Thus for the maintenance of malarial fever the co-existence of three animal organisms are essential, viz., of man, the mosquito, and the malarial protozoon; and it is obvious that even the temporary banishment of either of the three from any given locality will necessarily put an end to all possibility of the occurrence of fever; for man can be infected only by the mosquito, and the mosquito by man, and the presence of both of the others is necessary for the maintenance of the species for the parasite. For the proper contrivance, then, of measures of prevention it is essential that we should be well acquainted with the life-history of our two partners in this curious cycle of development, and as that of the parasite has already been sufficiently described for our purposes, it remains only to describe the leading parts of the life-history of the mosquito. Mosquitoes, or, as they are called in England, gnats, are small two-winged insects whose appearance is quite familiar to most people, though midges so closely resembles them, in general appearance, that they are commonly confused with them, but mosquitoes may be easily distinguished by their possessing a long, trunk-like proboscis which is wanting in the midge, as well as by the fact that, if examined with a strong magnifier, it will be seen that their wings and the greater part of their surface are covered with minute, downy scales exactly like those of butterflies, while midges are quite devoid of any such covering. The males carry a pair of beautiful plume-like feelers, while those of the females, though quite as long, show only a few inconspicuous hairs. Over five hundred species of mosquito have been described; and, as might be expected, their habits vary to some extent; but, speaking generally, they are mainly twilight and nocturnal insects which remain hidden in sheltered places during the day, and come forth to feed and disport themselves in the open at dusk. Owing to the peculiar structure of their mouth-parts, which consist of a long, delicate tube, supported in the midst of a group of lancets, they are incapable of taking solid food, and subsist almost entirely on the juices of living plants and animals, which are sucked up by means of the tube which is introduced into a puncture made by the lancets. With one or two doubtful exceptions, the males of all species live entirely upon the juices of plants, but the females of perhaps the majority of species are not content with so simple a diet, and attack animals of all sorts, puncturing the skin and filling themselves with blood till they are scarce able to fly. No animal is safe from their attacks, and improbable though it may appear, it is recorded, on the authority of a skilled naturalist, that young fish are killed in large numbers by the crowds of mosquitoes which pounce on them as they show their heads and backs at the surface of the water.