THE MOSQUITO THEORY 379 



collections of rain-water. The larvse cannot live when the puddles 

 in which they breed dry up. Nor does Anopheles appear to favour 

 swamps containing deep water. The puddles they select are in 

 immediate proximity to houses, where the adult mosquitoes may 

 frequently pass to find human beings or cattle, from whom they 

 may derive their nourishment. As is well known, it is the female 

 mosquito which is the blood-sucker. After she has filled herself 

 with blood, she retires to some dark, sheltered spot near a stagnant 

 puddle, and after a few days deposits her eggs (from 200-400 in 

 number) in a mass on the surface of the water. Then she dies and 

 falls into the water beside her eggs. The eggs give rise (sometimes 

 in sixteen hours time) to the tiny swimming larvse, which feed 

 greedily and grow rapidly, shed their skins, and become nymphce or 

 pupce. Eventually, the shell of the nympha cracks along its dorsal 

 surface, and the young mosquito emerges. Standing on the raft of 

 its empty pelt, it dries its wings and flies away.* Very soon it also 

 lays eggs. The entire cycle from egg to egg is about fifty days. 

 The three conditions necessary for the multiplication of malarious 

 mosquitoes are (a) high atmospheric temperature, 75 to 104 F. ; 

 (&) collections of water, fresh or brackish ; and (c) the presence in 

 the breeding pools of low forms of animal and vegetable life. 



Having now gathered the outstanding facts concerning the 

 mosquito, we may return to the part which it plays in the propaga- 

 tion of malaria. The method which nature elects for the liberation 

 of any given parasite is generally one that is reasonably regular and 

 frequent in its operations. Hence, it occurred to Laveran and 

 Manson that as the malarial organism is a passive blood parasite, 

 its escape from the human body might be effected on the same 

 principle that the escape of the passive muscle parasites is effected. 

 As the latter obtain their opportunity by being swallowed by some 

 flesh eater, Dr Manson reasoned that the blood parasite might obtain 

 similar liberty by being swallowed by some blood-eater, some 

 suctorial insect such as the sand-fly or mosquito. He was still 

 further led to the mosquito theory owing to the parallel conditions 

 which he had already found to exist in the case of the analogous 

 mosquito phase of Filaria sanguinis.^ The mosquito phase then is 

 the extra-corporeal stage of the life-history of the malaria parasite 

 which takes place within the body of the mosquito, and we may 

 now briefly follow the course of events in the further development 

 of the flagellated body inside the mosquito. 



Whilst the mosquito theory was largely the suggestion of Sir 



* For further particulars as to mosquito Anopheles, see Brit. Med. Jour. 1901, 

 i., p. 195; Jour, of Hygiene, 1901, i., pp. 4-77, 269, and 451, also vols. 1902 and 

 1903. 



t Goulstonian Lectures, 1896. Sir Patrick Manson. Brit. Med. Jour., 1896, i., 

 641 el seq., and 1900, i., 328. 



