56 



BIRDS L\ THEIR RELATIONS TO MAN. 



SI'HINX J.AKVA WITH COCUCJNS OF J'ARA.SITK 



ADULT PAKASITK AT RIGHT, 



Natural size ami inu,i:;nifie(l. (After Piley.) 



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 llir 

 body of the larva selected 

 as the Aictim : they soon 

 hatch into grubs that de- 

 velop at the expense of the 

 (issues of llir liosl. Some of the ichneumon-fly larv.-B are 

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

 others aHacli tlieiiiselves externally. In either case the host 

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

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

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

 fox in the fal3le — will gnaw away its vitals. When the ichneu- 

 mon larvie become fully grown, they generally spin slight silken 

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

 as adult Hies. 



There is a group of ichneumon-flies, commoidy called 

 Microgasl ers, wliidi spin their cocoons on the back and sides 

 of the lar\;e of butterllics and inoths, giving the host a most 

 singular appearance. A specimen of a connnon sphinx larva 

 bearing these cocoons is represented above. 



Ichneumon-flies are eaten to a considerable extent by many 

 birds, especially the llycatcliers. The tact that a bird may 

 eat a certain lunnber 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 

 ('hapter VI. 



Besides the insects proper there are many animals that are 

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



