PROTOZOA AND DISEASE 89 



the sexual cycle of the parasite may be initiated. This sexual cycle, 

 however, does not occur in the body of man but must take place in a 

 mosquito. If these sexual forms, which are the microgametocytes and 

 macrogametocytes, do not enter the body of a mosquito, they do not 

 further develop. 



The mosquito which is capable of transmitting the malarial parasite 

 belongs to the genus Anopheles. It is distinguishable from the common 

 mosquitoes, which belong to Culex, by the fact that it holds its body at an 

 angle to the surface on which it rests. The body of a culicid mosquito 

 is held parallel to such a surface. When the mosquito bites, it drills a 

 hole through the epidermis with its proboscis and penetrates the vascular 

 dermis. Then it injects into the wound saliva the effect of which is to 

 prevent coagulation of the blood and thus permit the mosquito to suck 

 until filled. It is the irritation caused by the saliva that produces the 

 itching which is so often a feature of these bites. 



If in the blood sucked up by the mosquito there are only ordinary 

 spores, the mosquito does not become infected and is not capable of 

 transmitting the infection. If, however, there are microgametocytes and 

 macrogametocytes, these give rise in the stomach of the mosquito, 

 respectively, to microgametes and macrogametes, which unite to form 

 zygotes. These zygotes become elongated, exhibit a gliding movement, 

 penetrate the wall of the stomach, and encyst just beneath the outer 

 layer. In these cysts are produced a great many spores, which, when 

 they are set free, make their way through the body of the infected mos- 

 quito to the salivary gland, in the cavity of which they accumulate. 

 When this mosquito bites another person these spores are injected into the 

 wound made by the proboscis, along with the saliva. In the blood 

 they enter the red blood corpuscles, become ameboid, and thus another 

 asexual cycle is begun. 



It is evident from this outline of the life cycle that after biting a 

 marlarial individual and acquiring the infection, the mosquito cannot at 

 once transmit the disease. It is necessary for such a transmission that 

 there shall be sexual spores in the blood of the person bitten and that 

 they shall be taken up by the mosquito. A sufficient length of time must 

 also elapse for the sexual cycle to be completed and for spores to form from 

 the zygote. This takes, on the average, about twelve days, though the 

 time varies with the form of the disease and environmental conditions, 

 such as temperature. 



In the absence of man the female mosquito feeds on the blood of other 

 animals or upon the juices of plants. The male mosquito does not feed, 

 nor does it bite, and therefore cannot become infected and transmit the 

 disease. The sexes may be distinguished by their antennae; those of the 

 male are complexly branched and have a feathery appearance, while those 

 of the female are simple, straight, and hairlike. 



