INSECTS AND HUMAN DISEASE 99 



liberated into the mosquito's body cavity by the bursting 

 of the envelope in which they were formed (fig. 21, H), and 

 about ten days after the meal of infected blood, they find 

 their way to the salivary glands of their host, which is 

 now in a position to infect the first human being on which 

 it feeds. With the first puncture of the human skin, 

 countless numbers of the sporozoites are injected into the 

 wound along with the mosquito's saliva. Each sporozoite 

 enters a red- blood corpuscle, loses its elongate form, and 

 assumes a " signet-ring " form (fig 21, J). After various 

 changes, accompanied by a deepening of colour, the parasite 

 appears to have assumed the form of a rosette within the 

 blood corpuscle (fig. 21, L) ; in reality, the original organism 

 has divided up into a number of smaller ones, known as 

 merozoites. At this stage the wall of the corpuscle bursts 

 once again, setting free the parasites in the blood stream. 

 Not long, however, do they remain at liberty ; they soon 

 attack healthy red corpuscles, and the process is repeated, 

 further rosette formations appear, and further organisms 

 are set free, as the parasitised blood corpuscles burst. 



Only when a large number of blood corpuscles have been 

 destroyed does the patient feel any ill effects; the inter- 

 vening period is the incubation period of the disease. 

 Eventually this form of increase by division, asexual re- 

 production, as it is called, gives place to sexual repro- 

 duction, and this can only take place in the body of a 

 mosquito, and a malaria mosquito at that. If no mosquito 

 of the requisite species comes along to taste the blood of 

 the malaria patient at this stage, the parasites perish, as 

 they do, too, if imbibed by some non- malarial mosquito. 

 The sexual changes of the malarial blood parasite can only 

 take place within the bodies of certain mosquitoes, though 

 why they should not be able to live within every species 

 of blood-sucking mosquito has never been satisfactorily 

 explained. It is one of nature's secrets, destined some day, 

 perhaps, to be learned as a result of careful research, or 



