OF WASHINGTON. 107 



had issued in large numbers from the puparia of Baccha 

 babista, a syrphus-fly whose large larvae he had found feeding 

 also on the Orange Aphis. Another series he had reared from 

 the large puparia of another Baccha, the larvae of which had 

 fed upon the Cotton Aphis (A. gossypii), and naturally both 

 of these instances only confirmed my suspicions that Prof. 

 Cook's species had come from syrphid larvae and not from 

 bark-lice. The idea was becoming fixed in my mind that 

 Pachyneuron is normally a parasite of dipterous larvae of the 

 family Syrphidae and its close allies, my experience with 

 other groups of Chalcididae having apparently shown me that 

 speedy generalizations of this particular character are not 

 rash. 



Subsequent developments, however, speedily proved that no 

 such absolute rule could be founded, and that, while the para 

 sitism of Pachyneuron is not of the miscellaneous or heteroge- 

 nous character found with Pteromalus or Eupelmus, species of 

 each of which have been bred from hosts of four or five 

 different orders, the former from larvae and pupae, and the 

 latter from eggs, larvae and pupae of many widely differing 

 insects, yet it is also not of a uniform, unvaried character. It 

 may be said, in fact, to be of a dual nature, its species attack 

 ing, apparently indiscriminately, insects of two entirely dis 

 tinct and well-defined groups, each belonging to a distinct 

 order, and, curiously enough, and very unfortunately for the 

 economic reputation of Pachyneuron, these two classes of 

 hosts partake of the mutual relation of prey and predator 

 injurious insects and their direct and peculiar enemies thus 

 counterbalancing the benefits which our parasites might have 

 produced had their tastes restricted them to the former class. 

 Such instances have been very rare in my experience, and I 

 only recall one other in the case of Elasmus, which attacks 

 both microlepidopterous larvae and the Microgasters which 

 parasitize them. These two classes of the host-insects of 

 Pachyneuron are, first, the closely-related homopterous families 

 Coccidtz, Aphidida and Psyllid<z, and second, their predatory 

 dipterous enemies the Syrphida and (in one instance) Antho- 

 myiidce. When infesting Aphididce, in no case has more than 

 one parasite issued from a single plant-louse, but in the case of 

 Mr. Hubbard's anthomyiid two specimens emerged 'from the 

 host puparium, while with the large Baccha, according to Mr. 

 Hubbard's published statemenf, the parasites "issue from the 

 puparium in numbers varying from six to eighteen, through a 

 number of small holes which they gnaw through its top and 

 sides." A glance at the specimens, however, contradicts the 

 latter portion of this statement, for there is but one hole 



