224 



LECTURE XVII. 



spiral filament is situated between the external cellular, and an in- 

 ternal delicate epithelial lining. 



The trachece commence either from lateral apertures, called spi- 

 racles and stigmata {fig- 102. 

 y), or from pneumatic tubes 

 (z), generally continued from 

 the anal segment ; these latter 

 are peculiar to insects which 

 live in water, as the Nepa 

 and Ranatra. 



The air is conducted from 

 the spiracles or the pneumatic 

 tubes, both which may co- 

 exist together, into a large 

 longitudinal tracheal trunk 

 (<7), which runs near each side 

 from one end of the body to 

 the other; they are connected 

 together by transverse tubes, 

 which run across the posterior 

 margin of each abdominal 

 segment, and distribute an in- 

 finitude of smaller tracheal 

 ramifications. Some of these 

 branches dilate into air re- 

 ceptacles (//), the number and 

 size of which, like the air-cells 

 in birds, are in direct relation 

 with the powers of flight. In 

 the Nepa these reservoirs of air are confined to the thorax : in other 

 insects, as the gras^shopper, they are frequently developed also upon 

 the transverse abdominal tracheee : they are very capacious in the 

 abdomen of the bee. 



The spiracles differ in number and form in different insects. In 

 the Coleoptera there is a spiracle at the interspace between every two 

 segments ; the Diptera have the fewest spiracles. In some insects the 

 orifice is situated upon an entire oval horny ring. In many insects, 

 especially those that burrow, the margins of the spiracles are de- 

 fended by a fringe of hairs, which prevent the entry of extraneous 

 particles. In the mole-cricket, the thickened margin of the spiracle is 

 strengthened by two horny half rings, and can be closed by the 

 action of the small sphincter muscle. 



Most of the aquatic larvae breathe by temporary gills ; in the Phry- 

 ganese tiiese consist of vascular lamiiiEe developed from each side of the 



Respiratory system, Nepa. 



