The process of excretion is carried on not only by the urinary tubes, but also, as Cuénot has recently shown (1896) in Orthoptera, by the pericardial cells and certain cells of the fat-bodies. In the last-named cells urates are stored throughout life; the pericardial cells apparently secrete but do not store waste products, which are finally eliminated by the urinary tubes, the latter constantly eliminating waste.
Primitive number of tubes.—Wheeler considers the primitive number of urinary tubules to be six, other authors regarding two pairs as the primary or typical number; and while Wheeler agrees that the more ancestral tracheate arthropods had but a single pair, Cholodkowsky supposes the primitive number in insects themselves to be a single pair. This view is strengthened by the fact that Scolopendrella has but a single pair (Fig. 15).
While Peripatus has no urinary tubes, in Myriopods a single pair arises, as in insects, from the hind-intestine.
Fig. 347.—Section of proctodæum of embryo locust, showing origin of urinary tubes (ur.t); ep, epithelial or glandular layer; m, cells of outer or muscular layer; a, section of a tube.
When in insects the number of these tubes is few, they are, with rare exceptions, arranged in pairs, so that Gegenbaur and others have considered this paired arrangement as the primitive one. When the tubules are very numerous in the adult, as in Orthoptera, the embryos and larvæ have a much smaller number, Wheeler stating that “in no insect embryo have more than three pairs of these vessels been found.” We have observed 10 primary tubes in the embryo of Melanopus (Fig. 347), from each of which afterwards arise 15 secondary tubules. In the Termites, only, do the young forms have more urinary tubes than the adults.
In Campodea there are about 16 urinary tubes and in Machilis either 12 (Grassi) or 20 (Oudemans); but in other Thysanura the number is much less, Lepisma having either four, six, or eight, according to different authors, and both Nicoletia and Lepismina having six, opening separately into the hind-intestine. On the other hand, these organs have not yet been detected in Japyx. Whether they exist at all in the Collembola, which are degenerate forms, is doubtful. The weight of opinion denies their existence, though they may yet be found existing in a vestigial condition. They are said by Tullberg and by Sommer to exist in Podura, but are of peculiar shape.
Coming now to the winged insects, in what on the whole is perhaps the lowest or most generalized order, the Dermaptera, the number is over 30, and their insertions regularly encircle the intestine. (Schindler.) In the most ancient and generalized family of Orthoptera, the Blattidæ, Schindler detected from 60 to 70 tubes, but in a nymph of Periplaneta not quite 10 mm. in length he found from 16 to 18, and in nymphs 4 to 5 mm. long there were only eight vessels; while Wheeler has found in the embryo of Phyllodromia germanica but four tubes. In the adult Acrydiidæ there are as many as 150, in the Locustidæ between 40 and 50, and in the Gryllidæ about 100.
The Ephemeridæ with about 40, the Odonata with 50 to 60 tubules, the Perlidæ with from 50 to 60, are polynephrious; while the Termitidæ and Psocidæ are oligonephrious, the former having from six to eight and the Psocidæ only four tubes. So also all the other orders not mentioned, except the Hymenoptera, have few of these tubes. The Hemiptera, with none in Aphidæ, a single pair in the Coccidæ, and two in all the rest of the order, have the fewest number.
In the Neuroptera there are from six to eight, while in a larva, possibly that of Chauliodes, Wheeler finds the exceptional number of seven.