18 ILLINOIS BIOLOGICAL MONOGRAPHS [48 



more like malformations than processes. Horns are sharp pointed and 

 unbranched, the caudal horn of the sphingids being the best example. 

 Protuberances are large rounded swellings without definite outlines and 

 are usually lateral in position. 



There are two or more large chitinized plates covering the dorsal 

 half of a segment. The first is the prothoracic shield (Figs. 7, 25, etc.), 

 a thickening of the cuticula covering the greater part of the dorsal half 

 of the prothorax. In a few cases shields are also developed on segments 

 II and 9. The dorsum of segment 10 bears the suranal plate, which 

 varies from a mere thickening of the body wall to a highly modified and 

 variously specialized structure. (Fig. 84.) 



A glossary giving all these terms will be found at the close of the 

 paper. Following it, the method of numbering the segments is described 

 and a list of the Greek letters used for setae is given. 



CLASSIFICATION 



In nomenclature Dyar's "List of North American Lepidoptera" 

 (1902) has been followed throughout. "While this list is not perfect, it 

 is widely distributed and is the most accurate one now extant. In a 

 few cases in which the genus has been divided since the publication of 

 that list the new names are used but the old ones are included in paren- 

 thesis. Certain species have also been described since 1902 and they 

 are the only ones for which the authority is given. Outside the Micro- 

 lepidoptera, the only important change in the families is in the trans- 

 ference of Apatelodes from Notodontidae to Eupterotidae and in the 

 rearrangement of the Saturnioidea. 



It has been necessary to revise the Microlepidoptera entirely, owing 

 to a rapid advance in our knowledge of the adults. The grouping and 

 the division into families is principally a compilation from Walsingham 

 and from published and unpublished work of August Busck. At the 

 same time we believe that nothing in the larval structure precludes this 

 arrangement and that it is a more natural classification than any pub- 

 lished more than a decade ago. 



The order of treatment is the reverse of that usually followed in 

 Lepidoptera. There seems to be no excuse for beginning lists and out- 

 lines of this order with the most specialized forms when a treatment of 

 Hymenoptera always starts with a consideration of the Tenthredinoidea, 

 Coleoptera with the Carabidae, Diptera with the Nematocera, etc. The 

 latter arrangement is the logical one and the most natural. For that 

 reason families are listed here in an order which proceeds so far as 

 possible from generalized to specialized. In most cases, consequently, 

 it is the exact reverse of that followed in Dyar's List. 



