66 CULICID^ [CH. 



general rule they feed on algse, but many species swallow any 

 kind of minute obiect that may be in the water. By means 

 of the mouth-brushes (Fig. 31, G, b) currents are set up towards 

 the mouth, and any small particles are carried with the stream 

 and swallowed. Certain larvae {e.g. Culex pipiens) seem to 

 thrive best when the water is charged with animal refuse, 

 whilst the larv£e of Psorophora, Liitzia and some other genera 

 are actively predacious, feeding upon other mosquito larvje. 



When food is abundant and the temperature favourable 

 the larva grows rapidly and may become full-grown in little 

 more than a week. The duration of the larval stage, how- 

 ever, is extremely variable, for the larvae of some species 

 [e.g. Wyeomyia smithii) can live for nearly a year without any 

 food. The duration is also greatly prolonged by low tempera- 

 tures and it is probable that some species pass through the 

 winter in the larval form. 



During its growth the larva moults four times, the last of 

 these moults giving rise to the pupa. 



The pupa is also aquatic, and swims actively through the 

 water, though of course it is unable to take in any food, the 

 appendages of the head and thorax of the future fly being 

 enclosed in a common chitinous covering. The pupa respires 

 by means of a pair of appendages on the thorax, the respiratory 

 trumpets, or horns (Fig. 8, tr). These trumpets communicate 

 with the anterior pair of thoracic spiracles and when the pupa 

 is at the top of the water they break through the surface film 

 and thus admit air to the spiracles. The pupa is kept floating 

 at the surface by means of a pair of fan-like tufts of hairs, 

 situated on the dorsal surface of the first abdominal segment. 

 The abdomen is composed of nine distinct segments ; the 

 eighth bears at its apex a pair of large chitinous plates, the 

 paddles or fins. The shape of the trumpets and paddles varies 

 in different species but these characters are of no generic value. 



The duration of the pupal stage as a rule does not exceed 

 more than two or three days, but as in the case of the other 

 stages may be prolonged by low temperatures. Towards the 

 end of this period the pupa becomes mflated with air and 

 when the adult is about to emerge it gradually straightens 



