In about six days the pupa (Fig. 18) de velops and in a day or two the mosquito begins its flight.
The larva of all mosquitoes have a breath ing-tube near the end of the tail and the pupa have a pair of breathing-tubes projecting from the body near the head.
The insect, both in the larval and the pupal stage, requires air, to obtain which the breath ing-tubes at frequent intervals protrude from the water surface into the air above.
The life cycle of the different mosquitoes varies, as to duration of the formative periods, according to conditions favoring rapid devel opment; and no hard and fast rules can be laid down as absolute in this regard. It may be said, however, that the cistern mosquito breeds, approximately, in one week, the gutter mosquito in two weeks and the swamp mos quito in three weeks.
Observe how both the larva and the pupa rise to the surface to breathe. This necessity suggested that they could be destroyed by pouring oil on the water surfaces so as to cut off the air. It is estimated that about two tablespOonsful of ordinary kerosene' will spread and film the water surface of an average cis tern, and the oil will positively not affect the taste or healthfulness of drinking-water. The placing of oil upon drinking-water for the de struction of mosquitoes is not a new idea, for it was suggested as early as 1812 and has been practised for many years.
How MOSQUITOES TRANSMIT DISEASE.
Mosquitoes and Malaria is one of the greatest scourges to which man is sub ject. Its cause i fully established to be a minute unicellular nimal parasite or hzmos poridium tb, (1, Sporozoa (q.v.), living within the red oioou-corpuscles and introduced by the bite of a mosquito. Each well-marked variety of malaria has its especial causal para site, which passes through a complicated life cycle. (See MALARIA). The parasites enter the blood of the mosquito as minute slender sporozoites. When mature the sporozoites are freed into the body-cavity by the rupture of the wall of the sporocyst, when they migrate to the salivary glands, penetrate their walls and reach the proboscis through the salivary duct. When a mosquito harboring the parasites in this stage bites a susceptible human being, some of the spores pass into the blood with the sa liva and induce an attack of malaria, mild or severe according to their number and other conditions. The malarial organism once intro
duced into the human tiysteiii mae continue to multiply indefinitely by the asexual method un less destroyed by drugs or some reaction of the organism, but that it can be trani,ferred natu rally to another person only through the inter mediation of a mosquito and the intervention of the sex ual generation. Properly speaking, man is the intermediate, the mosquito the final or ( host, though the latter appears to suffer no ill consequences trom the presence of the parasite. In pernicious tropical mala ria, or mstivo-autumnal fever, the period for the complete development of the characteristic sporozoites (Laverania malaria) in the mos quito is seven or eight days, but owing to the vast number of parasites of different broods present in the blood and their overlapping stages of development the exact time of schiz ogony is doubtful. Tertian and quartan fevers, the two best differentiated types of mild or be nignant malarial fevers, are caused by related parasites known respectively as Plasmodium vivax and P. malaria', which differ from Laver ania chiefly in the replacement of the crescent stage by an immediately spherical gametocyte. Asexual sporulation recurs in the first form at intervals of 48 hours, and in the latter of 72 hours, corresponding with rhythm of the fever's paroxysms. Daily or other intermediate recurrences are due to double or triple infec tions, in which the different broods sporulate on alternate days or in other combinations.
It was not until 1880 that Lsveran, a sur geon in the French army, discovered the amcebulx in the blood of malarial patients in Algiers; the development of the sexual genera tion in the mosquito and its relation to 'Laver an's parasite were traced by Ross in 1897-98, while to the Italian zoologist Grassi is due the first clear demonstration of the effectiveness of preventive measures directed at the mosquito.