Fig. 159.—Germ of a hind wing detached from its insertion, and examined in glycerine: i, pedicel of insertion to the hypodermis; tr, trachea; b, semicircular pad; e, enveloping membrane; c, bundle of capillary tracheoles; the large tracheæ of the wing not visible; they follow the course of the bundles of tracheoles.—After Gonin.
Gonin states that before the tracheæ reach the wing they divide into a great number of capillary tubes united into bundles and often tangled. This mass of tracheæ does not penetrate into the wing-germ by one of its free ends, but spreading over about a third of the surface of the wing, separates into a dozen bundles which spread out fan-like in the interior of the wing. (Fig. 159). These ramifications, as seen under the microscope, are very irregular; they form here and there knots and anastomoses. They end abruptly in tufts at a little distance from the edge of the wing. A raised semicircular ridge (b) surrounds the base of the wing, and within this the capillaries are formed, while on the other side they are covered by a cellular layer.
Landois, he says, noticed neither the pedicel of the insertion of the wing (i) nor the ridge (b). Herold only states that the tracheæ pass like roots into the wing. Landois believed that they formed an integral part of it. Dewitz and Pancritius used sections to determine their situation.
Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ and veins. A represents the germ of a hind wing attached to a trachea; c the elongated cells, in which, as seen at B, c, a fine tangled tracheal thread (t) appears, seen to be magnified at C. The cell walls break down, and the threads become those which pass through the centre of the veins.
Fig. 160.—Origin of the wings and their veins.—After Landois.
Fig. 161.-Section of the “rib” of a vein: c, cord; b, twig.—After Schaeffer.
The wing-rods.—Semper discovered in transverse sections of the wings, what he called Flügelrippen; one such rib accompanying the trachea in each vein. He did not discover its origin, and his description of it is said to be somewhat erroneous. Schaeffer has recently examined the structure, remarking: “I have surely observed the connection of this cellular tube with the tracheæ. It is found in the base of the wing where the lumen of the tracheæ is much widened. I only describe the fully formed rib (rippe). In a cross-section it forms a usually cylindrical tube which is covered by a very thin chitinous intima which bears delicate twigs (Fig. 161). These twigs are analogous to the thickened ridge of the tracheal intima. I can see no connection between the branches of the different twigs. Through the ribs (rippen) extend a central cord (c) which shows in longitudinal section a clear longitudinal streaking. Semper regarded it as a nerve. But the connection of the tube with the trachea contradicts this view. I can only regard the cord as a separation-product of the cells of the walls.”
