230 



Journal New York Entomological Society, t^'o'- ^^v. 



a similar condition occurs in the lowest Apterygota {Campodea, 

 Eosentomon, etc.). Although both types of cerci occur in the Pan- 

 plecoptera, I am inclined to regard those with numerous small ring- 

 like segments (fig. 4, " c") as the more primitive. The fact that the 

 eighth abdominal sternum does not overlap the following ones is 

 another primitive feature in the Plecoptera, and, on the whole, the 

 Plecoptera seem to have departed as little as any living insects, from 

 the original condition of the ancestors of the Pterygota. In many 

 respects they are fully as primitive as the lowest fossil Pterygota, and 

 a study of their structures is equally important from the standpoint 

 of phylogeny. Furthermore, in dealing with living forms, there is 

 the added advantage of being able to examine numerous details not 

 preserved in fossil specimens, and to take into account the various 

 biological data as well ! 



Before discussing the interrelationships of the three superorders 

 here mentioned, their principal diagnostic characters may be briefly 

 summarized in the following table. There are some exceptions to the 

 general application of these characters, but, in the main, they hold 

 good for the groups in question. 



