TACHINID PARASITES OF THE GIPSY MOTH. 203 



has been proved to be of preeminent importance abroad have 

 grown less bright, the tachinid parasites have gained in the favor 

 accorded to them, and from being considered as of secondary impor- 

 tance they have become of primary importance. 



This change in attitude toward them would have come about in 

 another way, even though it had not been forced through the compar- 

 ative failure of the hymenopterous parasites to make good as yet. In 

 nearly every instance in which the parasites of a native defoliating 

 caterpillar have been studied, the tachinids have been found to play a 

 part which was at least the equivalent of the part taken by the 

 Hymenoptera, while in more than half the instances the tachinids 

 have displayed superior efficiency. This is probably not true of 

 the parasites of any other order than the Lepidoptera, and of only a 

 portion of the larger representatives of that order. 



For the most part the tachinids are restricted in their choice of 

 hosts through purely physiological limitations, but to a material 

 extent they are restricted through purely physical causes as well. 

 The fall webworm offers a striking example of both. Literally 

 thousands of tachinid parasites have been reared from it in the 

 course of the past few years, and with the exception of an insignificant 

 number of the imported Compsilura concinnata, only a single species 

 has been found amongst them all. This species, at present known 

 as Varichseta aldriclii, through its habit of depositing living larvae 

 upon the food plant instead of depositing eggs or larvae upon the 

 caterpiUars, possesses a very distinct and powerful advantage in its 

 attack upon this particular host. If the leaves or stems near a colony 

 of young caterpillars are selected for larviposition, it is practically a 

 certainty that the caterpillars will enlarge their nest to include 

 these leaves; will thereby come in contact with the parasite larvae, 

 and thus complete the chain of circumstances through which para- 

 sitism comes about. A parasite having a similar habit would stand 

 an infinitesimal show of providing for the future of its young if the 

 webworm should suddenly change from a gregarious and nest 

 building to a solitary and wandering insect. At the same time that 

 the host escaped attack by Varichaeta, it would lay itself open to 

 attack by a variety of other species which are now only prevented 

 from attacking it on account of the protection which its web affords. 



But even though it were freely exposed to attack by all the species 

 of tachinids which deposit eggs or larvae directly upon or in their host, 

 it would be immune to such attack by all but a small percentage of 

 the species which might conceivably select it as a host. This is 

 proved through the occasional occurrence of caterpillars bearing 

 tachinid eggs, but with no evidences of internal parasitism showing on 

 dissection. It is not merely necessary that the host be exposed to 

 attack and acceptable to the instincts of the mother parasite; it is 



