OF WASHINGTON. 93 



issuing from ash trees on the streets of Washington in September. 

 He thought that there are two broods in this species. Mr. Ash- 

 mead stated that he had reared two annual broods of a Sesiid 

 from oak galls in Florida. Mr. Ashmead further discussed the 

 note on Ceresa, and gave in full the synonymy of C. uniformis 

 of Fairmaire. He called attention to the excellent character for 

 the separation of species in the last ventral segment of the female, 

 which varies in the intensity and form of a central indentation of 

 the hind margin. 



Dr. Gill, apropos of the difference in the sexual organs and 

 their use in separating species, asked as to the difference in the 

 genitalia of Belostoma and Benacus. Dr. Riley stated that, curi 

 ously enough, the differences in this respect between these two 

 forms are slight. 



Mr. Ashmead and Mr. Schwarz further discussed this point, 

 agreeing that in many groups whole families occur in which the 

 sexual characters are uniform, while in others they are very vari 

 able and afford criteria for the separation of species. 



Dr. Stiles stated that in the worms the genital characters are 

 the main ones in use. He said that in the Strongylidas, for in 

 stance, all classification was, until recently, based on the bursa 

 of the male. He now finds that the spicule affords a good char 

 acter, and in fact all through the Nematodes where workers are 

 lost on all other characters this spicule can be relied upon. 



Mr. Schwarz asked about the females, and Dr. Stiles replied 

 that it is dangerous to determine the females specifically. The 

 labia of the vulva afford some characters, but the determination 

 of species takes place mainly through association with the male. 



Dr. Gill stated that in the Helicidae we have an illustration of 

 the value of sexual characters. Helix hortensis and H. nemo- 

 ralis vary greatly in color, but in the color of the shell lip they 

 are constant, that of hortensis being invariably white, and that 

 of nemoralis brown, and this character is coincident with certain 

 genital modification. In fact, in the genitalia of this group we 

 have the best criteria for classification. Dr. Stiles asked as to 

 the reason for these sexual variations. Dr. Gill said that they 

 might be the coefficients of evolution, i. e., that these variations 

 might perpetuate varieties owing to structural difficulty in copu- 



