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 luteum is one of 

 the former tribe ; it plants its eggs in the skin of the 

 caterpillar of the puss-moth (Centra Vinula). 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 Iarva3 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 



a Linn. Trans, v. 102. b PLATE XX. FIG. 22. a. 



c De Geer ii. 850. d Reanm. ii. 444. 



