6 MISC. PUBLICATION 336, U. S. DEPT. OF AGRICULTURE 
structures that keep them afloat. The incubation period is short in 
warm weather (usually 2 or 3 days), but in certain species, particu- 
larly Aedes and Psorophora, the eggs are able to withstand long 
periods of drying; in fact, they appear to require a certain amount of 
drying, and sometimes exposure to cold, before they will hatch. 
B 
FIGURE 1.—Heads and appendages of mosquitoes: A, Side view of Anopheles 
female; 0, occiput; ft, frontal tuft; 1-5, palpal segments. B, Culex female 
(from above). C, Culex male. D, Anopheles male. 
FIGURE 2.—Seyeral species of mosquitoes, Showing difference in size: A, Wegar- 
hinus rutilus; B, Psorophora ciliata; C, Anopheles quadrimaculatus ; D, Aedes 
atlanticus; E, Culex quinquefasciatus; F, C. erraticus; G, Uranotaenia lowii. 
The larvae of all mosquitoes are aquatic and most of them free 
swimming. Although possessing tracheal gills, the larvae of most 
species must come to the surface for air, and an elongated air tube 
or other modified apparatus is provided for obtaining air through 
the surface film. During the period of development, which lasts 4 to 
