DIFFER A. 



37 



tervening space with hay not crammed too tight ; it will act as 

 a spring and reduce the effect of shaking ; the whole parceL 

 should be made thoroughly secure against damp." 



Fig. 31. 



Fig. 32. 



DIPTERA. 



FLIES, MOSQUITOES, BOT FLIES, ETC. 



There are about 2,500 species of North American flies de- 

 scribed, and it is probable that the number of living North 



American species amounts to 10,000. 



In Europe there are also about 10,000 



known species, be- 

 longing to about 



680 genera. 



The flies of this 



country, compared 



with the other 



groups, have been 



but little studied, though the habits of 

 many are so interesting and the species 

 very numerous. The different parts of 



Syrplms Fly. 



the body vary much more than in the Hymenoptera and Lepi- 

 doptera, and in such a degree as often to afford comparatively 

 easy characters for discriminating the genera. 



Their habits are very variable. Fresh water aquaria are 



Fig. 33. 



Fig. 34. 



Tachina Fly. 



Bot Fly. 



necessary for the maintenance of aquatic larvae. If quanti- 

 ties of swamp mud and moss with decaying matter are kept in 



