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 they can be 



easily deprived of 



breath and killed. 

 " In some aquatic 



larvae such as those 



of Dyticidce, Eristalis 

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



Epliydm, and also in some perfect insects, 

 as in Nepa and Ranatra, 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 branchiae 

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

 supposed that these false gills, or branchiae, "absorb 

 the air from the water, and conve}< it by the minute 



ramifications of the tracheal ves- (/ 

 sels, with which they are abun- 



dantly supplied, and which ter- 

 Fig. si. 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 branchiae there are three kinds. The first, as in 

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

 ments arranged in tufts arising from a single stem. Fi s- 53 - 

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



FIG. 49. Chamber leading into the trachea; a, a, external valve protecting the 

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

 valve closing the entrance into the trachea (Z, k); TO, conical occlusor muscle 

 closing the inner orifice. From Strait* Diirc/cliciin. 



FK;. 50. Portion of a trachea divested of its peritoneal envelope, a, spirally 

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

 tracheal branch. from Strait* Durclshchn. 



Fi<;. ") - 2. 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, permeated by 

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

 4* 



