THE FLY 89 



leaves, and ears of wheat, rye, and barley, and cause in some 

 years great destruction, especially in Scandinavia. 



Over against the injury wrought by the Diptera may be 

 placed certain benefits bestowed by them. In the first rank 

 come certain species which prey upon injurious insects, either 

 in the larval or adult stage. The robber-flies prey on the Dip- 

 tera, Hymenoptera, and certain beetles, but they are not care- 

 ful to choose alone injurious species. The larvae of the Syr- 

 phidiE prey on the injurious plant-hce; those of horse-fiies are 

 carnivorous, and feed on insect larva. The larvae of certain 

 small flies are internal parasites of bugs, beetles, and other 

 (mostly injurious) insects. Finally, the larvae of some crane- 

 flies, robber-flies, and Syrphidae, by boring into rotten wood, 

 help in the work of forming forest mold. The Diptera, like 

 almost every other insect group, has its economically injurious 

 and beneficial species. However, the good that they do is so 

 small in comparison with the destruction that they directly 

 cause to animals and plants by worrying or feeding upon them 

 or by bringing to them various noxious and fatal diseases 

 that, on the whole, the world would be a better place to five 

 in if there were no Diptera. 



