322 AGRICULTURE. 



flies,Chalcis Hies, and braconids. These generally 

 lay their eggs in or on the body of the larva of 

 other insects, but sometimes they deposit them 

 in the adult, the pupa (Fig. 1 14), or even the egg. 

 When th~ eggs hatch, the larva; feed upon the 

 substance of the host, thus destroying it, to- 

 gether with all of its posterity, which in a few 

 years might have been countless. 



One e;enus of the ichneumon-flies which is 



o 



often mistaken for an enemy of plants is the 

 thalessa, a yellow or black (according to the 

 species) insect, with a long, slender, though 

 powerful, ovipositor, with which it pierces into 

 the wood of a tree. It will be found upon ex- 

 amination, however ; that the tree is infested 

 with borers (Fig. 122), and that what the ichneu- 

 mon really does is to deposit its eggs in the 

 nest of the borer, where the larva, when it 

 hatches, fixes itself to the body of the borer, liv- 

 ing upon its juices and gradually killing it. 



The many species of Chalcis flies, as well as 

 the ichneumon, are parasitic upon a great num- 

 ber of different insects, one species feeding 

 upon the chrysalis of the cabbage-butterfly. * 



(2) KXKRCISK 14. (a) As many kinds of these insects 

 as can be obtained should be placed in the breeding-jars 

 and watched through their development. 



(/>) Experiment with the food of these insects in dif- 

 ferent stages of their development to ascertain in what 

 stage they are predaceous and what insects they will eat. 



* The larvae of Syrphus-flies do much good by destroying scale 

 insects and aphides, in whose colonies they live. 



