56 BIRDS IN THEIR RELATIONS TO MAN. 



adults are four-winged flies with slender bodies and long 

 antennae, and the larvae are soft, fleshy grubs. In many 

 species the females have long egg-depositors, by which they 



can reach caterpillars hid- 

 den in trunks of trees or 

 stems of herbaceous plants. 

 The eggs are usually de- 

 posited either on or in the 

 body of the larva selected 



SPHINX LARVA WITH COCOONS OP PARASITE ; aS tllC Victim I they SOO11 



ADULT PAEASITE AT EIOHT - hatch into grabs that de- 



Natural size and magnified. (After Riley.) 



velop at the expense of the 



tissues of the host. Some of the ichneumon-fly larvae are 

 internal parasites, living beneath the caterpillar's skin, while 

 others attach themselves externally. In either case the host 

 insect is doomed : it may be killed long before it gets its full 

 larval growth, or it may be allowed to complete that growth 

 and spin a cocoon, but sooner or later the parasites like the 

 fox in the fable will gnaw away its vitals. When the ichneu- 

 mon larvae become fully grown, they generally spin slight silken 

 cocoons, within which they change to pupae, to emerge later 

 as adult flies. 



There is a group of ichneumon-flies, commonly called 

 Microgasters, which spin their cocoons on the back and sides 

 of the larvae of butterflies and moths, giving the host a most 

 singular appearance. A specimen of a common sphinx larva 

 bearing these cocoons is represented above. 



Ichneumon-flies are eaten to a considerable extent by many 

 birds, especially the flycatchers. The fact that a bird may 

 eat a certain number of insects of this sort without necessa- 

 rily doing any injury to agriculture is indicated in the discussion 

 of the relations of vegetivorous and carnivorous insects in 

 Chapter VI. 



Besides the insects proper there are many animals that are 

 eaten by birds. The spiders are the most important of these. 



