Hoffbauer (1892) also has suggested that the elytra are not the homologues of the fore wings of other insects, but of the tegulæ. Kolbe describes the alula of Dyticus as a delicate, membranous lobe at the base of the elytra, but not visible when they are closed: its fringed edge in Dyticus is bordered by a thickening forming a tube which contains a fluid. The alula is united with the inner basal portion and articulation of the wing-cover, forming a continuation of them. Dufour considered that the humming noise made by these beetles is produced by the alulets. Hoffbauer finds no structural resemblances in the alulæ of Dyticus to the elytra. He does not find “the least trace of veins.” They are more like appendages of the elytra. Lacordaire considered that their function is to prevent the disarticulation of the elytra, but Hoffbauer thinks that they serve as contrivances to retain the air which the beetle carries down with it under the surface, since he almost always found a bubble of air concealed under it; besides, their folded and fringed edge seems especially fitted for taking in and retaining air. Hoffbauer then describes the tegulæ of the hornet and finds them to be, not as Cholodkowsky states, hard, solid, chitinous plates, but hollow. They are inserted immediately over the base or insertion of the fore wings, being articulated by a hinge-joint, the upper lamella extending into a cavity of the side of the mesothorax, and connected by a hinge-like, articulating membrane with the lower projection of the bag or cavity. The lower lamella becomes thinner towards the place of insertion, is slightly folded, and merges without any articulation into the thin, thoracic wall at a point situated over the insertion of the fore wing. The tegulæ also differ from the wings in having no muscles to move them, the actual movements being of a passive nature, and due to the upward and downward strokes of the wings. Comstock adopts Meinert’s view that the elytra are not true fore wings, but gives no reasons. (Manual, p. 495.) Dr. Sharp,[25] however, after examining Dyticus and Cybister, affirms that this structure is only a part of the elytron, to which it is extensively attached, and that it corresponds with the angle at the base of the wing seen in so many insects that fold their front wings against the body. He does not think that the alula affords any support to the view that the elytra of beetles correspond with the tegulæ of Hymenoptera rather than with the fore wings.