When full grown they are about a quarter of an inch in length, and vary in colour from bright green, through brown to black. Two principal forms may easily be distinguished by the difference between the structure of their breathing organs. The first and commonest kind, of which the common English gnat (_Culex pipiens, L._) is a good example, has a long breathing tube projecting from the back close to the tail, so that it looks much as if the hinder part of the body were forked. [Illustration: FIG. 7.--Larvæ and pupa of Culex mosquitoes, as seen when viewed from the side of a tumbler or other transparent vessel of water. Drawn from a photograph of the living insects.] As will be seen from the above illustration, only the tip of the breathing tube is kept at the surface of the water, while the body and head hang down into it in a slanting position, so that the head is the most deeply immersed portion of the insect. In the other kind there is practically no breathing tube, the air-vessels opening almost flush with the surface, though in the same part of the body; and hence, in order to keep these openings at the surface, the larvæ must needs lie horizontally at the surface, looking much like a small blackened straw, and on closer inspection are seen to have an outline not unlike the ornamental keyhole plates often seen in old-fashioned furniture. They generally lie with their tails supported against some solid object, such as the side of the saucer in which they have been placed for observation. [Illustration: _PLATE I._ Living _Anopheles_ larvæ. Photographed by Mr. T. H. ROYLE, of Rosa.] Owing to their being confused with the line of optical contact of air and water, and the latter being raised by adhesion into curves over their backs, they do not come out as clearly in the photograph as in a drawing, but they are, nevertheless, sufficiently clearly shown for easy recognition, a matter of some importance, as these are the larvæ of the sub-family _Anophelinæ_, to which belong all the mosquitoes that are concerned in carrying malaria, besides which they are the commonest carriers of blood-worms. The mosquito, however, that conveys yellow fever (_Stegomyia_) belongs to the Culex sub-family. [Illustration: FIG. 8.--(1) Larva commonly found in jhils and tanks (probably of _Myzorrhynchus sinensis_.); (2) frontal bristles of the same; (3) abdominal palmate tuft; (4) separate leaflet of palmate tuft.] For any one possessing a microscope these larvæ form most fascinating examples of “pond-life,” as many species, especially when young, are transparent enough to enable one to follow the entire mechanism of their internal organs; but a great deal of interest can be made out with a simple hand-lens.