150 FIELD ZOOLOGY. 



and all four -wings are provided with comparatively few- 

 branched veins. 



You may have found caterpillars with their bodies 

 quite thickly set with tiny white silken cocoons. These 

 cocoons mark the third life stage in the life of some 

 parasitic hymenopter, usually one of the ichneumon flies, 

 which laid her eggs on the body of this caterpillar, and 

 the greedy larvas hatching from the eggs, bore through the 

 skin of the host, and make many a meal off the protesting 

 caterpillar, which finds it difficult to eat enough for itself 

 and for all its uninvited guests also. The caterpillar 

 may still be crawing about, feebly trying to find a juicy 

 leaf and perhaps wondering why it feels so queer inside. 

 But more likely it is just dead or dying, though some of 

 them do manage to pupate in the face of such enormous 

 odds. The mother ichneumon usually lays her eggs at or 

 near the end of the caterpillar stage, and the fly has an 

 unusually short period of immaturity. 



The effectiveness of these parasites as aids to man, as 

 well as of all parasites in general, depends upon their not 

 being parasitized themselves, and also upon their not 

 becoming so numerous as to eat up all the available cater- 

 pillars, or other hosts, within reach. If the ichneumon 

 should do the latter, the succeeding season would witness 

 a decided decrease in its own numbers, while the few 

 remaining caterpillars would have a chance to increase. 



Another of the ichneumon flies, the big Thalessa, is 

 an interesting as well as a decidedly beneficial insect. The 

 Pigeon Horntail is accustomed to boring into trunks of 

 box elder, maple, or sycamore to lay its eggs. The larva 

 hatching from the egg, eats or tunnels its way into the 

 tree through the bark, and then turns downward along the 

 inner wood of the tree, eating as it goes. It then pupates. 



