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Fig. 155.—Wings of nymph of Psocus.

“After the last moult, however, when the supply of moisture is very much reduced in the wing-pouches, which are contracted at the bottom, their two layers become closely united, and afterward grow into one single, solid wing-membrane.

“These thick-walled blood-tubes arising above and beneath the upper and lower membrane of the wing are the veins of the wings; the development of the creased wings in the pupa of butterflies is exactly like that of cockroaches and bugs. The difference is only that the folds of integument furnishing the wings with an ample store of material for their construction reach in a relatively shorter time, that is the space of time between two moults, the same extent that they would otherwise attain only in the course of several periods of growth in the ametabolous insects.”

Fig. 156.—Nymph of Aphrophora permutata, with enlarged view of the wings and the veins: pro, pronotum; sc, mesoscutum; 1ab, 1st abdominal segment.

Ignorant of Graber’s paper, we had arrived at the same result, after an examination of the early nymph-stages of the cockroach, as well as the locusts, Termites, and various Hemiptera. In all these forms it is plainly to be seen that the wings are simply expansions, either horizontal or partly vertical (where, as in locusts, etc., the body is compressed, and the meso- and metanota are rounded downwards), of the hinder and outer edge of the meso- and metanotum. As will be seen by reference to the accompanying figures, the wings are notal (tergal) outgrowths from the dorsal arch of the two hinder segments of the thorax. At first, as seen in the young pupal cockroach (Fig. 152) and locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are continuous with the notum. Late in nymphal life a suture and a hinge-joint appear at the base of the wing, and thus there is some movement of the wing upon the notum; finally, the tracheæ are well developed in the wings, and numerous small sclerites are differentiated at the base of the wing, to which the special muscles of flight are attached, and thus the wings, after the last nymphal moult, have the power of flapping, and of sustaining the insect in the air; they thus become true organs of flight.