256 
PARASITES OF GIPSY AND BROWN -TAIL MOTHS. 
THE EG(x PARASITES OF THE BROWN-TAIL MOTH. 
THE GENUS TRICHOGRAMMA. 
The parasites belonging to the genus Trichogramma, of which sev- 
eral have been reared from eggs of the brown-tail moth, are* the most 
minute of any which have been handled at the laboratory, and are 
among the smallest of insects. The egg of the brown-tail moth is in 
form of a flattened spheroid, approximately as large in its greatest 
diameter as the printed period which ends this sentence. Normally 
two or three individuals of the parasite pass through all of their 
transformations from egg to adult upon the substance of a single 
host egg, and in exceptional instances as many as 10 perfect adults 
are known to have issued from one egg. This is the more remarkable 
when it is remembered that the female Trichogramma is sexually 
mature at the time of issuance, and ready to 
deposit a large number of eggs for a new 
generation. 
The mother parasite exhibits little discretion 
in the selection of host eggs for attack (fig. 57), 
and if any dependence is to be placed upon 
observations which have been made in the 
laboratory, she is quite as likely to select eggs 
which contain caterpillars nearly ready to 
hatch as those which are freshly deposited. 
The feeding habits of her young are such as to 
permit a considerable latitude in tins respect, 
but there is a certain limit, and after the em- 
bryological development has passed beyond a 
certain point in the host egg, the attack by the 
parasite is unsuccessful. It is much better 
that the host egg be dead than that it contain a living embryo in the 
later stages of its development. 
The life cycle, from egg to adult, varies very considerably in 
length in accordance with the prevailing temperature. In the summer 
it may be completed in as short a period as nine days, while in the 
fall three weeks or more may be required. If the temperature falls 
below certain limits the young parasites will hibernate or attempt 
to hibernate, and thereafter their development may be delayed for 
several weeks, or even months, even though they are exposed to 
continuous high temperature during this period. 
After about one-third of the time requisite for the completion of 
the life cycle has elapsed, the eggs begin to turn dark, and finally 
become -Inning, lustrous black (fig. 58). This change is brought 
about by the preparation of .the larva? for pupation. 
Fig. 57. — Trichogramma sp. in 
act of oviposition in an egg of 
the brown-tail moth. Great- 
ly enlarged. (Original.) 
