12 WHEELER— THE PARASITIC ACULEATA. 



In 1883 Perez published an important study of the parasitic 

 bees and republished his general conclusions in a separate article in 

 1884. After careful morphological investigation he concluded that 

 the parasitic genera must have evolved from the host genera and 

 was able in a few instances to point out the very species from which 

 the parasitic genus had originated. He recognized four distinct 

 lines of development from as many host genera: Pskhyrus from 

 Bombus, Sfelis from Anthidium, Ccclioxys and Dioxys from Mega- 

 chile and Sphecodes from Halictiis. The series of genera known 

 as the Nomadinae and comprising Epeolus, Melecta, Crocisa, Am- 

 mohatcs, Pasitcs, Philcrcmus, Biastcs and Nomada, he derived from 

 Ccclioxys on the supposition that this genus had given rise to a 

 whole series of parasitic forms which had acquired new hosts among 

 the various genera of recoltant bees. He contended that Latreille's 

 example in placing the parasitic genera next to their host genera 

 should be followed in any attempt at a natural classification of the 

 Apidae. The truth of his contention has since been conceded and 

 is clearly expressed in the classifications of Ashmead ( 1899) , Robert- 

 son (1899) and Cockerell (1910). Dalla Torre (1896) and Friese 

 in his work on the African bees (1909), however, adopt a com- 

 promise between the views of Latreille and Lepeletier, dividing the 

 bees into podilegid, gastrilegid and social sections and appending 

 to each a series of parasitic genera. Of the classifications I have 

 seen Robertson's seems to be the most natural, but he is dealing 

 with a limited fauna, in which the relations of the parasitic genera 

 are few and fairly well known, whereas Dalla Torre and Friese, 

 in an attempt to deal with the bees of remote regions or of the 

 whole world and with dozens of imperfectly known parasitic gen- 

 era, have some justification for the course they adopt. It is evi- 

 dent, nevertheless, that no satisfactory classification can be con- 

 structed till the precise affinities of all the parasitic genera to one 

 another and to the host genera have been thoroughly elucidated. 



The phylogenetic relationships even among the European and 

 North American parasitic bees are still in part very problematical. 

 Probably all agree that Psithyrus must be derived from Bombus, 

 Stelis from Anthidium (scnsu lafo), and Ccclioxys from Mcgachilc, 

 or some closely related, now extinct, genus. But the origin of the 



