56 OUR SHADE TREES AND THEIR INSECT DEFOLIATORS. 
ber of them. But the fly now to be described is fully as useful as any 
of the others. 
“Tachina-flies are very easily overlooked, because they resemble 
large house-flies, both in appearance and in flight, and their presence 
out of doors is not usually noticed on that account. Yet they play a 
very important role, living as they do in their larval state entirely 
in insects. During the caterpillar plague such flies were often seen 
to dart repeatedly at an intended victim, buzz about it, and quickly 
disappear. If the caterpillar thus attacked was investigated, from 
one to four yellowish-white, ovoid, polished, and tough eggs would be 
found, usually fastened upon its neck, or some spot where they could 
not readily be reached. These eggs are glued so tightly to the skin of 
the caterpillar that they can not easily beremoved. Sometimes as many 
as seven eggs could be counted upon a single caterpillar, showing a 
faulty instinct of the fly or flies, because the victim is not large enough 
to furnish food for so many voracious maggots. If the victim happens 
to be near a molt, it casts its skin with the eggs and escapes a slow but 
sure death. But usually the eggs hatch so soon that the small maggots 
have time to enter the body of the caterpillar, where they soon reach 
their full growth, after which they force their way through the skin 
and drop to the ground, into which they enter to shrink into a brown, 
tun-like object (known technically as the coarctate pupa), which con- 
tains the true pupa. The caterpillar, tormented by enemies feeding 
within it, stops feeding and wanders about for a long time until it dies. 
As arule not more than two maggots of this fly mature in their host, and 
generally but one. The caterpillar attacked by a Tachina-fly is always 
either fully grown or nearly so. 
‘‘Tachina-flies abounded during the whole term of the prevalence of 
the caterpillars. But it is impossible to state positively whether they 
were all bred from them or not, since the 
many species of this genus of flies resemble 
each other so closely that a very scrutiniz- 
ing investigation would have been necessary 
to settle such a question. But there is no 
doubt that they were very numerous during 
the summer. Some maggots obtained from 
caterpillars kept for this purpose in breeding 
| jars changed to the fly in six days; others 
Fic. 26,—A Tachina-fly. appeared in twenty-three days, and still 
others, obtained at about the same time, are 
still under ground, where they will hibernate. The maggots of tiese 
flies do not, however, always enter the ground, as some were found in- 
side cocoons made by caterpillars among rubbish above ground.” 
We have found, moreover, that three of these primary parasites of 
the Web-worm, viz, the Apanteles, the Limneria, and the Meteorus, were 
killed off at a serious rate late in the season by secondary parasites, 
