Class 3. Tnsecta. 241 



In order tliat the air sliall liave access to the most distant branches 

 ■of the system^ the trachea arising from the stigmaj possesses a 

 peculiar apparatus by which it may be completely closed ; when this 

 is effected^ and the abdomen contracts^ air cannot escape from the 

 stigma, and is, therefore, driven into the ultimate branches and into 

 the vesicles. When the abdomen is relaxed, and the closing 

 apparatus opened, a new supply of air streams in ; then by the 

 contraction of the abdomen, and the simultaneous closing of the 

 stigma, this fresh supj)ly is driven again into the finest branches and 

 vesicles. By repetition of these processes, the tracheal system is 

 filled with air, and the vesicles completely expanded; this is of 

 ■especial significance in its function as aerostatic apparatus ; and 

 laefore flight, individuals may be seen expanding with air thus 

 pumped in. 



The traoliesB arise as epidermal invaginations, wHoh branch freely, and 

 unite to form the large longitudinal ti-unks ; they secrete a cuticle just as does 

 the epidermis. The apertures of the invaginations form the stigmata. At an 

 ecdysis, the cuticle of the tracheae is also renewed, the old cuticle being thrown 

 out through the stigmata. 



In a number of aquatic insectan larv^ (Dragon-fiies, May-flies, Neuroptera), 

 the tracheal system is closed, i.e. without open stigmata. The oxygen s 

 'Obtained eudosmotically from the water by means of the "tracheal gills," 

 membranous appendages, with large sui-faces, and containing a close tracheal 

 network. 



In addition to the respiratory and aerostatic function, the tracheal system of 

 many Insects is also concerned in the production of sounds. Thin 

 membranous folds often occur within the tracheae, close to the stigmata ; they 

 are set in vibration by the air entering the tracheae, and thus produce a sound 

 (buzzing of Plies and of Cockchafers). The noises made by Insects are also 

 brought about in many other ways. rUes, Bees, and Gnats, all of which produce 

 sounds by the vibration of the tracheal folds, can also make humming noises by 

 the rapid movements of the wings. Others iTib vaiious parts of the surface 

 •of the body against one another. The male Grasshopper, for instance, rubs a 

 dentate ridge of the tibia of the last leg against the elytron of the same 

 ■side ; others again, strike some part of the body against a foreign object (the 

 Fiddler-beetles for instance, knock then- heads against the wall of the 

 passages they have gnawed in wood, and thus cause the well-known ticking 

 sound). 



Some Insects are luminous in the dark. The light proceeds from large 

 cells situated within the body beneath transparent arese of the skin, and depends 

 upon the oxidation of certain substances present in the cells, which are therefore 

 surrounded by a close tracheal network. 



The vascular system is but little developed, a circum- 

 istance correlated with the high specialisation of the respiratory 

 apparatus. Since air is carried direct to all parts of the body, the 

 importance of the blood as oxygen-carrier is necessarily limited. In 

 the dorsal region of the abdomen there is a tubular heart, closed 

 iDehind, open in front, and constricted into a series of chambers 

 corresponding with the abdominal segments ; each chamber is fur- 

 nished with a pair of o s t i a provided with valves, and there are 



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