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 « 
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 Varicheta aldrichi, through its habit of depositing living larve 
upon the food plant instead of depositing eggs or larve upon the 
caterpillars, 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 larve, 
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 Varicheta, 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 larve 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 
