The structure of the wing testifies to its origin as a fold of the chitinous integument. It is a double lamina, which often encloses a visible space at its base. The nervures, with their vessels and tracheal tubes, lie between the two layers, which, except at the base, are in close contact. Oken termed the wings of an Insect “aerial gills,” and this rather fanciful designation is in some degree justified by their resemblance to the tracheal gills of such aquatic larvæ as those of Ephemeridæ, Perlidæ, Phryganidæ, &c. In the larva of _Chloeon (Ephemera) dipterum_ (fig. 30), for example, the second thoracic segment carries a pair of large expansions, which ultimately are replaced by organs of aerial flight. The abdominal segments carry similarly placed respiratory leaflets, the tracheal gills, which by their vigorous flapping movements bring a rush of water against their membranous and tracheated surfaces. Gegenbaur[72] has argued from the resemblance of these appendages to wings, that the wing and the tracheal leaflet are homologous parts, and this view has been accepted as probable by so competent an observer as Sir John Lubbock.[73] [72] Grundzüge der Vergl. Anat. (Arthropoden, Athmungsorgane.) [73] Origin and Metamorphoses of Insects, p. 73. The leaflets placed most advantageously for propulsion seem to have become exclusively adapted to that end, while the abdominal gills have retained their respiratory character. At the time of change from aquatic to terrestrial life, which takes place in many common Insects when the adult condition is assumed, and which, according to Gegenbaur, was a normal event among primitive Insects, the tracheal gill is supposed to disappear, and in its place, at the next moult, an opening, the stigma, is formed by the rupture of an air-tube. Gegenbaur supposes that the primitive Insects were aquatic, and their tracheal system closed. The tracheal gill he takes to be the common structure which has yielded organs so unlike as the wing and the stigma. [Illustration: Fig. 30. Chloeon (Chloeopsis) dipterum. Larva in eighth stage, with wings and respiratory leaflets. × 14. Copied from Vayssière (loc. cit.).]