38 



ENTOMOLOGY FOR MEDICAL OFFICERS 



large in flies whose movements are particularly sudden and 

 quick. 



The posterior (metathoracic) wings are represented by 

 a pair of small club-shaped organs known as halteres, 

 or balancers (aXr^/oej = weights held in the hand to give 

 impetus in jumping), the use of which is supposed to be 

 either for maintaining equilibrium or for stridulating. They 

 are very constant, and are even, as a rule, present in wingless 

 flies. 



The venation of the wings is comparatively simple, but 

 the general plan varies a good deal in detail throughout the 

 order. It is enough for our purpose to compare the wing- 

 venation of a house-fly, a mosquito, and a gadfly as 

 standards. 



In the house-fly (Fig. 5) a vein — the costal — runs along 

 the front margin (the wing is supposed to be extended at 



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itcilm 



CO ^ 



Fig. 5.— Wing of House-fly. 



■"""TMialoii. 



mn/ 



right angles with the body) from the root of the wing almost 

 to the tip. A second vein — the subcostal, or auxiliary — -runs 

 parallel with the costal for a short distance and then curves 

 into the latter. Behind the subcostal, three veins — \^^ first, 

 second, and third longitudinal — arising from a common stem, 

 radiate, in the anterior half of the wing, to successive points 

 in the costal vein. Three longitudinal veins in the posterior 



half of the wing complete the series ; two of them the 



fourth and fifth longitudinal— a^nse together, and diverge to 

 the margin of the wing; the remaining one — the sixth 

 longitudinal — arises more or less independently and runs 



