FIG. 515. a, end of 



DIPTERA. 439 



molts changes into a club-shaped pupa, the head and thorax 

 being greatly enlarged (5 14, b). With this transformation a 

 remarkable change takes place in the re- 

 spiratory system. There are now two 

 breathing-tubes, and these are borne by 

 the thorax. One of these tubes is repre- 

 sented greatly enlarged by Figure 515, b. 

 At the tail-end of the body there is a pair 

 of leaf-like appendages, with which the bfeattme-mbe of larva: 



6, breathing-tube of 



insect swims; for the pupee of mosqui- p"P a - 

 toes, and also of certain midges, differ from the pupae of 

 other insects in being active. The pupa state lasts only a 

 few days ; then the skin splits down the back, and the 

 winged mosquito carefully works itself out and cautiously 

 balances itself on the cast skin, using it as a raft, until its 

 wings are hardened so that it can fly away. 



In recent years it has been found that the transmission 

 of several diseases is due to mosquitoes, different kinds of 

 mosquitoes transmitting different diseases. Malaria is 

 caused by a protozoan organism, Hcemamaeba, which de- 

 stroys the red blood corpuscles of man. It has been dem- 

 onstrated that when blood is taken by a mosquito of the 

 genus Anopheles from a person suffering with malaria 

 the Hcemamceba passes through certain stages of its life 

 history within the body of the mosquito and finally reaches 

 the salivary glands of the mosquito, from which the ma- 

 larial germs are injected into persons bitten by the infected 

 mosquito. And there is no reason to believe that man 

 becomes infected with malaria in any other way. People 

 have lived in extremely malarious regions and escaped 

 taking the disease by merely spending the evenings and 

 nights in mosquito-proof houses. 



It often happens that plagues of these pests are bred in 

 receptacles of rain-water standing near dwellings ; such re- 

 ceptacles should not be left open unnecessarily. When the 

 breeding-places are ponds of limited extent the larvae and 



