DIPTERA. 
439 
5 «—end of 
molts changes into a club-shaped pupa, the head and thorax 
being greatly enlarged (514, 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, 
At the tail-end of the body there is a pair 
of leaf-like appendages, with which the breatfun*Sbe ofTarva- 
. . r r 1 r . ** breathmg-tube of 
insect swims; for the pupae of mosqui- pupa- 
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, Hcemamceba , 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 Hcztnamceba 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 <arvae and 
