

INSECTS AND HUMAft < DISEASE 101 



hunch-backed. The wings, ioo/ $hbw ; marked aifierehces, 

 those of Anopheles being more or less spotted, and in Culex 

 they are clear. But it is in the egg and larvae that the 

 most striking differences are observed. Anopheles lays 

 its eggs singly and often, as will be described later ; they 

 take up almost geometrical positions on the surface of the 

 water. Culex, on the other hand, lays groups of eggs glued 

 together to form rafts. The Anopheles larva, when un- 

 disturbed, lies almost parallel with the surface of the water 

 (fig. 23), whereas the Culex larva hangs head downwards, 

 almost at right angles to the water level (fig. 24). There 

 are other important differences, of course, but we have 

 quoted sufficient to show how the ordinary observer may 

 recognise the two mosquitoes. 



The life-histories of all mosquitoes are of the greatest 

 interest. The mother insects lay their eggs on or at the 

 edges of water, or sometimes on floating objects ; the larval 

 and pupal stages are spent in water, and the adult stage 

 only is passed away from this element. The eggs of all 

 mosquitoes, when first laid, are white in colour, but in a 

 short time they change to dark brown or black. When 

 laid upon water some provision is always made for keeping 

 the eggs afloat, for if they become submerged they fail 

 to hatch. The malaria mosquitoes (Anopheles), and some 

 species of the yellow fever mosquitoes, lay their eggs in 

 piled-up masses on the surface of the water, or on floating 

 substances, whilst the eggs of the common gnat are laid 

 on the water, in groups of from two hundred to four 

 hundred, cemented together in such a manner that each 

 egg floats perpendicularly with its broader end pointing 

 downwards, the whole mass forming an oval-shaped raft. 



Let us examine these eggs and egg masses in some 

 detail. When the Anopheles egg mass is laid on the 

 surface of water the eggs separate, and, on account of 

 purely physical causes, assume at times very regular and 

 beautiful patterns; sometimes they are arranged in star- 



