126 
ZOOLOGY, 
the egg state, from five days to a week as a maggot, and 
from five to seven days in the pupa state. It breeds 
about stables. 
The Tachina-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 poll his) mimic wasps; their mag- 
FiG. m.— Syrphus poUtus gots ai'c inost uscful 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 {Cecidomyia, etc.. Fig. 162, 
C. tritici, Hessian-fly) the body is small and slender, with 
long antennae. The crane-flies (Tipiila) are large flies, 
standing near the head of the order, and, like the flea and 
Fi». 162.— Hessian-fly. a, larva; &, pupa; c, incision in wheat-stalk for larva. 
gall-fly, the chrysalis is enclosed in a cocoon, there being 
no pupariiim or pupa-case, as in the lower flies. Lastly, 
we have the mosquito (Figs. 163, 164), whose larva is 
aquatic, and breathes by a process on the end of the body, 
containing an air-tube. 
