15] LEPIDOPTEROUS LARVAE — FRACKER 15 



DEFINITIONS 



According to the Standard Dictionary, homology refers either (a) 

 to ' ' the correspondence of a part of one animal with another, determined 

 by agreement in derivation and development from a like primitive 

 origin," or (h) to homotypy, which is "the correspondence of a part or 

 organ of one region with that of another region in the same animal." 

 Smith, in the "Glossary of Entomology", adds the usual provision that 

 "the organs must be identical in general structure and origin, tho they 

 may have developed in different ways for different purposes. ' ' 



There is an important difference between the homology of crustacean 

 appendages with each other and the homology of the setae in insects. 

 In the former case, work is based on similarity in the fundamental struc- 

 ture and development of the homologous organs ; in the latter, only pos- 

 ition can be considered, as the setae are all similar in structure. For 

 that reason it is necessary to secure a little more accurate definition as a 

 basis of work. With this in view, I suggest the following: 



Two organs on different segments of the same animal are Jiomotypic, 

 regardless of their jjositions at the present time, when they have developed 

 from homotypic organs of a generalized ancestor. In a generalized type, 

 two similar organs on different segments are homotypes when they hear 

 the same relations to the other organs of their respective segments. 



To show that a prothoracic seta, for example, is homologous with 

 one on the mesothorax, it is necessary to show that at one time the anlagen 

 from which these setae were developed were in similiar positions on 

 their respective segments. On the other hand it is equally true that, when 

 two setae of a specialized form are in similar positions on their respective 

 segments, a demonstration that they diverge farther and farther as we 

 study the more and more generalized types shows that they are not true 

 homologues but have converged in phylogenetic development. This gives 

 to ancestry the primary importance and makes necessary a consideration 

 of the nature of the evidence which bears on phylogeny. 



DIFFICULTIES 



The meagerness of the results thus far obtained on this problem is 

 due partially to peculiar difficulties in its solution. Certain obstacles 

 stand in the way of an accurate and complete demonstration of the 

 homology of the setae. They are mentioned, not to emphasize the magni- 

 tude of the task, but to explain the fact that so many doubtful points 

 remain unsettled. 



The first of these difficulties is caused by the absence of intermediate 



