4. Lining the internal canal a chitinous layer penetrated by pore-canals, the intima often wanting.
The secretory cells are usually of the same size, but in many cases are relatively small; sometimes four to six or more form the periphery of the canal, sometimes three or only two. In some insects the cells are so very large that a single cell forms the entire periphery. The nuclei in the Lepidoptera (Papilio, Pontia, Cossus) are large and irregularly branched.
The excretions of the Malpighian vessels, derived from the blood and from the fat-body, are more or less fluid and granular, sometimes pulpy. From the cells they pass into the canal, thence into the intestine, and thence out of the body. How, says Kolbe, the secretion passes into the intestine, whether by the contraction of the fine fibrillæ of the peritoneal membrane, or by the external pressure of the other organs, or by the pressure of the secretory matter behind, is not yet known. Grandis observed in living Hydrophilus that the urinary tubes moved, without the muscles seeming to show what caused the motion. Moreover, the cells incessantly changed their form. At a lower temperature such motions ceased. The tracheæ, ending freely in the cells, did not anastomose. (Kolbe.)
The different colors of the tubes (white, yellow, red, brown, or green) is due to the hue of the excretions, and is independent of the color of the blood and of the urinary substances held in the secreted matter.
Schindler found that insects of different stages, collected in winter, differed very much in their urinary secretions, the tubes in the adults being entirely empty, while in the larvæ they were filled full, so that he concluded that in the former the process of excretion during the winter hibernation is very slow, but in the latter very rapid.
As to the activity of the urinary vessels the following experiments will throw some light. Tursini fed a Pimelia with fuchsin; its urinary tubes were consequently colored red. Schindler fed insects with indigo-carmine, which was excreted by the urinary tubes; Kowalevsky arrived at the same results, which seems to prove that these vessels are analogous to the kidneys of vertebrates. Moreover, Schindler injected through the side of the first abdominal segment into the cavity of the body of a Gryllotalpa a concentrated solution of sodium salt of indigotin-disulphonic acid. After one or two hours the external portion of the epithelium of the urinary vessels was stained deep blue, while the inner portion remained of the normal transparency; the nuclei being for the most part deeply stained. Between one and two days after, the staining matter had not yet wholly passed through the central canal, the surface recently stained still appearing light blue.
The solid contents of the urinary tubes consist partly of crystals, which occur singly in the epithelial cells, or form scattered masses when situated in the central canal. Besides tabular rhombic crystals, there occur concretions which contain uric acid, and probably consist of urate of soda, also octahedral crystals of chloride of soda, and quadro-pyramidal crystals of oxalate of lime. Also acicular prisms occur; besides chloride of soda, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, coloring matters, etc.; while the fluid secretion also contains urea (?), uric acid, and abundant urates; uric acid crystals were precipitated by the addition of acetic acid, and by adding hydrochloric acid crystals belonging to the dimetric system were formed. The often numerous spheroidal small granules are biurate of soda and biurate of ammonia. Pale, concentrically banded concretions are leucine pellets.
According to Kölliker the contents of the urinary vessels[[55]] in general are: (1) round granules of urate of soda and urate of ammonia; (2) oxalate of lime; and (3) pale transparent concretions of leucine. Crystals of taurin are also said to occur. (Claus’ Zoölogy, p. 531.)
Although uric acid is characteristic of the urinary tubes, yet sometimes it is wanting in them, while uric acid substances in quantity occur in the fat-body or in the mid-intestine.
In the living insect the urinary tubes remove urates from the blood; “the salts are condensed and crystallized in the epithelial cells, by whose dehiscence they pass into the central canals of the tubules and thence into the intestine.” (Miall and Denny.)