THE RECTAL TR AC HEAL GILLS 



463 



These differences in the number of functional spiracles are in direct relation 

 with the surroundings of the insects, the physical conditions of existence evi- 

 dently determining the position of the active functional open spiracles and the 



closure of those useless to the organism. 



i. The rectal tracheal gills, and rectal respiration of larval Odonata 



and other insects 



The remarkable mode of respiration by tracheal gills situated 

 within the intestine of the nymphs of dragon-flies was first described 

 by Swammerdain and afterwards by Reaumur. The most complete 

 and best illustrated modern account is that of Oustalet. In these 

 insects the large rectum is lined with six double longitudinal ridges, 

 in .Eschna bearing numerous delicate tubes or papillae, each of 

 Avhich contains very numerous (by estimate 24,000) tracheal branches 

 (Fig. 431) ; while in Libellula the gills are lamellate (Fig. 432). The 

 tracheae arise both from the main dorsal and visceral longitudinal 



FIG. 431. A small part of one leaflet, highly 

 magnified, showing many fine tracheal branches. 

 The portion shown is marked by a small circle in 

 Fur. 480, lower left-hand comer. After Miall. 



FIG. 432. Leaves, in!i, from a lamellate 

 tracheal frill of Libellula : t, trachea. -TMs and 

 Fijr. 429, after Oustalet. 



trunks, which give rise to secondary branches passing into the walls 

 of the rectum and sending into the branchial papillae fine twigs,, 

 which, extending to the distal end of the papilla or lamella, recurve 

 and anastomose with the efferent twigs. 



The anal opening is externally protected by the suranal and lateral 

 triangular chitinous plates, three to five in all. When open, the water 

 passes into the rectum and bathes the rectal gills, where it may be 

 forcibly expelled as if shot out from a syringe, thus propelling the 

 insect forward. In Libellula the anus affords direct access to the 

 intestinal cavity, but in ^Eschna Oustalet describes " a sort of vesti- 

 bule separated from the rectum by a circular valvule." He also 

 states that the inspiration and the repulsion of water is produced at 

 irregular intervals, and rather by the movements of the dorsal and 



