FLIES 215 



birds. Culex is an ancient genus, which already existed, as the fossil 

 record proves, in the Oligocene period thirty million years ago. It is 

 now mainly tropical or subtropical, only a few species penetrating into 

 the temperate zone. Four of these are British, but only one, the most 

 common and familiar of all mosquitoes, \h&]io\i?>G-gndit {Culex pipiens), 

 is an important parasite of birds (Plate XXX). In India, Ross used 

 the related C. fatigans for his world famous experiments proving the 

 transmission of bird malaria by these insects. 



Only the female house-gnat bites, and she does so principally at 

 night. It is when swallows and martins gather in the reeds in communal 

 roosts prior to migration (see Plate XXVI) that they are severely 

 attacked, and mass infection with bird malaria frequently follows. In 

 the southern seas these insects are said to cause entire colonies of pelicans 

 to desert their nests. The behaviour of mosquitoes in the dark is difficult 

 to observe unless they attack man himself, but recently an extremely 

 ingenious invention has made this task much easier. Large numbers of 

 the insect are captured and then sprayed with fine luminous adhesive 

 dust. Subsequently they are released and their movements can be 

 followed in the dark like aeroplanes with lights attached to their wings. 

 It is not known if roosting birds are frightened by the pipe of a female 

 mosquito. There is, however, some evidence that cattle have an in- 

 herited fear of the hum of the warble-fly {Hypoderma bovis) and an 

 inherited fear of gnats might well have survival value in birds. 



Mosquitoes are as a rule very fussy about the conditions in which 

 they will mate. Some choose the evening before dark but only when 

 the light intensity has fallen below 2.0 foot-candles. A bright light 

 will put them off. As for the house gnat, it refuses to mate in a 

 confined space. If the air is still the males swarm, just after sunset and 

 again immediately after dawn, about six to nine feet from the ground 

 to the leeward of some prominent object, like a high hedge or the stone 

 coping on a roof. In the case of Culex pipiens about 50 to 100 males take 

 part and the whole swarm moves rhythmically up and down — in the 

 case of some other gnats it oscillates from side to side. The female is 

 attracted by the hum of the swaying column and in the excitement her 

 natural reserve is broken down and she is drawn into the swarm. She 

 is then seized by a male, and the couple drop out of the dance and 

 copulation takes place on the ground. 



The house-gnat lays her eggs in the form of a boat-shaped raft which 

 floats (Plate XXXd). In order to do so she stands on the surface 



