THE ORGANISM AND ITS ENVIRONMENT 361 



Insects, particularly Diptera, are of considerable importance in the 

 transmission of disease, flies by the mechanical conveyance of in- 

 fected materials and mosquitoes by the inoculation of the host with 

 the micro-organisms of disease. It is not our purpose here to discuss 

 the hfe cycles of insects in reference to their importance to public 

 health. However, the mosquito has played such an important role 

 in the spread of disease that an understanding of its life history is 

 properly a part of general information. 



All Diptera, including the mosquitoes, are insects of complete 

 metamorphosis. The eggs of the mosquito (Fig. 217) are laid in 



HONEY BEE HOUSE FUT 



Fig. 216. — Common honey bee and common house fly. 



boat-shaped bunches in water or moist places. The eggs hatch in a 

 few days, the time required varying with the variety of mosquito 

 and with the temperature. The larvx are aquatic and are harmless 

 to other animals. They live in water but are air breathers; next to 

 the last abdominal segment is equipped with a slender breathing 

 tube. They hang head downward in the water with this tube ex- 

 truded at the surface. The larvse grow rapidly and, after moulting, 

 change into pupse. The pupae of the mosquito, unlike the pupal 

 stage of any other insect, are active. They, too, have breathing 

 tubes but these are on the thorax and are two in number. After a 

 short pupal stage the skin splits and the adult emerges, using the 

 cast pupal case as a raft. As soon as its wings harden the adult flies 



