124 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 1 



from some of the forms closely related to parasites which control it in 

 its native environment. We have abundant proof however that such 

 is not usually the case, and it is a matter of general agreement that 

 the likelihood of any sudden variation appearing in the life history of 

 a parasite, due to an introduced host, is very slight. Such a general- 

 ization is not universally true, especially among members of the dip- 

 terous family Tachmidce, but seems to ordinarily apply, and does so 

 particularly well, to the groups of parasitic Hymenoptera to which I 

 shall confine most of my remarks. 



Upon what, then, does this mutual adjustment between parasitic 

 species and host species depend? 



Entering the field of speculation, it is evident that there are a num^ 

 ber of possible factors which may determine it, and I shall endeavor 

 to consider the more important in turn. 



That the physical form or size of a species has an important bearing 

 ■on the matter of parasitism is undoubted. Parasites which live singly 

 in the bodies of their hosts must necessarily confine their attacks to 

 species which will furnish them with a proper amount of food to ma- 

 ture. On the other hand, it is imperative that the body of the host be 

 •entirely consumed at maturity in the case of parasites which pupate 

 in situ; or in the case of species which leave the host for pupation, 

 that the emergence of the delicate parasitic larva may proceed without 

 accident. 



Many of the smaller parasitic species, particularly certain Chalci- 

 didcB, may develop in large numbers within a single host. Such species 

 ofter undergo remarkable multiplication during development, and the 

 number of young is regulated to suit the food supply. The adapt- 

 ability of certain forms in this respect has often been observed. For 

 ■example, the well-knoMTi and widespread Pteromalus puparum attacks 

 a considerable series of butterfly larvae, ranging in size from rather 

 small to large species, and the number of specimens derived from a 

 single caterpillar is roughly proportionate to its size. 



Such species are however decidedly in the minority, and within rea- 

 sonable bounds, the size of an insect is a limiting factor in the deter- 

 mination of its parasites. This does not necessitate that all of its 

 parasites be of uniform size, since the number of eggs laid in a single 

 tost usually determines the number of emerging parasites. For ex- 

 ample, there may issue from a parasitized cocoon of the common Cecro- 

 pia moth, but a single specimen of the large Ichneumon-fly, Eremoty- 

 lus macrurus, while dozens of specimens of the small Cryptus extreme- 

 tatis regularly issue from a single cocoon of the same moth. This sort 

 of adjustment is almost universal, and most insects which have been 



