ORGANS OF EESPIRATION. 



41 



Fi?. 50. 



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 tliorax they can be 

 easily d e p r i v e d o f 

 breath and killed. 



" In some aquatic 

 larvffi such as those 

 of Dyticidoi, Eristalis t 

 Fis*9- (t'ig- 51, pupa), and 



Epliydra, and also in some perfect insects, j 

 as in Nepa and lianatra, the parts sup- 

 porting the stigmata are prolonged into slen- 

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

 breathes the atmospheric air. 



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

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

 supposed that these false gills, or branehise, "absorb 

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

 ramifications of the tracheal ves- 

 sels, with which they are 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 branchiffi there are three kinds. The first, as in 

 the larvae and pupse of Gnats, consist of slender fila- 

 ments arranged in tufts arising from a single stem. Fig. 52. 

 In the larva of Gyrinus and the aquatic caterpillar of a moth. 



Fig. 49. Chambei- leading into the trachea; a, a, external valve protecting the 

 outer opening of the stigma, or breathing hole; 6, c, c, inner and more coniplicatcil 

 valve closing the entrance into the trachea (l, k); vi, conical occlusor muscle 

 closing the inner orifice. — From Straus Durckheim. 



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

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

 tracheal branch. — From Straus Durckheim. 



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

 pupa of Agrion enlarged, consisting of a broad leaf-like expansion, permeated by 

 tiachejE which take up by endosmosis the air contained in water.— Original. 



