230 DISEASES or INSECTS. 
place with respect to the whole inless than half an hour. 
When entirely disengaged, they place themselves close 
to the sides of the caterpillar : even before this they be- 
gin spinning, and draw unequal threads in different di- 
rections, of which they form a cottony bed, which serves 
as the base of the separate cocoon of each individual, 
which they next construct of a beautiful silk thread of a 
lovely yellow, which, if it could be unwound and in suffi- 
cient quantity, would yield a silk unrivalled in lustre and 
fineness a . 
De Geer has recorded a very singular fact which de- 
serves your notice. An Ichneumon, appropriated to one 
of the Tortrices, had deposited its eggs in two of their 
caterpillars ; each produced a considerable number ; but 
those that emerged from one were all females, and those 
from the other, ?nales b . He observed a similar fact take 
place with Misocampus Puparum c . One might conjecture 
from this circumstance, that as in the queen-bee d , so in 
these Ichneumons, the eggs producing the two sexes 
were arranged separately in the ovaries. Reaumur has 
related, that in one instance three or four males were 
produced to one female ; and in another four or five fe- 
males to one male e . 
But though the great majority of insects are subject to 
this Scolechiasis f in their larva state, yet sometimes they 
are not attacked by the Ichneumon till they have become 
pupa. Of this kind is one just mentioned (M. Puparum\ 
which commits its eggs to the chrysalis of the butterfly 
of the nettle, Vanessa Urticae : the moment this cater- 
1 Reaum ii. 41!)—. " De Geer i. 583—. 
c lhid. ii. 884. A See above, p. 164. 
r Reaum. vi. 312. ' Vol. I. p. 99. 
