MOSQUITOES AND FLIES
more northern one common about Washington, D. C., where it breeds in rock pools along the Potomac River.
The larva of Aëdes (Fig. 178 A) resembles a Culex larva, but it feeds more habitually at the bottom of the water and may spend long periods below without coming to the
Fig. 179. Mosquito pupae in natural position resting against the under
surface of the water
A, Aëdes atropalpus. B, Anopheles punctipennis
surface for air. In its search for food it noses about in the refuse at the bottom of the water and voraciously consumes dead insects and small crustaceans. The pupa likewise (Fig. 179 A) does not differ materially from a Culex pupa. When quiet it floats at the surface of the water with the entire back of its thorax against the surface film and the tips of its breathing tubes above the surface. Probably no mosquito pupa hangs suspended from its respiratory tubes in the manner in which the pupae of various species are often figured.
Aëdes aegypti is the only known natural carrier of the virus of yellow lever from one person to another. The disease can be taken only from the bite of a mosquito of this species that has become infected by previous feeding on the blood of a yellow-fever patient. The organism that produces yellow fever is perhaps not yet definitely known, though strong evidence has been adduced to show
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