LARVAE OR "WRIGGLERS" 431 



mosquitoes where water is plentiful. Just as a man runs less 

 risk of ruin if he deposits his money in a number of insecure 

 banks rather than in a single uncertain one, so it is with mos- 

 quitoes and the places where they deposit their eggs. The 

 gamble for life in a dry climate would be too risky if all eggs were 

 deposited in one place, and species with this habit have probably 

 long since been weeded out in the struggle for existence. An- 

 other remarkable adaptation of dry-climate mosquitoes is the 

 variation in the hatching periods of the eggs in the same batch; 

 not all hatch with the first drops of moisture, but some lie 

 over until subsequent immersions, thus insuring a much better 

 chance that some of them, at least, will not waste their life on 

 the desert air with too little water to enable them to reach 

 maturity. 



The eggs of mosquitoes never hatch except in the presence of 

 water. The larvae, which are always aquatic, are very active 

 wormlike creatures, well known as " wrigglers " or " wriggle-tails" 

 (Fig. 196). When first hatched they are almost microscopic, 

 but they grow rapidly to a length of from a quarter of an inch to 

 almost an inch. The bunches of long bristly hairs on the body 

 take the place of legs, and aid the larva in maintaining a position 

 in the water. The " rotary mouth brush " is a brush of stiff 

 hairs which is used to sweep small objects toward the mouth; 

 in predaceous species these are sometimes modified into rakelike 

 structures or into strong hooked bristles for holding prey. The 

 trumpet-shaped breathing tube (Fig. 196 A) is present on all 

 mosquito larvae except Anopheles (Fig. 196B), in which it is 

 undeveloped. It is used to pierce the surface film of the water to 

 draw air into the air tubes or tracheae inside the body, for, al- 

 though aquatic, mosquito larvae are air breathers, and make 

 frequent trips to the surface to replenish their air supply, re- 

 maining suspended by the breathing tube from the surface of the 

 water while breathing. The leaf like " tracheal gills " on the last 

 segment of the abdomen differ from true gills in that air tubes 

 or tracheae instead of bloodvessels ramify in them. In one 

 species of mosquito, Mansonia, the larvae absorb air from the air- 

 carrying tissues in the roots of certain aquatic plants, piercing 

 them with the apex of the breathing tube and thus avoiding the 

 necessity of rising to the surface of the water. In well-aerated 

 water the larvae can live without surface air for a long time by 



