74 GENERAL ZOOLOGY 



the surface of the water early in the morning. Within a day 

 they hatch, and the larvae are the familiar, active creatures 

 known as "wrigglers." The next to the last somite bears 

 a long respiratory tube through which the larva breathes 

 air when at the surface ; the last somite is provided with 

 four flaps (tracheal gills) which act as organs of respiration 

 when the larva is beneath the surface. In addition to these 

 methods of obtaining air, the skin is capable of absorbing 

 oxygen, and a network of tracheae lines the posterior part 



of the alimentary 

 canal, so that oxy- 

 gen may be obtained 

 from the water taken 

 in at the anal open- 



Anopheles Culex . 



jTinr The 1j3t*v?p fppd 

 Fig. 50. Resting positions of mosquitoes. s * 



(Slightly enlarged) on small particles of 



After Grassi decaying matter in 



the water, thus be- 

 ing useful as scavengers. After several molts and a life of 

 about a week, if the weather is warm they then pass into 

 the pupal stage, breathing by means of two air tubes aris- 

 ing from the thorax. In about two days the pupal skin 

 splits down the back and the imago works itself out of its 

 old skin, dries its wings, and flies away. Cold weather 

 retards these changes considerably. 



Great interest attaches to the mosquitoes by reason 

 of the fact that malaria and yellow fever are both wholly de- 

 pendent upon mosquitoes for transmission from one per- 

 son to another. Malaria is carried by mosquitoes belonging 

 to the genus Anoph'eles, the females of which may be distin- 

 guished from the females of Culex by the greater length of 

 the palpi. Anopheles shows a tendency, especially on hori- 

 zontal surfaces, to alight with the hind end of the body 

 raised at a considerable angle to the surface ; Culex holds 



