DISEASES OF INSECTS. 231 
pillar quits its skin to assume that state, while it is yet 
soft they pierce it and confide to it their eggs a . De Geer 
and others have supposed that this same Ichneumon at- 
tacks the Cocci and Coccinellce b ; but this probably is an 
erroneous supposition. Cryptus Compunctor also at- 
tacks the pupae of butterflies c . 
If we consider the great purpose of Providence in 
giving being to this tribe of destroyers — the keeping of 
insects within their proper limits, — we may readily con- 
ceive that this purpose is more effectually answered by 
destroying them in their preparatory than in their ulti- 
mate state, since at that time the laying of their eggs and 
a future progeny could not so effectually be prevented ; — 
this will account for there being few or no Ichneumons 
appropriated to them in their latter state. 
The next tribe of insect parasites are to be found in 
the Diptera Order. The species that has been particu- 
larly noticed as such is the Tachina Larvarum ; its larva 
is polyphagous, laying its eggs upon the bodies of cater- 
pillars of different kinds. Sometimes a pair is placed on 
the first segment, sometimes on the head itself, and some- 
times near the anus. These eggs are very hard, convex, 
of an oval figure, polished and shining like a mirror. 
They are fixed so firmly that if you attempt to remove 
them with a penknife the skin comes off with them. 
When hatched, they enter the body and feed on the in- 
terior, and, undergoing their metamorphosis within it, 
do not emerge till they enter their perfect state. The 
caterpillar thus attacked lives long enough to spin its 
cocoon, when it dies d . Sometimes, however, these ani- 
a DeGeer ubi supr. b Ibid. 883. c Linn. Fn. Sua: 160!). 
" Reaum. ii. 443. De Gcer i. 196—, 550—. vi. 24. 
