The abdominal spiracles present quite a different plan of structure. The external orifice is permanently open, owing to the absence of valves, but communication with the tracheal trunk may be cut off at pleasure by an internal occluding apparatus. The external orifice leads into a shallow oval cup, which communicates with the tracheal trunk by a narrow slit, or internal aperture of the spiracle. The chitinous cuticle, surrounding this internal aperture, is richly provided with setæ, which are turned towards the opening.[151] Fig. 87_C_ represents a spiracle seen from within, and shows that the slit divides the cup into two unequal lips, the smaller of which inclines away from the middle line of the body, is movable, and is strengthened on its deep surface by a curved chitinous rod, the “bow” of Landois. From the opposite lip, a pouch is thrown out, which serves for the attachment of the occlusor muscle. The muscle is inserted into the extremity of the bow, and when it contracts, the bow is pulled over into the position shown in fig. 87_D_, and the opening is closed. The antagonist muscle, which exists in all the abdominal spiracles, is shown in fig. 88; it arises from the supporting plate of the spiracle, and is inserted opposite to the occlusor, into the extremity of the bow. [151] In the first abdominal spiracle the setæ are developed only on that lip which carries the bow. [Illustration: 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._)] [Illustration: Fig. 87.--Four views of the First Abdominal Spiracle (left side). × 70. The bow is shaded in all the figures. (_P. americana._) _A_--The spiracle, seen from the outside; _p_, lateral pouch; _I_, internal aperture. _B_-- Do., side view. _C_-- Do., seen from the inside, the aperture open. The occlusor muscle is shown. _D_--The spiracle, seen from the inside, the aperture shut.] Each of the eight abdominal spiracles is constructed on this plan; the first merely differs from the others in its larger size and dorsal position, being carried upon the lateral margin of the first abdominal tergum, whereas the others are placed on the side of the body, each occupying an interspace between two terga and two sterna. The bow is of about the same length in all; hence the apparent disproportion in the figures of different spiracles. The external aperture of the abdominal spiracles is oval or elliptical, placed vertically and directed backwards. [Illustration: Fig. 88.--Abdominal Spiracle (left side) in side view, showing the bow: × 70; _p_, lateral pouch of spiracle, seen from within. The tesselated structure of the spiracle and trachea is shown at _A_ (× 230), and the margin of the external aperture at _B_ (× 230). (_P. americana._)]