. A manual of zoology. Zoology. IV. INSECTA: HEXAPODA. 469 much probability connected with hearing. These are thin drum- like parts of the chitin, framed in thicker portions (figs. 492, 493), beneath which is a tracheal vesicle, with a nerve ending in a ' crista acustica.' The power of producing sound is widely distributed and often highly developed, the organs for this purpose var^ang widely in character. Stridulating organs are formed by ridges on wings and legs, which are rubbed against each other or against similar ridges on the body. Humming is produced by the action of the. Fia. 492. Flu
. A manual of zoology. Zoology. IV. INSECTA: HEXAPODA. 469 much probability connected with hearing. These are thin drum- like parts of the chitin, framed in thicker portions (figs. 492, 493), beneath which is a tracheal vesicle, with a nerve ending in a ' crista acustica.' The power of producing sound is widely distributed and often highly developed, the organs for this purpose var^ang widely in character. Stridulating organs are formed by ridges on wings and legs, which are rubbed against each other or against similar ridges on the body. Humming is produced by the action of the. Fia. 492. Flu. 4u;j. Fig. 492.—Sirle view of gras-shopper. r, spiracles ; t, tympanic ort^an. Fig. 493.—Anterior tibia of a Locustid with tympanum, t. (From Hatschek, after Fischer.) wings or by the passage of air through the spiracles, which are often provided with vibrating membranes which also serve to close these openings. The trachese (figs. 477, 494) are usually united, just inside the spiracles, by longitudinal trunks from which fine branches extend, enveloping and penetrating all the organs with delicate silvery threads. This connexion of tracheae renders it possible for the spiracles of some segments to disappear. The spiracles of the abdomen are the most constant, usually occurring in the soft mem- brane between the sternites and tergites; the thorax at most has but two pairs, the head none. In insects with good powers of flight many of the tracheal trunks are exjoanded to large air sacs, which may be of value as reservoirs of air, so that the ordinary respiratory motions are less necessary during flight. Au interesting adaiitation of the tracheal system to aquatic life occurs in the larva3 of many Archiptera (Odonata and Mayflies) and Neuroptora, and even among Lepidoptera {Paraponijx) and Ooleoptera (Gyrinidte). The spiracles here are usually closed, and the taking of oxygen occurs either through the skin or by means of so-called tracheal gills—bushy or leaf-like appendage
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