The latest researches are those of E. Holmgren, who has studied the branching of the tracheæ in the spinning-glands of caterpillars. He prefers to call the end-cells “transition cells,” as they lead from the tracheal tubes proper to the capillary network. This latter is formed by slender nucleated cells, often with an intracellular lumen, and, according to the author, probably constituting a respiratory epithelium. He finds that both large and small tracheæ may penetrate the gland-cells. (Anat. Anzeiger, xi, 1895, pp. 340–6, 3 figs.; Jour. Roy. Micr. Soc., 1896, p. 182.)

b. The spiracles or stigmata

The spiracles are segmentally arranged openings in the sides of the thorax and abdomen, through which the air passes into the air-tubes. In its essential structure a spiracle, or stigma, is a slit-like opening surrounded by a chitinous ring, the lips or edges of the opening being membranous and closed by a movable valve of the spiracle attached by its lower edge, which is closed by an occlusor muscle (Fig. 401). The aperture when open forms a narrow oval slit; and in most insects the slit is within guarded by a row of projecting spines or setæ, which form a lattice work or grate to keep out dust, dirt, fluids, etc.

Fig. 401.—Horizontal section of left third stigma and trachea of Melolontha vulgaris, showing the chamber or drum leading into the trachea: a, a, external frame or valve protecting the outer opening of the stigma; b, c, c, inner frame closing the entrance into the trachea (l, k); m, occlusor muscle closing the inner orifice.—After Straus-Dürckheim.

Krancher[[64]] has described five leading types of stigmata, not, however, taking into account those of the Synaptera.

I. Stigmata without lips (Primitive or generalized stigmata).

a. The simplest stigma is an aperture which is kept open by a chitinous ring (Acanthia). The opening may be round or elliptical. There are no lips nor any movement of the edges to be observed. Such air-holes occur in the abdomen of bugs (Hemiptera) and beetles (Coleoptera); within the opening of the stigmata in the same insects is a funnel-like contraction. Also in the Diptera the abdominal stigmata are of the same type.[[65]] The stigmata of the Pulicidae (Siphonaptera) are more complicated, as the edges of the openings are provided with setæ (Fig. 402).

Fig. 402.—First abdominal spiracle with a part of the trachea of the cat-flea: sp, spiracle; t, trachea.