372 



GENERAL PRINCIPLES 



the Diptera and Hemiptera. The larvae of the Diptera are often 

 parasites. The botfly, Hypoderma, develops under the skin of 

 cattle. The botfly of the horse (Gastrophilus) deposits its eggs 

 about the shoulders and head of the horse. The horse gnaws 

 them off or they fall into his food, and thus get into the stomach, 

 where the larvae remain attached to the wall of the stomach. 

 762. The botfly larvae of Cephalomyia ovis in the frontal 

 sinuses of sheep produce bhnd-staggers. The larvae of the 



Fig. 235. — Ichneumon fly, Thalessa lunator, depositing eggs in the burrow of 

 the wood-boring Tremex upon whose larvae the larvae of the Thalessa feed. 



ichneumon fly, a Hymenopter, are parasitic in the cater- 

 pillars of various butterflies. The adults of a large number of 

 flies are temporarily external parasites, as are also many mos- 

 quitoes. The flea is also closely related to the flies, and its 

 wingless condition is probably the result of degeneration 

 through parasitism. 



763. The Hemiptera, or bugs, are, as a group, parasitic. 

 They are often evil smelHng because of the secretion of a peculiar 

 gland. The bed bug and squash-bugs exemplify this point well. 

 The plant lice, scale insects, the water striders, water boatmen 



