MOSQUITOES AND CATERPILLARS 



87 



or less dense than the water ? that is, is it heavier or lighter 

 than water? Note that some of them hang quietly from 

 the siirface, and that each one comes occasionally to the 

 surface and rests 

 there for a while to 

 breathe. Every ani- 

 mal has to breathe; 

 that is, to take up 

 oxygen from the air 

 and to give off. from 

 its body carbon di- 

 oxide (CO2). There 

 is always some air 

 mixed with or dis- 

 solved in water, 

 and most aquatic 

 animals — fishes for 

 exampl e — have 

 special structures 

 called gills which 

 enable them to take 

 up this dissolved 

 oxygen, and thus to 

 breathe under wa- 

 ter. But the gills 

 of most mosquito 

 larvae are too un- 

 developed, and 

 therefore they have 

 to come occasional- 

 ly to the surface to 

 breathe. 



Fig. 36. A mosquito, Culex sp.; showing eggs 

 (on surface of the water), larvae (long and 

 slender, in the water), pupa (large-headed at 

 surface), and adult (in the air). (About three 

 times natural size; from living specimens.) 



Examine with a hand lens one of the larvae in a watch- 

 glass of water. Distinguish the head end of the body; 

 note the eyes (two small black spots), the feelers, or antennas, 



