62 CULICID^ [CH. 



pairs of large thoracic, situated on the mesothorax and meta- 

 thorax respectively, and six pairs of abdominal, on the second 

 to the seventh segments. 



(d) The circulatory system. As in the majority of insects 

 the, circulatory system consists of a dorsal tube or heart con- 

 tained within a pericardium. The heart is provided with 

 valves and by its rhythmic contractions the blood, which 

 bathes all the organs, is kept circulating round the body. 



{e) The nervous system. This consists of a double ventral 

 nerve-cord uniting a series of ganglia, one to each segment. 

 The anterior or cephalic ganglion, situated in the head and 

 supplying nerves to the appendages of this region, is remark- 

 able for its large size in comparison with that of the insect. 

 From this ganglion the ventral nerve-cords pass one on each 

 side of the oesophagus, to unite in an infra-oesophageal ganglion. 

 The double ventral nerve-cord continues posteriorly from this 

 ganglion and in each segment, except the last, is enlarged into 

 a segmental ganglion. 



Life-cycle. The life-cycle of all species of mosquitoes may 

 be briefl}^ outlined in a few words. The female, after being 

 fertilized, deposits her eggs on, or near, the surface of some 

 water. After a short interval these hatch out giving rise to 

 young aquatic larvae, which grow until they reach a certain 

 size and then pupate. The resulting pupa is actively motile 

 and swims about in the water as vigorously as the larva. 

 After a comparatively short existence the anterior dorsal 

 region of the pupa sphts and the adult insect emerges. 



The egg. The number of eggs laid by the female varies 

 considerably, from 40 to 100 {A. maculipemiis) , up to as many 

 as 400 {Culex pipiens). The eggs may occur singly, as in the 

 case of Anopheles and Stegomyia, or adhere together forming 

 boat- or raft-shaped masses, as in Culex and many other Cuh- 

 cinEE, and being provided with air-chambers, they invariably 

 float on the surface of the water. The shape of the eggs is 

 different in each genus ; those of Anopheles are boat-shaped 

 with a distinct float at the sides, growing broader in the 

 middle (Fig. 31, A, B), whilst those of Culex are fusiform (Fig. 30). 

 In the great majority of mosquitoes the eggs are laid on the 



