88 MOTHS AND BUTTERFLIES. 
closely watch a colony of the tent caterpillars when out of their nest 
or a large number of larve of any other kind, he may see a small 
swarm of ichneumon flies constantly hovering about and over them. 
These are on the watch for a chance to deposit their eggs in or on the 
bodies of the caterpillars ; and when one considers their numbers and 
persistency, one wonders that a sufficient number of the larvae escape 
to propagate the species. Here is an instance which gives some idea 
of their ravages. One of my brothers had one hundred caterpillars 
of a large species he wished to raise which he placed on a young tree 
and enclosed the whole in mosquito netting. Under this net they 
remained until nearly full grown, when one day a small rent was dis- 
covered in one corner of the net presumably made by a bird; and 
when the larvie were taken out and examined, of the entire number 
but three had escaped the ichneuman flies which had found their 
way into the net and laid their eggs on the caterpillars. 
While a species is comparatively scarce, its chances of escape 
from birds and parasitic insects are fairly good; and if it is a prolific 
Clisiocampa disstria. 
species well adapted to its surroundings and has plenty of food, it 
will increase until its posterity may be reckoned by millions. It now 
begins to attract attention, both from the birds and the parasites, as 
a large amount of food going to waste. The parasites attack it, and 
having an abundance of food begin to increase at a rapid rate; and 
the birds finding an easily obtained food-supply neglect other foods 
and apply themselves to this. The consequence is that the demand 
soon equals and finally exceeds the supply, and an entire region 
previously infested with an injurious insect may be so depopulated 
in one or two seasons thata careless observer might conclude that the 
Species was extinct or had migrated to other parts. This, in my 
opinion, is the explanation of the sudden disappearance of many an 
