ORGANS OF RESPIRATION. 



41 



serous, and an internal mucous membrane, inclosing between 

 them a spirally convoluted fibre, thus giving great strength 

 and flexibility to the tube." 



Nearly all the air enters through the thoracic and first 

 abdominal spiracles, so that on pinching most insects on 

 the thorax the}' can be 

 U easily deprived of 

 breath and killed. 



" In some aquatic 

 larvjB such as those 

 of Dyticidm, EristaUs 

 Fig. 49. (Fig. 51^ pupa), and 



Epliydra, and also in some perfect insects, 

 as in Nepa and Bcmatra, the parts sup- 

 porting the stigmata are prolonged into slen- Fig. 50. 

 der tubes, through which the insect, on rising to the surface, 

 breathes the atmospheric air. 



Agrion (Fig. 52) affords a good instance of branchiiVi 

 or gill-like expansions of the crust, or skin. It is 

 supposed that these false gills, or branchiae, "aljsorb 

 the air from the water, and convey it by the minute 

 ramifications of the tracheal ves- 

 sels, with Avhich thej^ arc abun- 

 dantly supplied, and which ter- 

 minate in single trunks, into the 

 main tracheae, to be distributed over the whole body, 

 as in insects which live in the open atmosphere." 

 (Newport.) 



Of branchiiie there are three kinds. The first, as in 

 the larvse and pupae of Gnats, consist of slender fila- 

 ments arranged in tufts arising from a single stem. 

 In the larA'a of Gyrinus and the aquatic caterpillar of a moth, 



Fig. 40. Cliambev leading into the trachea; a, a, external valve protecting the 

 outer opening of the stigma, or breathing hole; b, c, c, inner and more complicated 

 valve closing the entrance into the trachea (7, Ic); m, conical occlusor muscle 

 closing the inner orifice. — Fmm Strfins Durckhcim. 



Fig. 50. Portion of a trachea divested of its peritoneal envelope, o, spirally 

 ronvohited fibre, closely wound around the trachea, as at e ; c, origin of a secondary 

 tracheal bran(rh. — From Strniix Durckheim. 



Fig. 52. One of the three gill-like appendages to the abdomen of the larva and 

 pupa of Agrion enlarged, consisting of a broad leaf-like expansion, i)ernieated by 

 trachea; which take up by endosmosis the air contained in water.— OrUjiiud. 

 A* 



Fig. 51. 



Fig. 52. 



