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The development and structure of the tracheæ and veins of the wing.

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.