192 



ARTHROPODA 



Fig. 89. Yellow-fever mosquito, Stegomyia fas- 

 ciata. (Howard, F. B., 1354, U. S. D. A.) 



situated between the 

 other two, secretes the 

 poison. If a mosquito 

 punctures the skin, but 

 does not reach blood, 

 the poison is not injected. 

 An anticoagulant is pro- 

 duced by the mosquito. 



The gall gnats {Ceci- 

 dofnyiidae) are terrestrial 

 forms attacking many- 

 plants and producing 

 " galls." The Hessian- 

 fly, a small black form, 

 is estimated to damage 

 wheat and rye to the ex- 

 tent of many millions of 

 dollars annually. The 

 crane flies ( Tipulidae) 

 somewhat resemble mo- 

 squitoes. They are often 

 quite large, with long, 

 loosely attached legs. 



Fig. 90. Male and female fruit flies {Drosophila melanogaster) . (Morgan, Physical 

 Basis of Heredity. Courtesy of J. B. Lippincott Co.) 



