INSECTS. 133 



host by bringing the tip of its abdomen (where the 

 main stem of the air-tube system opens) into con- 

 nection with one of the host's spiracles. Since the 

 larva feeds altogether upon perfectly digestible sub- 

 stances an anus is superfluous and is absent. Many 

 ichneumon larvse are ready to become pupse when 

 the host is about to pass into the same condition; 

 the larva then bores through the skin of the latter, 

 which quickly dies. Other species do not attack the 

 organs of the host so soon, but allow it to become a 



Fig. 90. — The Yellow-legged Ichneumon 'Fly (^Afic^'ogaster glomeratus) of the Cabbage 

 Caterpillar. Left, the adult insect ; right, the larva (both enlarged). In the 

 middk, Cabbage Caterpillar and a heap of Microgaster pup^e. 



pupa in peace, and then themselves become pupge 

 inside it; later on, one or several ichneumon flies 

 come out of this pupa instead of a moth or butterfly. 

 It is obvious that ichneumon flies are very serviceable 

 by destroying a large number of harmful insects. 

 They cannot, indeed, prevent the increase of any 

 particular noxious insect, but, when this takes place, 

 they themselves increase to a greater extent, and 

 finally appear in such numbers as to make an end 

 of the pest. 



I shall not enumerate here all the various ichneumon 

 flies which benefit agriculture, but simply mention 

 the Small Cabbage Caterpillar Ichneumon Fly (Micro- 

 gaster glomeratus), depicted in Fig. 90, which, like 

 its host (the large cabbage white), appears in two 

 generations. The larvae of the ichneumon fly live in 

 large numbers in the large cabbage caterpillar, and at 

 their last moult acquire minute teeth, with which 

 they bite through the skin of their host ; they then 



