102 INSECTS AND MAN 



shaped groups, sometimes ia equilateral triangles, and at 

 other times in regular rows. If the water in which these 

 free-floating eggs are found be disturbed, their arrange- 

 ment is of course upset, but, when the surface is once 

 more quiet, they will re-arrange themselves in the original 

 designs. With one exception, the eggs of these malaria 

 mosquitoes are boat-shaped their upper surface is flat, 

 and the lower one deeply convex. Running round the 

 margin of the upper surface is a gleaming white frill, 

 forming, as it were, the gunwale of the boat; whilst, on 

 either side of the egg, there is a remarkable structure, 

 oval in shape, and transversely corrugated ; this structure 

 contains air cells and acts as a float (fig. 25, A). We have 

 mentioned that the eggs of one Anopheles species differ 

 from the typical form. In this species the eggs are devoid 

 of floats and exhibit only a rudimentary frill ; they are 

 always deposited on floating objects, never on the water ; in 

 fact, if they accidentally fall into the water they sink and 

 perish (fig. 25, B). 



From the larval stage we pass to the nymphs. They have 

 relatively large globular bodies made up of head and thorax 

 with small tails ; roughly, they are comma-shaped (fig. 26). 

 From the globular part of the body the thorax, to be exact 

 two breathing tubes arise, and the shape of the orifices 

 of these tubes forms the easiest means of distinguishing 

 our examples. Those of Anopheles have square, truncated 

 ends; in Stegomyia they are broadly triangular; and in 

 Culex, in addition to being longer and narrower than in 

 the other species, they have slit-like openings. The differ- 

 ences between the nymphal stages of the various mosquito 

 species are not nearly so marked as they are in the egg and 

 larval stages. The nymphs, when undisturbed, like the 

 larvae, lie with their breathing tubes out of water ; on the 

 slightest alarm they rapidly dart away from the surface, 

 but they do not stay away from it for long. Some of the 

 darker-coloured nymphs are less active than the others, 



