1924] . The Biology of Trichopoda pennipes Fob. 15 
early August. When these pupae yield adult flies during August 
and the first part of September, many young squash bugs have 
become adult, and more have reached the fourth and fifth 
instars. Upon these bugs the parasite lays its eggs, and it is in 
this generation that the work of the fly is seen to be effective 
beyond question, since no parasitized bug appears to live to 
sexual maturity. Many nymphs (counts have shown about 
50 per cent) die before becoming adult, and those which reach 
the adult state and pass the winter safely in hibernation, are 
subsequently killed by the parasite before laying eggs. 
No parasites appear to reach maturity in nymphs which 
die. In this regard the egg-laying flies seem unable to discriminate 
closely between nymphs which are sufficiently developed to 
support the parasite maggots and those which contain too little 
substance. However, the flies seem to realize that their progeny 
will find too little nourishment in nymphs younger than the 
fourth instar, since third instar nymphs were rarely molested in 
the cages, and parasitized third instar nymphs were not col- 
lected in the field. 
It was interesting to note that occasionally a nymph would 
escape parasitism by molting, leaving an unhatched parasite 
egg on the molted skin. This does not account for any great 
loss, however, since the egg of the parasite hatches in thirty 
hours, while the fourth and fifth instars of the squash bug re- 
quire about six days and sixteen days respectively for their 
completion. 
But one parasite has ever been observed to issue from one 
host. Several maggots have been observed to enter one host, 
but this has always seemed to result in the early death of the 
host and of the parasites within. 
Collections of squash bugs in the fall give no indication of 
the true percentage of parasitism, since many nymphs die, 
and others slough off the empty egg-shells of the parasite with 
their molted nymphal skins. In midsummer, however, collections 
of overwintered adult bugs have indicated a parasitism as high 
as 80 per cent. There is no reason to suppose that the percentage 
of parasitism may not often run as high among the older nymphs 
