac- 

 ters in the larvae. One always faces the fact that he is working with 

 genera and families established on adult characters and that the un- 

 known species of a particular group may be very different from those 

 with which he is acquainted. 



Another element of doubt is caused by the necessity of breeding. 

 One can not breed a specimen and keep it too. Larvae must always be 

 identified from the adults into which their associates develop except 

 when they are raised from fertilized eggs laid by a known female. The 

 danger of mistaken records can be greatly reduced by careful work but 

 when handling large numbers of species can never be wholly eliminated. 



It should also be noted that the adult state represents a single instar 

 while the larva undergoes several molts, changing materially at least 

 once. Wherever possible a description should include all the instars. 

 In this paper, only the more mature larvae were considered, as a rule, 

 but in most families the characters used apply to all except the first or 

 the first and second stages. Usually only the colors change after the 

 second molt. 



These sources of error were reduced in importance by the confirma- 

 tion of observations on long series of individuals. The larvae of several 

 large collections were examined, with the result that several mistakes 

 due to these causes were corrected. 



CHARACTERS USED 



Adult insects are identified mainly on characters of the wings, seg- 

 mented appendages, body sclerites, copulatory apparatus, and vestiture 

 of the body and legs. Of these, aU except the segmented appendages 



