under two heads, viz. the external organs by which the air is admitted into the body and expelled from it, and the internal ones by which it is distributed.
The former differ according as the place inhabited is land or water; in other words, according as the air is received directly, or through the medium of another element.
Nearly all insects inhabiting the land and the air, as well as the amphibious species (Dytiscidæ, &c.), receive their supply immediately from the atmosphere which surrounds them on all sides, and it is admitted by apertures named spiracles or stigmata. These are small perforations, commonly of an oval or rounded form, placed along the lateral margins of the body. In number, and almost every other respect, they vary greatly, but their arrangement is always symetrical, one on the right side, and another on the left; each segment, for the most part, being thus furnished with a pair. (Pl. III. fig. 3.) They never exist in the head, and there are never more than two pair in the thorax, consequently the greater number are to be found in the abdomen. They are usually surrounded by a horny ring, and their aperture can be closed at the will of the insect by means of a muscular apparatus. To enable it to do this more effectually, the mouth is sometimes furnished with plates which close like shutters, or it is fringed with hairs, ciliæ, &c. Two of their most simple forms are represented on Plate III. figs. 4 & 5. A more complicated structure is exhibited by the hinder stigmata of Dytiscus marginalis, (Pl. III.
