104 INSECTS AND MAN 



parent globular body ; the thick end floats downwards in 

 the water, and through it the larvae emerge. 



So much for the eggs ; and now, turning to the larvae, we 

 shall see that they also show striking differences. With 

 anatomical details we are not concerned, but we may 

 mention that each larva is composed of three parts, a head, 

 thorax, and abdomen, divided into segments, at the tail end 

 of which is a tuft of gills; and there may, or may not, 

 according to the species, be a breathing tube. The larva 

 of Anopheles is easily distinguished from all others, 

 because, when at rest, it lies flat along the top of the water, 

 and some of its hairs actually pass into the air, so that the 

 larva appears as though it had its back out of water. The 

 reason for this position is, that Anopheles has no breathing 

 or " spiracle " tube, respiration taking place through a pit, 

 on the eighth abdominal segment. With one exception, it 

 is only when about to pupate that the Anopheles larvae 

 forsake their horizontal resting position for a more vertical 

 one; the exceptional species is the one whose eggs are 

 unprovided with floats, and these larvae, although they rise 

 to the surface in a horizontal position, then slowly sink till 

 their tails alone touch the surface. As in the egg, so in 

 the larval stage, the yellow fever mosquito (Stegomyia) 

 exhibits an intermediate structure between Anopheles and 

 Culex. The former has no siphon tube ; the latter has a 

 relatively long one, whilst Stegomyia has a short, thick 

 one. When at rest, at the surface of the water, this larva 

 assumes an almost vertical position, hanging head down- 

 wards, with the tip of its siphon tube projecting into the 

 air. Unlike the majority of mosquito larvae, however, it 

 spends a considerable time at the bottom of the water, and 

 then it usually lies horizontally. The gnat larva at rest 

 does not assume quite such a vertical position as the pre- 

 ceding example, nor is it so long and worm-like, although 

 provided with a longer siphon tube. A word may here be 

 said about these structures. 



