42 



INSECTS INJURIOUS TO STAPLE CROPS. 



beneficial insects, feeding jis tliey do upon sucli pests as 

 the Codling-moth, Web-worms, Plum-cnrciilio grub. Plant- 

 lice, etc. Some of the more common forms of this family 

 belong to the genus Microgaster, and their small white 

 cocoons may frequently be seen almost covering one of 

 our large tomato- or tobacco-worms (see page 237), the 



Fig. 23. — The Long-tailed O^hion {Ophion macrurum). a, adult; 

 h, maggot. (After Riley.) 



pupae of which are often known as "' horn-blowers.^' Many 

 mistake these cocoons for the eggs of the worms, and there- 

 fore destroy some of their best friends. Though some 

 thus spin their cocoons on the outside of the host, others 

 remain inside of the parasitized insect until the adult fly 

 emerges. Thus dead plant-lice may often be found with 

 a large round hole in the abdomen — the only evidence of 



