3. Amphipneustic type.—Larvæ with a pair of open spiracles situated at each end of the body, the intermediate spiracles being closed. (Most dipterous larvæ, Musca, after the first moult, Œstridæ, Asilidæ, and Syrphus.)
Fig. 429.—Branchial tuft of nymph of Æschna.
4. Peripneustic type; with prothoracic and abdominal spiracles, the mesothoracic pair atrophied or closed. (The larvæ of Neuroptera, Mecoptera, Trichoptera, Lepidoptera, of most Coleoptera,[[72]] of most Diptera, and of most of the Hymenoptera.[[73]])
Fig. 430.—Part of three rows of respiratory folds from cuticular living rectum of Æschna. The shaded parts are abundantly supplied with tracheal tubes. The leaflets appear to be connected with a central trachea, but this is not really the case.—After Miall.
These differences in the number of functional spiracles are in direct relation with the surroundings of the insects, the physical conditions of existence evidently 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 Swammerdam and afterwards by Réaumur. 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 Æschna bearing numerous delicate tubes or papillæ, each of which contains very numerous (by estimate 24,000) tracheal branches (Fig. 431); while in Libellula the gills are lamellate (Fig. 432). The tracheæ arise both from the main dorsal and visceral longitudinal trunks, which give rise to secondary branches passing into the walls of the rectum and sending into the branchial papillæ fine twigs, which, extending to the distal end of the papilla or lamella, recurve and anastomose with the efferent twigs.
