220 



NATURAL HISTORY. 



The ichneumon-flies are to be classed as beneficial forms ; for they sare 

 true parasites, passing their larval lives inside the bodies of other insects, 



and in this way they destroy a host 

 of forms which otherwise might cause 

 great damage to the farmer. Of these 

 ichneumon-flies there are many thou- 

 sand species, a few large, but the great 

 majority small. In all the habits are 

 much the same, and a description of 

 one will answer for all, with the prefa- 

 tory remarks that there is scarcely a 

 species of insects which is free from 

 their attacks, and that some affect the 

 eggs, while others begin with the larva? 

 or even the adult hosts. In some the 

 eggs are placed within the host, in 

 others they are placed on the larva;, 

 and then the grubs which hatch out 

 bore their way through the skin, where 

 they begin to feed on the tissues. 

 There is a wonderful relation between 

 these parasites and their hosts. .Both 

 grow, but the parasite is careful not to attack any vital part. It is nec- 

 essary for his existence that the larva on which he feeds should continue 



alive so as to furnish him food, and hence it is only 

 when the parasites emerge that the host dies. Our 

 figure shows one of the adult ichneumon-flies greatly 



enlarged, and by its side a cater- 

 pillar from which the ichneumon 

 larva? are emerging. It is no 

 uncommon sight with the collec- 

 tor to find caterpillars thus cov- 

 ered, and the only wonder is that 

 the host can so long endure with 

 so many parasites feeding upon 



Fig. 220. — A caterpillar covered with the larvte of an its tissues. 

 ichneumon-fly (Microgaster) which have hatched from .-. -,- n • 



c--s laid by the perfect insect, represented at the right. Oil a preceding page allUSlOD 



was made to a balance of nature, 

 and this is well exemplified in the case of the ichneumon-flies. When any 

 insect appears, like the army-worm or the Rocky Mountain locust, in vast 

 swarms, there soon appears a corresponding number of these parasitic 



Fig. 219. — Ichneumon-fly (Macrocentrus) ■ 



