294 DISEASES OF INSECTS. 
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 Musca Larvarum L.; 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 cater- 
pillar thus attacked lives long enough to spin its cocoon, 
when it dies*. Sometimes, however, these animals quit 
their prey sooner. Reaumur saw a grub of one of the 
Muscide come out of a caterpillar, and then become a 
pupa, which was so large that he wondered.how it could 
have been contained in the animal it had quitted>. __ 
We have now done with those parasites that produce 
in insects the disease I have called Scolechiasis*: the 
rest, which belong to the Aptera Order, will afford us 
examples both of Phthiriasis and Acariasis‘. 
I begin with the frst. Mr. Sheppard once brought 
me a specimen of a bird-louse (Nirmus) which he took 
upon a butterfly (Vanessa Jo): and should such a capture 
be more than once repeated, it would afford a certain 
instance of the first of these diseases amongst insects;—but 
* Reaum. ii. 443. De Geer i. 196—, 550—. vi. 24, 
> Reaum. ii. 440—. ON Gibal Ss Ve WER d Tbid. 84, 97. 
