THE DIPTERA 309 



Different species of Anopheles appear to be connected with different 

 types of malaria. Anopheles quadrimaculatus has been proved to carry 

 the organisms causing both the tertian and quartan forms while Anopheles 

 crucians is a carrier of the organisms producing the sestivo-autumnal 

 type of the disease, and, in other parts of the world, other species play 

 similar roles in relation to these forms. 



The animals causing malaria are believed to be of three closely 

 related kinds, belonging to the Protozoa. In its form when introduced 

 into the blood of man, the animal is a rather long and slender spindle 

 with pointed ends. It now assumes an amoeboid form and attacks a 

 red blood corpuscle, working into it, feeding on its haemoglobin contents 

 and producing black granules. It feeds on the haemoglobin in the 

 corpuscle until this has all been consumed and grows until it nearly fills 

 the corpuscle. It now divides into many parts, each similar to the one 

 which first entered the corpuscle, and these proceed to attack other 

 corpuscles in a similar way. This breaking up of the animal into parts 

 coincides with the " chill" of the disease and the interval of time between 

 successive chills determines which type of malaria is present, a period of 

 2 days indicating the tertian type; 3 days the quartan type, while a 

 varying period indicates the sestivo-autumnal type. As the parasites 

 increase in abundance and consume more of the corpuscles, the patient 

 becomes anaemic and weaker. 



Some of the products of division in the corpuscle, however, do not 

 proceed to attack other corpuscles and increase in numbers but are of 

 two different kinds which are the sexual stages. When these are taken 

 into the stomach of an Anopheles which attacks a person having malaria, 

 the two kinds fuse and the resulting animal penetrates the cells of the 

 stomach wall of the mosquito and there remains, forming a cyst. Divi- 

 sion of the animal here results finally in the production of cells like those 

 which enter human blood, and these now escape into the body cavity of 

 the mosquito and gradually gather in its salivary glands whence, they 

 are expelled into the wounds caused by the feeding of the mosquito there- 

 after. The time which must elapse after a mosquito has received the 

 malarial parasites before it can transmit these to man varies, but is 

 usually at least 10 or 12 days and may in some cases be more than this. 



The Yellow Fever Mosquito (Aedes .cegypti L.). This insect, for- 

 merly known as Stegomyia fasciata, is the carrier of Yellow Fever. It 

 occurs in the tropics throughout both hemispheres and, during warm 

 weather, may extend to the temperate regions but can survive there 

 only while the temperature is fairly high. 



The adult (Fig. 328) is a small mosquito with silvery lines along the 

 back of the thorax and its legs are banded with white. It flies in the 

 daytime and occurs mainly in towns and cities, being only rarely found 

 in the country. Its eggs are laid singly or in small clusters on, or close 



