Insects have six jointed legs, a pair of antennæ or horns (as they are called) and generally one or two pairs of wings. The head is joined to the body by a constricted neck, the part of the body to which it is joined is called the thorax, and to this is added the posterior part or abdomen; this part is extremely various in form in different insects, in some it is round and full, in others long and extended. The antennæ arise from the head, and are generally composed of eleven pieces variously disposed; these wonderful organs are possessed of great sensibility, and they certainly serve to convey information to the insect, of the nature of one of the special senses; it was formerly thought to be simply that of touch very much refined, or of smell, but it is now generally considered to be that of hearing, or a modification of it. The forms of the antennæ are very various, fig. 17 represents that of the Cockchaffer (Melolontha vulgaris). The legs proceed from the thorax, as do the wings, the abdomen giving rise to none of the extremities; the feet of insects are all pretty much upon the same model, some being more developed than others, they have a pair of hooks or claws for catching and clinging to rough surfaces, and a pair of cushions or pads, covered in some cases with suckers. The foot of the Fly (fig. 18) is well developed in this particular, enabling it to walk with perfect ease even on glass or the smoothest surface, and in any position. Insects do not breathe by lungs or gills, but by means of branched tubes called "tracheæ," which convey the air to the interior of the animal. On the surface of these, the blood vessels ramify, giving out the carbonic acid of the blood and receiving oxygen; the openings to these air-tubes are arranged along the sides of the body, and are called "spiracles" (fig. 19). They are curious objects under the microscope, some of them being closed by a perforated membrane, others have a sort of sieve or network of fibres, and most of them a contrivance to catch and exclude minute particles of matter floating in the air, and thus prevent their entry into the tracheæ. Insects have two eyes, one on each side of the head, which are of great size, often forming complete hemispheres; each eye is made up of several thousand separate eyes or "ocelli," hence they are called compound eyes; these ocelli are placed closely together, so as to form a sort of honeycomb arrangement; the appearance of a part of one of these eyes is shown in fig. 20. [Illustration: FIG. 19.--SPIRACLE.] [Illustration: FIG. 20.--COMPOUND EYE. (1, perpendicular section; 2, surface.)]