LIFE HISTORY AND SEASONAL PREVALENCE 129 



less for a quarter of an hour or more, would be taken to be 

 such by any one not specially looking for them. As already 

 noted, they exhibit a great preference for the sides of the 

 tank or pool in which they may be, and may also be found 

 along floating sticks, leaves, or other such objects, lying at 

 right angles to, and with their tails touching it, as if moored 

 by the stern. When lying thus at the surface, they have 

 a quaint custom of screwing their heads completely round, 

 so that its ventral surface looks right upwards, and, though 

 otherwise motionless, if closely watched it will be seen that 

 the head is being slowly rotated and swept to and fro, while 

 by the rapid vibrations of the bristles of the " whorl organs," 

 a current of water with its suspended nutritive particles is 

 kept flowing to it. 



It is perhaps rather a misnomer to speak thus of the 

 moustache-like bunches of bristles that project from a pair 

 of plates hinged on to the fore corners of the clypeus, as the 

 bristles all work together, and do not bend, one by one in a 

 wave, like the cilia of the Rotatoria, from a fancied resem- 

 blance to which they have got the name. Although capable 

 of being moved separately, the two brushes when in action 

 are brought together, as if grasping at something, and then 

 open out again, and repeating this movement with great 

 rapidity ; according to Nuttall {loc. cit.), three times a 

 second. 



He describes, too, how they employ the peculiar bristles 

 on their mandibles to comb off, into the mouth, the nutri- 

 tive particles that have collected on the brushes. The 

 mouth-organs of Culices are used in exactly the same way, 

 only instead of sweeping the surface, for floating particles, 

 they depend on such as are suspended in the water beneath. 

 When disturbed the larvae dart backwards into the depths 

 of their puddle, and hide themselves among the loose par- 

 ticles at the bottom, and then, after a few minutes, float 

 slowly back to the surface when the coast is clear. A good 

 deal has been written about these movements, and those 

 of the pupas, but as they are obviously purposive, they 

 naturally vary, and hardly merit detailed description. 



The nymphs, as may be seen in Fig. 31, float in 

 9 



