11] LEPIDOPTEROUS LARVAE — FR ACKER 11 



PART ONE. THE HOMOLOGY OF THE SETAE 



I. Introduction 



One of the most serious difficulties in the path of scientific progress 

 is the lack of a means of expression common to all the workers in a single 

 field. When a particular term means one thing to one scientist and 

 something else to another, no amount of learning will make the opinions 

 of these men intelligible to each other until they understand the differ- 

 ence. In human anatomy, the large number of workers, the excellent 

 figures, and the antiquity and narrow limits of the subject have to a 

 large extent removed this confusion but in other fields of biology the 

 mistakes it causes are still apparent. This is true in entomology and 

 very noticeably so in the study of larvae. 



The various systems of numerals which have been applied to the 

 setal arrangement of lepidopterous larvae are all based on the simple 

 plan of numbering the setae from the dorsomeson. Except in the most 

 conspicuous cases, little consideration is taken of the relations of the 

 different segments to each other. Several authors have already intro- 

 duced confusion by applying the numbers in a slightly different way 

 from that first suggested, but no careful investigation has been made 

 of the real relations of the larval chaetotaxy of one group to that of 

 another. 



Realizing the conflicts in the application of the numerals now in use 

 and the nature of the objections to them, the writer began the study 

 of caterpillars with an investigation of the homology and homotypy of 

 the setae. The object of the former was a determination of the changes 

 which have taken place in the ancestral history of any particular body 

 segment, such as the prothorax, and the application of a given name to 

 the same structure throughout the entire order. The study of the latter, 

 homotypy, was taken up for the purpose of finding the relation of the 

 setal pattern of the different body segments to each other, in the hope 

 of applying the same name to the same structure throughout the entire 

 body. All the segments behind, but not including, the head were studied 

 and satisfactory results were obtained for all except the tenth or last 

 abdominal segment. 



