INSECTS AND DISEASES 399 



is secured. When disturbed, the larvae frequently sink to 

 the bottom but can remain only a few minutes, being 

 obliged to return to the surface for air. 



The pupa, into which the larva develops, differs notice- 

 ably from the larva in form and possesses two breathing 

 tubes situated on the thorax. The insect now rests with its 

 head uppermost. The pupa is active, being an exception 

 in this respect to the general rule among insects. The 

 insect remains in the pupal stage only a few days ; the skin 

 splits and the adult emerges, resting on the old skin at the 

 surface of the water until its wings become dry and hardened. 



Under favorable conditions the whole life history may be 

 completed in ten days, one day in the egg, seven days in 

 the larval state, and two in the pupal state. During cool 

 spells of weather this time may be greatly prolonged. The 

 development of the malarial mosquito takes from fifteen to 

 twenty-four days. 



Several hundred eggs are laid in a single mass and as each 

 may develop into an adult in ten days, it is clear that the 

 possibilities of multiplication are enormous. Professor 

 Hodge has calculated that the descendants from a single 

 mosquito (on the supposition that one half are females and 

 that each of these lives and lays two hundred eggs) might 

 amount in one hundred and eighty days to a number repre- 

 sented by the figure two followed by thirty-nine ciphers. 

 Of course this never actually happens, on account of the 

 insect's enemies and the lack of food, but it suggests the 

 possibilities of growth. 



Breeding places. Mosquitoes breed in a great variety 

 of places, such as rain barrels, tin cans in dump heaps, dishes, 

 and almost any receptacle containing water, in pools of 

 water, in ditches, depressions, footprints, in cesspools, 

 hollows of trees, sewer catch basins, railroad ditches, swamps, 

 and woodland pools. In fact they may breed in almost any 

 stagnant pool of water that stands for a week or more. They 



