14 ILLINOIS BIOLOGICAL MONOGRAPHS [14 



homotypic with iv of the abdomen, it is shown to be really iii a, a seta 

 which Dyar had either overlooked or considered of no importance. 



W. T. M. Forbes' work (1910) did not cover the subject of the 

 homotypy of the setae. The few figures he labels include errors for 

 which he was not responsible as he had not given the subject con- 

 sideration. (See discussion of theta, page 34.) 



Siltala (1907) made a study of the chitinous armature, especially 

 the setae, of caddice-worms. He finds considerable variation in the order 

 (Trichoptera) but carefully compares the different arrangements. The 

 setae of the first larval stage are very sparse while those of later instars 

 are obviously special and secondary developments. In neither case can 

 satisfactory comparisons be made with the Lepidoptera. 



THE CHAETOTAXY OF A TYPICAL SEGMENT 



While previous workers have drawn so-called type segments, these 

 have never included all the setae found in larvae of this order. Their 

 significance has been limited to particular segments or to particular 

 groups. For example, Dyar's "typical segment" refers to the abdomen 

 only, altho it has been extended to cover the last two thoracic segments of 

 the larvae of Frenatae. 



In order to make the following pages clear, attention should be 

 called at this point to the diagram (Fig. 1) which shows all the pri- 

 mary and subprimary setae normally present on the segments of any 

 generalized lepidopterous larva. The evidence on which this diagram is 

 based and also the reasons for the adoption of Greek letters instead of 

 numerals in naming the setae are given on later pages. This hypothetical 

 type is mentioned here as a point of reference in order that it may be 

 possible to use the setae as illustrations of the general principles on 

 which this study of homology is based. On the same and the follow- 

 ing plates are shown some of the modifications of the plan. 



It has been necessary to introduce certain new terms in this paper 

 and in a few cases to use the old terms in a special sense. A glossary 

 will be found at the close of Part Two, giving the meaning of all words 

 used in a special sense in this discussion. The explanation of plates 

 (p. 147) gives the names and grouping of all the setae. 



II. Nature of the Evidence 



It is necessary first to outline the principles underlying attempts 

 at any determination of homotypes. The evidence on which decisions 

 in doubtful cases were based is discussed here, for some of the principles 

 are not axiomatic. Their correctness will not be disputed, I am sure, 

 by those who consider the broad basis of fact on which they rest. 



