THE FLY AND ITS ALLIES 73 



was imported in the straw bedding of the troop-ships which 

 brought over the Hessian mercenaries in 1775, has spread, 

 within a century, over the eastern half of the United States, 

 and has at various times injured the wheat crop to the 

 value of millions of dollars. The larvae of certain Muscidse, 

 especially the genus Chlorops, 1 attack, in Europe, the 

 stems, leaves, and ears of wheat, rye, and barley, and 

 cause in some years great destruction, especially in Scan- 

 dinavia. 



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 Diptera, Hymenoptera, and certain beetles, but they 

 are not careful to choose alone injurious species. The 

 larvae of the Syrphidse prey on the injurious plant-lice; 

 those of horse-flies are carnivorous, and feed on insect larvse. 

 The larvse of certain small flies are internal parasites of 

 bugs, beetles, and other (mostly injurious) insects ; while 

 those of various other Diptera are useful in acting as scav- 

 engers, mosquito larvse, for example, serve in this way. 

 The carrion-fly devours putrid animal substances, which 

 might otherwise be a source of discomfort or disease. The 

 insects which feed on decaying vegetable matter are also 

 not to-be despised. Finally, the larvse of some crane-flies, 

 robber-flies, and ' Syrphidse, by boring into rotten wood, 

 help in the work of forming forest mould. The Diptera, 

 like almost every other insect group, has its economically 

 injurious and beneficial species, and it is impossible to say 

 whether mankind would be better or worse off were the 

 group to be exterminated. 



green ; oty, eye. 



