INSECTS AND MANKIND 433 



transmitting the sporozoites of the bird-infesting Plasmodia, 

 he failed to obtain from experiments with those insects 

 similar results as to the parasites of human malaria ; later 

 it was shown that these can undergo the sexual and sporula- 

 tion phases of their life-cycle only in female Culicidae of 

 the Anopheline group, whose palps are nearly as long as 

 the proboscis (Fig. 87, c). It was thus proved that the 

 spread of malarial disease is dependent on the association 

 of infected human subjects with a definite section of the 

 mosquito family. 



The conclusion that these diseases, so serious and 

 so frequently fatal as they are wherever prevalent, can 

 be transmitted only by means of the bite of an infected 

 anopheline mosquito, is of the very highest importance in 

 preventive medical practice. The use of mosquito-nets or 

 other means that may prevent the insects from sucking 

 human blood (which they seek to do by night), and the 

 elimination in the neighbourhood of towns and settlements 

 of pools or reservoirs of stagnant water in which mosquitoes 

 lay their eggs and where their larvae (Fig. 87, b, d) and 

 pupae live, have proved effective safeguards in many districts 

 once considered hopelessly unhealthy. It is an immense 

 advantage when causes of disease are known which indicate 

 the possibility of precaution. The term '' malaria " suggests 

 the belief that these diseases were caused by '' bad air," the 

 damp exhalation of marshes. It is now known that the 

 marshes are dangerous because they serve as breeding-grounds 

 for blood-sucking insects which act as alternative hosts of the 

 micro-organisms actually causing the disease. The Roman 

 Campagna was for long notorious as one of the most malarial 

 districts in the world ; within the last thirty years it has 

 been experimentally shown that perfect immunity may be 

 secured by sojourners there if they take precautions, by 

 surrounding themselves with nets, to avoid being bitten by 

 mosquitoes. The anopheline section of the Culicidae, whose 

 members alone act as hosts of the malarial parasites, are 

 represented in Great Britain and Ireland by three species 

 which are widespread and in some seasons abundant. 



2 F 



