INSECTS
that it is one of the minute, non-filterable organisms called spirochetes. The virus will not develop in the mosquitoes at a temperature below 68° F., and Aëdes aegypti will not breed
Fig. 180. The female malaria mosquito, Anopheles punctipennis
in latitudes much beyond the possible range of yellow fever. Yellow fever, therefore, is a disease ordinarily confined to the tropics and warmer parts of the temperate zones. Seasonal outbreaks of it that have occurred in northern cities have been caused probably by local infestations of infected mosquitoes brought in on ships from some southern port.
The malaria mosquitoes belong to the genus Anopheles, a genus represented by species in most temperate and tropical regions of the world, which are prevalent wherever malaria occurs. Our most common malaria species is Anopheles punctipennis (Fig. 180), characterized by a pair of dull white spots on the edges of the wings. The Anopheles females lay their eggs singly on the surface of the water, where they float, each buoyed up by an air jacket about its middle.
The larvae of Anopheles (Fig. 178 B) differ conspicuously from those of Culex and Aëdes both in structure and habits. Instead of a respiratory tube projecting from near the end of the body, as in Culex (Figs. 174 E, 175), there is a concave disc (Fig. 178 B, f) on the back of the next to
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