232 ILLINOIS ACADEMY OF SCIENCE 



idea, which contains five families, the Oligoneura, which con- 

 tains the Cecidomyiidae, and the families Bibionidae and 

 Scatopsidae, all the families in the Nematocera are aquatic 

 either wholly or in large part. The aquatic species in the 

 Brachycera are contained in five families — Leptidae, Stratiom- 

 yiidae, Tabanidae, Empididae, and Dolichopodidae. As al- 

 ready indicated in the foregoing general discussion, the larvae 

 of some of these families are predaceous and may justly be 

 considered beneficial ; the others feed upon algae and decaying 

 vegetable matter, and while their presence in water that is in- 

 tended for drinking purposes is undesirable, it is not neces- 

 sarily harmful unless the vessel containing them is small and 

 they are numerous enough to foul the water, either with ex- 

 creta or exuvia. With the exception of some Chironomidae 

 and Culicidae there are few species that frequent reservoirs or 

 cisterns, most of them preferring lakes, ponds, or streams. 



My information regarding the habits of the order in gen- 

 eral leads me to the conclusion that as a whole their beneficial 

 and injurious activities practically offset each other. The fact 

 that there are injurious species which cause great recognized 

 damage, such as the malarial and other disease-breeding mos- 

 quitoes and the Hessian fly, very largely outweighs in the 

 mind of the uninitiated the benefits — few of which are ap- 

 parent except to a student of the Diptera — that are directly or 

 indirectly due to the presence of other forms. With advance 

 in a knowledge of the biology of the insects of this order will 

 come a realization that their injurious and beneficial effects 

 are practically balanced. 



