50 ECONOMIC ENTOMOLOGY 



A nopheles Mosquito and Malaria: 



Malarial fever and ague were common a generation ago, and our 

 fathers vaguely attributed the disease to the presence of swamps 

 whose numbers have fortunately been greatly reduced by drainage. 



The story of the discovery of the causal organism and of its life- 

 history in connection with the Anopheles mosquito is one of the in- 

 teresting chapters in modern biological investigation. The organism 

 belongs to the amoeboid Protozoa and was discovered by Laveran, 

 a French army surgeon, in 1880. The part of Anopheles as a second 

 or intermediate host of the malarial organism was worked out later 

 by Doctors Manson and Ross. (A full account of the discovery will be 

 found in Kellogg's "American Insects.") 



The malarial plasmodium on gaining access to the human body 

 lives within a red blood-corpuscle and thrives at the expense of the 

 haemoglobin. A characteristic excretory product in affected cells 

 is the black granules of melanin. In 48 hours the plasmodium reaches 

 maturity and divides into many spores, termed merozoites, which are 

 set free in the blood. These soon enter new blood-corpuscles and 

 reach maturity in 48 hours as before. This production of spores coin- 

 cides with the characteristic "chill" of ague, and is followed by a fever 

 when the spores enter the blood-corpuscles. As a result of the de- 

 struction of the red blood-cells the patient becomes anamic. Certain 

 of these spores, however, make no attempt to enter new blood-corpuscles 

 and may remain in the blood for an indefinite period. These are the 

 gametes — the micro- and macro-gametes — which, if taken into the 

 stomach of an Anopheles mosquito, will, however, undergo further 

 development. The male or micro-gamete produces a number of 

 whip- like threads or fiagellce, which are capable of uniting with the 

 female or macro-gametes, producing vermicules or ookinetes. These 

 penetrate into the wall of the stomach of the mosquito where they 

 rest as cysts, forming little lumps on the outer surface. These cysts 

 mature in about ten days and burst, liberating large numbers of 

 sporo-blasts into the body cavity, whence they find their way to the 

 salivary glands. When such an affected mosquito bites a human 

 being these spores are injected into the blood and enter the red cor- 

 puscles. It will be seen, therefore, that ordinarily the Anopheles 

 mosquito is the only agency for the transmission of malaria to man 

 (Fig. 43)- 



