Finally, Wheeler has discussed at length these embryonic organs, which he regards as glandular structures, and calls pleuropodia, their primitive function having been that of limbs. He has detected them in the embryo of Periplaneta orientalis, Mantis carolina, Xiphidium ensiferum (Fig. 387); also in the Hemiptera (Cicada septemdecim, Zaitha fluminea), and in Sialis infumata. He discards the view that they were once gills or sense-organs, and concludes that they were glands. But, as we have suggested, their function once that of gills, and still respiratory in Synaptera, has perhaps become in the winged insects glandular and repugnatorial. Instead, then, of being modified abdominal limbs afterwards serving as glands, as Wheeler claims, we are inclined to believe that they functioned as blood-gills.
k. Tracheal gills of adult insects
Tracheal gills are known to be retained by a few insects in the imago stage, the nymphs in all stages breathing by them. The most notable example is the perlid genus Pteronarcys, in which, as Newport states, there are eight sets, comprising 13 pairs of branchial tufts distributed over the under surface of the thoracic and first two abdominal segments.
The first set, consisting of three pairs of tufts, partly encircling the neck like a ruff, arises from the soft membrane connecting the head and prosternum. The thoracic tufts originate between and behind the coxæ, as well as on the front margin of the meso- and metathoracic segments. The number of filaments in each tuft varies from about 20 to 50 or more, the densest tufts being those of the two hinder thoracic segments. Each filament is usually simple, though in a few cases they are branched (Fig. 457, A).
The adult Pteronarcys is nocturnal, flying only at dewfall or in the night, and Mr. Barnston observed it when on the wing, “constantly dipping on the surface of the water”; by day it hides “in crevices of rocks which are constantly wetted by the spray of falling water, under stones and in other damp places.” It may thus be compared with the Amphibians, Necturus and Proteus, whose gills are retained in adult life. A similar large Chilian Perlid (Diamphipnoa lichenalis Gerst.) differs in completely lacking the thoracic gills, though there are four pairs on the abdomen, i.e. a pair on each of the first four segments. In this form the number of individual filaments in the largest tufts may amount to about 200.
Another Perlid (Dictyopteryx signata) is said by Hagen to have two pairs of gill-tufts on the under side of the head; the first pair situated on the base of the submentum, the second on the membrane connecting the head and prosternum.
Kolbe states that in the imagines of Perla marginata and P. cephalotes on the hinder edge of the thoracic stigmata arise three very small chitinous plates, which, on their under side and on the edges are beset with numerous short white filaments. These completely correspond to the filaments of the tuft-like larval gills. Persistent anal gills also occur in the imagines of Perla.
Fig. 457.—Under side of Pteronarcys regalis, showing the situation of the gills (g, b, f) and the sternal orifices: A, a branchial filament showing the direction of the current of blood; c, d, tracheæ. B, end of the abdomen enlarged.—After Newport.
In Nemoura lateralis and cinerea the tracheal gills are differently disposed. On each side of the anterior edge of the prosternum arise delicate tightly twisted filaments, like those of the larva. (Einführung, p. 536.)