INSECT PARASITES 



123 



nified in Fig. 50 a. It looks much like a common house-fly, 

 and is of about the same size. This fly deposits a whitish egg 

 upon the skin of the caterpillar, generally after it is more*than 

 half-grown. The egg soon develops into a tiny grub that bur- 

 rows through the egg-shell 

 and the skin of the cater- 

 pillar into the inside of the 

 body. There it remains, 

 absorbing the tissues of its 

 host, and gradually increas- b 



ing in size. In due time it P'°- 50-— Tachinid Parasite of Forest Tent 

 . r 11 1 1 1 • Caterpillar: li, fly; i, pnparium. 



becomes iully developed in 



this grub state, and breaks through the skin of the caterpillar. 

 It is then a good-sized, white, oval, footless grub. The cater- 

 pillar has generally spun its cocoon before the parasite comes 

 from its body, so that the parasite finds itself inside the cocoon 

 when it gets outside the caterpillar. The grubs of two species 

 of these Tachinid parasites work their way through the cocoon 

 and drop to the ground, while the other seems to remain in the 

 cocoon. 



The parasitic larvae have now to enter upon the quiet pupa 

 stage. For this purpose the outer skin turns brown, and 

 becomes hard, forming a protective covering for the insect 

 inside. This is called the puparmm (Fig. 50 i). Within this 

 covering the insect changes to a pupa, to change again about 

 two weeks later into an adult fly. 



These parasites were very abundant last season, and are, 

 doubtless, one of the most efficient checks upon the caterpil- 

 lars. In some localities they seem to have nearly brought the 

 outbreak to an end, and it is probable that they will do still 

 more good this year. A correspondent at Campton, N. H., 

 writes that in June great numbers of strange flies were seen in 

 the woods and pastures. They were probably these tachinids. 



The Ichneumon flies of the genus Pimpla^also did good ser- 

 vice in destroying the caterpillars. The adult is a four-winged 

 fly that deposits eggs in the caterpillars or newly-formed chrysa- 



1 /■/)«//« cmguisi/or vias the most abundant species; a (evi P. pedalis were also 

 reared. 



