Page:Insects - Their Ways and Means of Living.djvu/377

 to insure their acting as a single wing (D). The moths clearly show, therefore, as do the grasshoppers and the beetles, the effciency of a single pair of flight organs as opposed to two. The moths, however, have attacked from a different angle the problem of converting their inherited equipment of four wings into a two-wing mechanism—instead of suppressing the flight function in one pair of wings, they have given a mechanical unity to the two wings of each side, thus attaining functionally a two-winged condition.

The wasps (Fig. 133) and bees, likewise, have evolved a two-winged machine from a four-wing mechanism on the principle of uniting the two wings on each side. The bees have adopted a particularly efficient method of securing the wings to each other, for each hind wing is fastened to the wing in front of it by a series of small hooklets on its anterior vein that grasp a marginal thickening on the rear edge of the front wing (Fig. 168 E). Moreover, the bees have so highly perfected the unity in the design of the wings that only on close inspection of it to be seen that there are actually two wings on each side of the body.

Finally, the flies, including all members of the order Diptera, have boldly executed the toaster stroke by completely eliminating the second pair of wings from the mechanism of flight. The files are literally two-winged insects (Figs. 167, 168 F). Remnants of the hind wings, it is true, persist in the form of a pair of small stalks, each with a swelling at the end, projecting from behind the bases of the wings (Figs. 167, 168 F, Hl). These stalks are known as "balancers", or halteres, and in their structure they preserve certain features that show them to be rudiments of wings.

The giving over of the function of flight to the front pair of wings has necessarily involved a reconstruction in the entire framework and musculature of the thorax, and a study of the fly thorax gives a most interesting and instructive lesson in the possibilities of adaptive evolution,