Since that time there have been many practical demonstrations of the fact that malaria is transmitted exclusively by the bite of mosquitoes and that the destruction of the mosquitoes means the elimination of the disease.
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 numbers upon the surface of the water. Eggs lie upon their sides and float by means of lateral expansions ([fig. 127]). | Deposited in clumps in the form of a raft (Culex group) or deposited singly in the water or on the ground in places which may later be submerged. |
| Larva: When at rest floats in a horizontal position beneath the surface film. No respiratory tube but instead a flattened area on the eighth abdominal segment into which the two spiracles open ([fig. 128]). | When at rest (with few exceptions) floats suspended in an oblique or vertical position, or more rarely nearly horizontal, with the respiratory tube in contact with the surface film ([fig. 128]). |
| Adults: Palpi in both sexes nearly or quite as long as the proboscis. Proboscis projecting forward nearly on line with the axis of the body. When at rest on a vertical wall the body is usually held at an angle with the vertical ([fig. 128]). Wings frequently spotted ([fig. 130]). | Palpi short in the female, in the male usually elongate. Proboscis projects forward at an angle with the axis of the body. When at rest on a vertical wall the body is usually held parallel or the tip of the abdomen inclined towards the wall ([fig. 128]). Wings usually not spotted. |
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.