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 appendages of the surface or the rectum, richly permeated by 

 tracheal branches (fig. 495). In such cases the tracheal system has two 

 portions, one which receives oxygen from and gives off carbon dioxide to 

 the water ; the other which supplies the tissues with oxygen and receives 

 carbon dioxide. 



