98 INSECTS AND MAN 



frequent inhabited places ; in fact, their name Stegomyia is 

 derived from two Greek words meaning, collectively, a 

 house fly. As carriers of yellow fever they are of the 

 utmost importance, but we shall deal with them later. 



Into the clinical aspects of malaria in its three forms, 

 malignant, quartan, and tertian, we cannot enter here, but 

 a few words concerning the blood parasite causing the 

 disease may make the role of the malaria mosquito a little 

 more easy to understand. Let us suppose that a female 

 malaria mosquito has just imbibed a drop of blood from 

 an infected man : along with the blood, and in the blood 

 corpuscles, several exceedingly minute structures, known 

 as gametocytes, pass into the stomach of the insect (fig. 

 21, A). These blood parasites are not all of the same size, 

 the smaller ones, called microgametocytes, carry out male 

 functions, whilst the larger macrogametocytes may be re- 

 garded as females. Numerous other forms may be taken 

 up along with the blood, but, as they perish, we will dis- 

 miss them for the moment. The female parasite undergoes 

 various changes, which we need not detail, and eventually 

 sends out a small bud-like structure, in which is enclosed 

 some of the original cell contents : at this stage the organism 

 is ready for fertilisation (fig. 21, c $). The male parasite, 

 meanwhile, has also undergone changes, resulting in the 

 formation of several thread-like outgrowths (fig. 21, c ), 

 which eventually become free and wriggle about actively. 

 One of these free-swimming thread-like bodies, micro- 

 gametes, fuses with the female parasite (fig. 21, D), and the 

 resulting body is an active organism, with worm-like move- 

 ments, hence called a vermiculus (fig. 21, E). The vermi- 

 culus penetrates the wall of the mosquito's stomach and 

 passes to the external muscular layers, where it becomes 

 stationary, but grows rapidly, and its nucleus becomes 

 much divided (fig. 21, G), so that eventually the whole 

 structure is filled with a large number of somewhat rod- 

 like bodies known as sporozoites. These sporozoites are 



