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 Fig. 430, lower left-hand corner.—After Miall.
Fig. 432.—Leaves, mh, from a lamellate tracheal gill of Libellula: t, trachea.—This and Fig. 429, after Oustalet.
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 Æschna Oustalet describes “a sort of vestibule 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 sternal arches of the abdomen than by the contractions of the rectum, since the walls of this organ are less muscular than is supposed.
Fig. 433.—Larva of a mosquito (Culex nemorosus) of middle age, seen from above, the tracheal system omitted: at, antennæ; ab, their middle joint; eg, elastic articular membrane; atm, antennal muscle; atn, antennal nerve; zau, compound; eau, simple eye; os, brain; oex, extensor; ofl, flexor of labrum; ha, neck; œ, œsophagus; spd, salivary gland; mau, cœca; ch, chyle stomach; di, contents of intestine; mg, urinary tubes; dd, ileum; ed, rectum; a, anus; s, sipho; z″, its bristles; kb, tracheal gills: k1, k2, k3, closing lobes of the sipho; kn, basal tubercle of tactile hair; g, its ganglion cell; th, tactile hair of the siphon valve.
The nymph of Calopteryx (and probably of all the group Calopteryginæ) possesses rectal gills besides external caudal tracheal gills. There are three double rectal longitudinal folds or ridges, interpenetrated by tracheal twigs. (Dufour, denied by Poletaiew, but confirmed by Hagen.)
Dewitz claims that the caudal gills of the Agrionidæ are not their sole means of respiration, since he cut off the caudal tracheal gills of an Agrionid nymph, which continued to live for a week. Hence he thinks that there may be a rectal respiration, since under the microscope he saw a stream of water pass in and out of the end of the intestine.
Dewitz’ experiments prove that in young Ephemerids there may be besides branchial, both rectal and skin respiration. He saw under the microscope the anus for a while opened and then closed, causing the rectum to move; powdered carmine mixed with water was drawn into and then expelled from the rectum. There was, however, no enlargement and contraction of the abdomen as in the rectal respiration of Æschna. (Zool. Anz. 1890, p. 500.)
