DRAGON-FLIES 



421 



of Odonata are still very incomplete, but some interesting points 

 have been ascertained, the most important of which is perhaps 

 the existence in some forms of a 

 respiratory system in connexion 

 with the posterior part of the ali- 

 mentary canal (Fig. 269). In the 

 nymphs of Anisopterides the system 

 consists of four main tracheal 

 trunks, traversing the length of 

 the body, and by their ramifica- 

 tions and inosculations forming an 

 extensive apparatus. Connected 

 with the fou.r main trunks we 

 have described, there is a shorter 

 pair confined to the abdomen, 

 where it supplies a large number 

 of branches to the walls of the 

 stomach. The dorsal pair of the 

 main tubes give numerous sub- 

 sidiary branches to the outside of ^^^_ 269.-Portion of tracheal system 

 the rectum, and the ventral pair of nymph of Aeschna cyanea. 



furnish a smaller number. The 

 walls of the gut are penetrated by 

 the branches, which inside the 

 rectum form numerous loops ; these, covered by a membrane, 

 project into the interior in the form of multitudinous papillae 

 (j^^schninae). In the Libellulinae the papillae are replaced by 

 more flattened processes or lamellae. The structures attain a 

 remarkable development, there being in Aeschna cyanea upwards 

 of 24,000 papillae. 



These rectal gills obtain air from water admitted into the 

 rectimi for the purpose ; the extremity of the body being armed 

 with projections of variable form, that can be separated to 

 allow ingress and egress of the fluid, or brought into close 

 apposition so as to close the orifice. The water so taken in 

 can, by some species, be ejected with force, and is used occa- 

 sionally as a means of locomotion. These rectal branchiae can 

 absorb free air, as well as air dissolved in water. If the fluid 

 in which the creatures are placed has been previously boiled, 

 so as to expel the air from it, the nymphs then thrust the 



Rj R, R, R, rectum ; A, anus ; td, 

 dorsal ; tv, ventral, tracheal tubes ; 

 il/, Malpighian tubes. (After Ous- 

 talet.) 



