We have said that the malarial parasite is able to undergo its development only in certain species of mosquitoes belonging to the Anopheline group. It is by no means certain that all of this group even, are capable of acting as the definitive host of the parasites, and much careful experiment work is still needed along this line. In the United States, several species have been found to be implicated, _Anopheles quadrimaculatus_ and _Anopheles crucians_ being the most common. The characteristics of these species and the distinctions between them and other mosquitoes will be discussed in Chapter XII. In antimalarial work it is desirable to distinguish the anopheline mosquitoes from the culicine species in all stages. The following tabulation presents the more striking distinctions between the groups as represented in the United States. _Anopheles_ _Culex, Aedes, etc_. _Eggs_: Laid singly in small Deposited in clumps in the numbers upon the surface of the form of a raft (Culex group) or water. Eggs lie upon their sides deposited singly in the water or and float by means of lateral on the ground in places which expansions (fig. 127). may later be submerged. _Larva_: When at rest floats in When at rest (with few exceptions) a horizontal position beneath the floats suspended in an surface film. No respiratory oblique or vertical position, or tube but instead a flattened more rarely nearly horizontal, area on the eighth abdominal with the respiratory tube in segment into which the two contact with the surface film spiracles open (fig. 128). (fig. 128). _Adults_: Palpi in both sexes Palpi short in the female, in nearly or quite as long as the the male usually elongate. proboscis. Proboscis projecting Proboscis projects forward at an forward nearly on line with the angle with the axis of the body. axis of the body. When at rest When at rest on a vertical wall on a vertical wall the body is the body is usually held parallel usually held at an angle with the or the tip of the abdomen inclined vertical (fig. 128). Wings frequently towards the wall (fig. 128). spotted (fig. 130). Wings usually not spotted. [Illustration: 128. (_a_) Normal position of the larvæ of Culex and Anopheles in the water. Culex, left; Anopheles, middle; Culex pupa, right hand figure.] These malarial-bearing species are essentially domesticated mosquitoes. They develop in any accumulation of water which stands for a week or more. Ponds, puddles, rain barrels, horse troughs, cess-pools, cans, even the foot-prints of animals in marshy ground may afford them breeding places. [Illustration: 128. (_b_) Normal position of Culex and Anopheles on the wall.]