306 The Two-winged Flies 



of jcllow fever and filariasis, and to Anopheles belong the malaria brecdin" 

 and distributing mosquitoes. 



All the mosquitoes agree in having strictly aquatic immature stages. The 



eggs are laid on the surface of standing or .slowly moving water, usuallv 



fresh, although several .s])ecies breed abundantly and ])robably exclusively 



in brackish water. These eggs are in small one-layered packets or rafts (usual 



in Cule.x) (Fig. 413) or are scattered singly (in Stegomyia and Anopheles) (Fig. 



414) and hatch in from one to four days, varying with the species, and in 



^^ the same species with the temperature and light 



^^^ff^ conditions. The water oviposited on may be, for 



f'H^^Wi Culex, that of a pond, a pool, or any temporary 



tllAJti'F' jiuddle, or even that in an exposed trough, barrel, 



[/ pail, or can. With Anopheles only natural, usually 



Fig. 414.— The eggs of permanent, pools are selected. I have found the 



Anopheles sp. (After esas of Ciilex iiicideiis on the surface of a bubbling 



Giles; much enlarged.) , . . _ ,.f . , r r- . . 



soda-spnng in California, and of btegomyia in water 



held in slight depressions in a number of ship's metal parts in Samoa. 

 The brackish-water species of Culex usually lay their eggs on the small 

 clear pools scattered through the marshes. A few entomologists have 

 recorded their belief, based on various indirect observations, that the eggs 

 of Anopheles at least may be deposited on the soil, but no direct proof of 

 this is yet on record. 



The larvK (Figs. 413 and 415) of mosquitoes are the familiar wrigglers of 

 ponds and ditches. The long, slender, squirming body, with its forked posterior 

 extremity and thick head end, is thoroughly characteristic. The head is 

 provided with a pair of vibratile tufts or brushes of fine hairs which are 

 kept, most of the time, in rapid motion, creating currents of water setting 

 toward the mouth, and thus bringing to it a constant supply of food, which 

 consists of organic particles and microscopic animals. Breathing is accom- 

 plished by the wrigglers coming to the .surface and hanging head downward 

 from it with the open tip of the respiratory tube, one of the prongs of the 

 posterior forking of the body, projecting just through the surface film. If a 

 mosquito wriggler is prevented from coming to the surface, or if, once there, it 

 finds some impediment which restrains it from getting its rcsi)irator)' tube 

 into connection with the free air above the surface, it will drown. And 

 this fact partly explains the fatal effectiveness of a film of kerosene spread 

 over the surface of a pool in which mosquitoes are breeding. The larval 

 stage lasts from one to four weeks, varying in different species and also 

 varj'ing in the case of each species at different seasons and under different 

 conditions of food-supply, temperature, and light. Larva; of Culex have 

 lived in breeding-jars in my laboratory for three months. The larva" moult 

 twice, and on the third casting of the skin appear as active, non-feeding 



