The spiracles of the Cockroach are by no means of complicated structure, but their small size, and the differences between one spiracle and another, are difficulties which cost some pains to overcome.

Fig. 85.—First Thoracic Spiracle (left side), seen from the out­side. × 70. V, valve; I, setose lin­ing of valve (mouth of tra­cheal tube) × 230. The oc­clus­or mus­cle is shown. The arrow indi­cates the direc­tion of air enter­ing the spir­acle. In the nat­ural posi­tion this spir­acle is set oblique­ly, the slit being inclined down­wards and back­wards. (P. ameri­cana.)

Fig. 86.—Second Thoracic Spiracle (left side), seen from the out­side. × 70. V, lower (mov­able) valve. The oc­clus­or mus­cle is shown. The arrow indi­cates the direc­tion of air enter­ing the spir­acle. (P. americana.)

The first thoracic spiracle (fig. 85) is the largest in the body. It lies in front of the mesothorax, between the bases of the first and second legs. It is placed obliquely, the slit being inclined downwards and backwards, and is closed externally by a large, slightly two-lobed valve, attached by its lower border. The aperture immediately within the valve divides into two nearly equal cavities, each of which leads to a separate tracheal trunk; and between these cavities is a septum, thickened on its free edge, against which the margin of the valve appears to close. A special occlusor muscle arises from the integument below the spiracle, and is inserted into a chitinous process which projects inwardly from the centre of the valve. A second muscle, whose connections and mode of action we have not been able to make out satisfactorily, lies beneath the first, and is inserted into the thickened edge of the septum.

The second thoracic spiracle (fig. 86) lies in front of the metathorax, between the bases of the second and third legs. It is much smaller and simpler than the first. Its valve is nearly semi-circular, and the free border is strengthened on its deep surface by a chitinous rim, which terminates beyond the end of the hinge of the valve in a process which gives insertion to the occlusor muscle.