ZOOLOGY, ENTOMOLOGY, AND MEDICINE 231 



plants. In reducing the bulk of putrefying substances, which, 

 absorbed by the growing larvae, is transformed into the bodies 

 of the resultant imagines, they remove what is noxious to man. 

 It is chiefly when scavengers such as the common house fly 

 contaminate our food by contact, after feeding on foul sub- 

 stances which are impregnated with disease germs, that there 

 is real danger from these insects. Rarely the screw-worm fly 

 and some of the flesh-flies deposit their eggs or larvae in 

 wounds, either on man or on animals, and in this manner pro- 

 duce serious ulcerations ; and the larvae of the former has been 

 known to cause the death of persons by penetrating the brain, 

 which it entered by way of the nasal passages. The flesh-flies 

 and some other groups sometimes cause myiasis in man, the 

 larvae finding their way into the stomach with food in which 

 the flies have deposited their eggs or larvae and which has not 

 been prepared for consumption by judicious cooking, or care- 

 fully examined so as to exclude infested portions. 



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

 as Tachinidae (sens, lat.), Dexidae, and Pipunculidae, are 

 highly beneficial, and others, as Gastrophilidae, Hippoboscidae, 

 and Oestridae, are distinctly injurious. The parasites of this 

 order destroy many injurious species of insects, and, next to 

 the parasitic Hymenoptera, constitute 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 Culcidae 

 in part — are injurious as imagines, turning their attention 

 from insect larvae, on which they chiefly prey in the earlier 

 stage, and giving it largely to mammals, including man. This 

 ladical change of habit is, however, exceptional, as nearly all 

 other predaceous families in this and other orders feed upon 

 insects in both the larval and imaginal stages. Many Syr- 

 phidae are aphidophagous as larvae, the greater portion of the 

 species being scavengers, while the imagines are flower-fre- 

 quenters. 



The aquatic families, with the exception of the Sciomyzidae 

 and Ephydridae, which are in large part aquatic, belong to 

 the Orthorrhapha. With the exception of the Mycetophilo- 



