270 INDIRECT BENEFITS DERIVED FROM INSECTS. 



the great majority of these animals oviposit within the 

 body of the insect to which tliey are assigned, from 

 whence, after having consumed the interior and become 

 pupae, they emerge in their perfect state. An idea of 

 the services rendered to us by those Ichneumons which 

 prey upon noxious larvae may be formed from the fact, 

 that out of thirty individuals of the common cabbage 

 caterpillar (the larvae o^ Po7itia Brassiccc) which Reau- 

 mur put into a glass to feed, twenty-five were fatally 

 piei'ced by an Ichneumon [Microgaster glohatus^). And 

 if we compare the myriads of caterpillars that often attack 

 our cabbages and brocoli with the small number of but- 

 terflies of this species which usually appear, we may con- 

 jecture that they are commonly destroyed in some such 

 proportion — a circumstance that will lead us thankfully 

 to acknowledge the goodness of Providence, which by 

 providing such a check has prevented the utter destruc- 

 tion of the Brassica genus, including some of our most 

 esteemed and useful vegetables. 



The parasites are not wholly confined to the order 

 Hymenoptera : some insects of other orders, though com- 

 paratively very few, destroy our little enemies in the 

 same way. TacJiina Larvarnm, and another like it de- 

 scribed by De Geer, lay their eggs in caterpillars and 

 other larvas'' ; and Reaumur describes several other flies 

 of similar habits '^. The order also of Strepsiptera, lately 

 established'', appears to be altogether parasitic; but 

 with this difference from the Fupivora^ that these ex- 

 traordinary animals are found only upon Hymenoptera 

 in their perfect state, and do not appear to destroy the 

 insects upon which they prey, but probably prevent their 



' Rcamii. ii. 419. " DeGeer, i. 190. vi. 14. 24, 



• Reaum. ii. 440-4. '' Lhu\. Tram. xi. 80. 



