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the stomach with food in which the flies have deposited their eggs or 

 larvae and which has not been prepared for consumption by judicious 

 cooliing, or carefully examined so as to exclude infested portions. 



We may class as true parasites nine families, some of which, as 

 Tachinidae [setts, lat.), Dexiidae, and Pipunculidae, are highly bene- 

 ficial, and others, as Gastrophilidae, Hippoboscidae, and Oestridae, are 

 distinctly injurious. The parasites of this order destroy many injuri- 

 ous species of insects, and, next to the parasitic Hymenoptera, con- 

 stitute the most important check upon their increase. 



Another group of highly beneficial species is that containing the 

 predaceous forms. Two of the families which are to some extent 

 beneficial in the larval stage — Tabanidae, and Culicidae in part — are 

 injurious as imagines, turning their attention from insect larvae, on 

 which they chiefly prey in the early stage, and giving it largely to 

 mammals, including man. This radical change of habit is, however, 

 exceptional, as other predaceous families in this and other orders feed 

 upon insects in both the larval and imaginal stages. Many Syrphidae 

 are aphidophagous as larvae, the greater portion of the species being 

 scavengers, while the imagines are flower-frequenters. 



The aquatic families, with the exception of the Sciomyzidae and 

 Ephydridae, which are in large part aquatic, belong to the Orthorrha- 

 pha. With the exception of the Mycetophiloidea, which contains five 

 families, the Oligoneura, which contains the Cecidomyiidae, and the 

 families Bibionidae and Scatopsidae, all the families in the division 

 Nematocera are aquatic either wholly or in large part. The aquatic 

 species in the division Brachycera are contained in five families — 

 Leptidae, Stratiomyiidae, Tabanidae, Empididae, and Dolichopodidae. 

 As already 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 intended for drinking pur- 

 poses is undesirable it is not necessarily harmful unless the vessel con- 

 taining them is small and they are numerous enough to foul the water, 

 either with excreta or exuvia. With the exception of some Chiro- 

 nomidae 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 general 

 leads me to the conclusion that as a whole their beneficial and injuri- 

 ous activities practically offset each other. The fact that there are 

 injurious species which cause great recognized damage, such as the 

 malarial and other disease-bearing mosquitoes and the Hessian fly, 

 very largely outweighs in the mind of the uninformed the benefits— 



