THE ARTHROPODS 391 



parasitized by larvae of certain flies. The ichneumons 

 (Hymenoptera), of which more than ten thousand species are 

 named, are famous parasites. Some of these insects have 

 ovipositors three and four inches long and are able to bore 

 deeply into trees in order to lay their eggs in the larvse of 

 another insect. " Tomato- worms " (larvae of hawk-moth) and 

 similar moth larvse are often seen with their skins covered 

 with small white cocoons. The 

 parasitic larvae live inside the 

 moth larvae and crawl out to the 

 surface of the skin when ready to 

 pupate. The result in most cases 

 is the death of the parasitized 

 caterpillars. 



The above are simply illustra- FlG - 137 - ichneumon-fly. One 



,. , , .. ,, of a group of insects whose 



tive examples chosen from the eg gs are laid, by means of long 

 thousands of Cases in which in- ovipositors, in the larvae of 



sect parasites destroy other in- 



sects. In all such cases where the insect host is harmful the 



parasite is beneficial. 



Finally, it must be mentioned that insects are useful as 

 foods for numerous species of birds. True it is often stated 

 that the birds are useful as destroyers of insects ; but in- 

 telligent people are beginning to recognize that many birds 

 would be useful and well worth supporting for aesthetic 

 purposes even if there were no injurious insects to be de- 

 stroyed. Aside, then, from the great outbreaks of certain 

 insects, a limited number of them are desirable as food for 

 interesting birds. 



Also, it should be mentioned that many fishes eat large 

 quantities of insects, both adults and larvae. This is why 

 imitation insects are made as bait for trout and other fishes. 



328. Injurious Insects. Those insects which injure or 

 destroy useful plants and animals, or organic products (e.g., 

 foods) which are of value to man, or which injure man him- 



