36 ILLINOIS BIOLOGICAL MONOGRAPHS [36 



a subprimary seta on one segment the liomotype of a primary one on 

 another. The fact that in Hepialus eta is present on both the prothorax 

 and abdomen but that there is no seta with which it can possibly be 

 homologous on the mesothorax and metathorax, shows that in the latter 

 place its homotype has either been lost or has never appeared. The 

 same conclusion is indisputable after a study of first-stage Frenatae. 

 Since the separation of the two suborders a new seta has appeared on 

 the caudal thoracic segments in line with eta of other parts of the body. 

 This new seta bears the same relation to the other structures of the 

 segment as eta does and associates with kappa in the same characteristic 

 way. While its later appearance makes it doubtful whether the meso- 

 thoracic seta should be called a homotype of the one on the abdomen, 

 it is clear that no object would be served by merely attaching to it a 

 new name. We therefore depart to that extent from a strict interpre- 

 tation of our definition of homotypy. 



Pi. Considerable confusion might exist at first in regard to the 

 homology of this and neighboring setae. The description of the ventral 

 half of the larva of Hepialus shows that the homology there is evident 

 enough. Any other interpretation would involve one in endless compli- 

 cations and would necessitate a total neglect of the generalized first 

 larval stage. 



In Frenatae, however, the prolegs are much farther mesad than 

 the thoracic legs and the setae of the two regions have a less apparent 

 relation to each other. This is complicated by the arrival of mu which 

 is directly caudad of the thoracic pi and appears to be analogous with 

 it. Fortunately the condition in the newly hatched larva proves the 

 analogy to be more apparent than real, for mu is not present in the 

 first instar. This is so constant a character that Dyar distinguishes the 

 Jugatae from the Frenatae by the presence of this seta, "vi", after the 

 first molt in the latter group while it never appears in the former. In 

 spite of this fact, Dyar, Forbes, and others usually label pi on the 

 thorax, "vi", indicating that it may be homotypic with mu of the abdo- 

 men. For that reason the group requires more extended discussion 

 than the others have received. In the following summary of the evi- 

 dence on this point, ''Pi" is used for the Pi group of one or two setae 

 at the base of the thoracic legs, "mu" for the subprimary on the abdo- 

 men, and "vii" (following Dyar) for the lateral two of the group of 

 three setae on the prolegs which we are convinced are homotypic with 

 ''Pi". 



Pi must be homotypic with vii in Hepialus for mu is not present. 

 (Figs. 5 and 6.) 



