100 INSECTS AND MAN 



maybe by lucky chance. At any rate this cursory con- 

 sideration of the malarial blood parasite shows us that two 

 hosts are essential to its well-being a human host for its 

 asexual forms, and a mosquito host, of the sub-family 

 Anopheles, for its sexual forms. We learn, too, that the 

 malaria mosquitoes cannot, at once, infect mankind ; after 

 a meal of infective blood, ten days or so must elapse 

 before they are able to do so till the sporozoites are 

 formed. 



We will not attempt to detail the classification of the 

 mosquitoes, for the subject is one of great complexity, a 

 state of affairs that is not made easier by the fact that 

 the classification is at present undergoing revision. All 

 the malaria-carrying mosquitoes, however, belong to the 

 sub-family Anopheles, a name that means harmful, though 

 certain mosquitoes of the genus Stegomyia are equally 

 noxious. Approximately, twenty species of Anopheles are 

 known to transmit malaria from man to man : four species 

 are European, two North American, two tropical American, 

 one Australian, four tropical African, while no less than 

 eight species hail from Southern Asia. It is cold comfort, 

 perhaps, to know that the members of the sub-family Ano- 

 pheles are, in many ways, the most easily recognised of all 

 the mosquitoes. Let us compare a typical malaria mosquito 

 with, say, the common gnat of this country, Culex pipiens. 

 At rest, these two mosquitoes may be easily distinguished. 

 Anopheles assumes an attitude with its head and beak in 

 the same plane as its body, and its hind legs often stretched 

 straight out, either stationary or gently waving to and fro 

 (fig. 22, A). Culex, on the other hand, shows a hunch-backed 

 silhouette ; its head and beak are not in the same plane as 

 its body, and its hind legs rest on the support (fig. 22, B). 

 The resting position of mosquitoes forms a ready means of 

 rough distinction. Some Anopheles, when resting on a 

 wall, project almost at right angles ; other genera lie nearly 

 parallel to their support ; others again, like Culex, appear 



