126 



Z00L0O7. 



the egg &tate, from five days to a week as a maggot, and 

 from five to seven days in the pnpa state. It breeds 

 about stables. 



The Tacl)iiia-fly is beneficial to man, from its parasitism 



in the bodies of caterpillars and other injurious insects. 



The bot-fly (Fig. 160, Hypoderma 



hovis) is closely allied to the house-fly, 



but the maggot is much larger. The 



larval bot-fly of the horse lives in the 



stomach; that of the sheep in the 



frontal sinus, a cavity in the forehead. 



The Syrphus flies (Fig. 161, Syr- 



phus politjts) mimic wasps; their mag- 



FiG. \6\.— Syrphus potitus gots are most useful in devouring 



'*^' aphides. 



The fleas are wingless flies, allied to winged forms which 



are intermediate between the house flies and crane-flies. 



In the two-winged gall-flies (C'ecidomyia, etc., Fig. 163, 

 C. tritici, Hessian-fly) the body is small and slender, with 

 long antennae. The cvaue-flies {Tipula) are large flies, 

 standing near the head of the order, and, like the flea and 



Fia. 162.— Hessian-fly. a, larva; ft, pupa; c, incision in wheat-stalk for larva. 



gall-fly, the chrysalis is enclosed in a cocoon, there being 

 no pupnrium or pupa-case, as in the lower flies. Lastly, 

 we have the mos(|uito (Figs. 103, 164), whose larva is 

 aquatic, and breathes by a process on the end of the body, 

 containing an air-tube. 



