Page:Popular Science Monthly Volume 68.djvu/254

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Parallel Evolution.—A similar need among the several related families of the order Orthoptera to which the cockroaches belong has been met by the development of identical structures. Thus both the plications and the fold of the hind wings originated independently in the cockroach and some other families of the order.

Mechanical Principle.—Both the plications and the longitudinal fold of the Orthoptera wing seem to have been developed in response to the mechanical need of some means of caring for the largely expanded inner area of these wings.

Specialization by Reduction.—The reduction of the long ovipositor of early cockroaches to the short specialized ovipositor of modern forms is apparently an illustration of the law of specialization by reduction.

Loss of Organs Through Disuse.—The reduction of the wings of several modern species to such an extent that, although presenting normal adult features of venation- and articulation to the body, they are so far vestigial as to be practically functionless, is doubtless to be attributed largely to lack of use, and results from the ground habits of these insects in which they use the wings comparatively little. Not infrequently functional wings are retained by the males, even when lost by the probably less active females of the same species.

Arrestation of Development.—The wings of not a few modern forms remain as nothing more than wing-pads similar to the wing-pads of the larval stages. This further reduction is probably an instance of what has been called arrestation. Although other organs of the body have reached maturity, the wings, checked in their development, have not passed beyond the larval stages. Lack of use of the wings by the ancestors of the species, and consequent insufficient blood and food supply to this part of the body, is probably in this case an indirect cause of arrestation.