LIFE HISTORY AND SEASONAL PREVALENCE 117 



water ; or that thick spongy growth which clothes the sides 

 of fountains in which there are no fishes, and gets detached 

 and floats like a thick, dark green scum. The principal 

 animal food is the cast-off skins and pupa cases of water 

 larvae, including those of its own kind, and the remains 

 of dead mosquitoes and other small insects. These often 

 collect in patches on the surface of a pond, and one dip 

 under such a patch will secure hundreds of larvae after you 

 have searched the rest of the pond in vam. Now the larva of 

 Anopheles, unlike that of Culex, floats flat on the surface of 

 the water, and it is much more unwilling to go down than 

 Culex. If green food is to be had not more than two or 

 three inches deep, it will go down and feed, but it comes up 

 again very soon, and would evidently rather not go down 

 at all. As it floats you will see two little organs on the 

 front of its head incessantly stirring the water. These are 

 the " whorl-organs." They are crowned with little brushes 

 of bristles, and their function is to keep up an eddy, by 

 which every little floating particle which passes by is sucked 

 in towards the insect's mouth. With a lens you can see 

 this quite plainly and observe it seizing the little particles 

 with its jaws, sometimes eating them and sometimes throwing 

 them away with an angry toss of its head. I do not think 

 that even living objects, if small enough, are refused, and, 

 in fact, I am almost sure that the larger larvae sometimes 

 eat the little ones. They are all very ill-natured and bite 

 savagely at each other when they get the chance. This way 

 of feeding explains why Anopheles likes a certain amount 

 of motion in the water, for it brings food, and why it must 

 starve in deep water unless there chances to be a great deal 

 of food, animal or vegetable, floating on the surface. The 

 larvae of Culex dive much more freely and are more pro- 

 miscuous in their diet, and, since they float with their heads 

 down, they do not care much for floating matter. Hence 

 they dislike any motion. The more stagnant the water is 

 and the more dirty, the better it pleases them. Lastly, 

 Anopheles^lsirvse must have sunlight, though they will hate 

 it more cordially than other mosquitoes when they come to 

 mosquitos' estate." 



