8o 



Insects and Disease 



the breathing-tube above the surface most of the 

 time have very small tracheal gills, while those that 

 feed mostly on the bottom have them well de- 

 veloped. 



When first hatched the larvae are of course very 

 small. If the weather is warm and the food is 

 abundant they grow very rapidly. In a few days 

 the outer skin becomes rather firm and inelastic so 

 it will not allow further growth. Then a new skin 

 forms underneath and the old skin is cast off. This 

 process of casting off the old skin is called molting, 

 and is repeated four times during the one, two, 

 three or more weeks of larval life. 



PUPA 



With the fourth molt the active feeding larva 

 changes to the still active but non-feeding pupa 

 (Fig. 59). The head and thorax are closely united 

 and a close inspection will reveal the head, antennae, 

 wings and legs of the adult mosquito folded away 

 beneath the pupal skin. Instead of the breathing- 

 tube on the eighth segment of the abdomen as in 

 the larva, the pupa has two trumpet-shaped tubes 

 on the back of the thorax through which it now gets 

 its air from above the surface. The pupal stage 

 lasts from two to five or six days or more. When 

 the adult is ready to issue the pupal skin splits 



