DISEASES OF INSECTS. 221 
their egg state, and could not be brought to perfection 
by the food that a single one would furnish 3 . 
The Ichneumons that are parasitic upon larva are 
the most numerous of all. Some of them are deposited 
by the parent fly on the outside of their prey, and others 
introduced into its interior. Ophion lutcum is one of 
the former tribe; it plants its eggs in the skin of the 
caterpillar of the puss-moth (Cerura Vinuld). Each 
egg is furnished with a footstalk terminating in a bulb b , 
which is so deeply and firmly fixed that it is impossible 
to extract it without detaching a portion of the animal 
with it, and even when the caterpillar changes its skin it 
is not displaced. After it is hatched, the grub, while 
feeding, keeps its posterior extremity in the egg-shell, 
to which it adheres so pertinaciously, that it is scarcely 
possible to disengage it without crushing it. It fixes 
itself by its mandibles to the skin of the caterpillar, and 
keeps constantly sucking the contents of its body till it 
dies : sometimes nine or ten of these larvae inhabit a 
single caterpillar . Reaumur has given an account of 
other external Ichneumons. Upon one caterpillar that 
he examined, they were so numerous as to render the 
poor animal quite a spectacle, and they underwent their 
metamorphosis attached to it d . One species of this de- 
scription avenges the cause of insects upon their most 
pitiless foes, the all-devouring spider — for in the midst 
of her toils and lines of circumvallation it makes her its 
prey. De Geer, meeting one day with a young spider 
of a common kind, observed with surprise, engaged in 
sucking it, a small white grub, which was firmly attached 
to the abdomen near the trunk. Putting it by in a 
■ Linn. Trans, v. 102—. ■ " Plate XX. Fig. 22. a. 
' De Goer ii. 850—. " Reaura. ii. 444—, 
