wings, would be most successful in the struggle for existence. Thus also, not being so much needed in larval life before the reproductive organs are developed, they would not be transmitted except in a very rudimentary way, as perhaps masses of internal indifferent cells (imaginal discs), to the larva, being the rather destined to develop late in larval and in pupal life. Thus the development of the wings and of the generative organs would go hand in hand, and become organs of adult life.[33] =The development and structure of the tracheæ and veins of the wing.=—The so-called veins (“nervures”) originate from fine tracheal twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing. [Illustration: 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. [Illustration: FIG. 160.—Origin of the wings and their veins.—After Landois. ] [Illustration: FIG. 161.-Section of the “rib” of a vein: _c_, cord; _b_, twig.—After Schaeffer. ]