The capsule of the mature zygote ruptures about the tenth day and the sporozoites are thrown off into the body cavity. They make their way to the salivary glands and thence, by way of the veneno-salivary duct, in the hypopharynx, they are introduced into the circulation of the person bitten by the mosquito, and start a nonsexual cycle. As the sexual life takes place in the mosquito, this insect is the definitive host and man only the intermediate host. The sexual cycle or _sporogony_ in the mosquito takes about ten to twelve days. =Efficient Mosquito Hosts.=—It must be remembered that only certain genera and species of Anophelinae are known malaria transmitters; thus Stephens and Christophers, in dissecting 496 mosquitoes of the species _M. rossi_, did not find a single gland infected with sporozoites. With _M. culicifacies_, however, 12 in 259 showed infection. A mosquito which is capable of carrying out the complete sporogonous cycle is an efficient host and in the case of malaria the mosquito is the definitive host (sexual life of parasite). =Malarial Index.=—Mosquito dissection is one method of determining the endemicity of malaria or the _malarial index_. There are two other methods: 1. by noting the prevalence of enlarged spleens, and 2. by determining the number of inhabitants showing malarial parasites in the blood. This index is best determined from children between two and ten years of age, as children under two show for a general average too high a proportion of parasites in the peripheral blood while those over ten years of age show too great an incidence of enlarged spleens. Barber working in the Philippines with children from five to ten years of age obtained a spleen index of 13.3 and a parasitic index of 11. =As Before Stated there are Three Species of Malarial Parasites:= 1. _Plasmodium vivax_, that of benign tertian—cycle, forty-eight hours; 2. _Plasmodium malariae_, that of quartan—cycle, seventy-two hours; and 3. _Plasmodium falciparum_, that of aestivo-autumnal or malignant tertian—cycle of forty-eight hours. =Multiple Infections.=—Variations in cycles may be produced by infected mosquitoes biting on successive nights, so that one crop will mature and sporulate twenty-four hours before the second. This would give a quotidian type of fever. In aestivo-autumnal infections anticipation and retardation in the sporulation cause a very protracted paroxysm, lasting eighteen to thirty-six hours; this tends to give a continued or remittent fever instead of the characteristic intermittent type.