in bridging, in thought, the gap between these two methods of wing-growth, and has put forward an ingenious suggestion to meet it (1902).  Reference has already been made to insects of various orders in which one sex is wingless, the Vapourer Moth (p. 96) for example, or all the individuals of both sexes are wingless, as the aberrant cockroaches mentioned in Chapter II (p. 15), or certain generations of virgin females are wingless, for example aphids (pp. 18-19) and gall-flies (pp. 94-5).  Insects may thus become secondarily wingless, that is to say be manifestly the offspring of winged parents, and such wingless forms may on the other hand give rise to offspring or descendants with well-developed wings.  Frequently, as in the case of the aphids, many wingless generations intervene between two winged generations.  A striking illustration of this fact is afforded by an aquatic bug, Velia currens, commonly to be seen skating over the surface of running water.  The adults of Velia are nearly always wingless, but now and then the naturalist meets with a specimen provided with functional wings, the possession of which enables the insect to make its way to a fresh stream.  Moreover there are whole orders of parasitic insects, such as the lice and fleas, which, showing clear affinity to orders of winged insects, are believed to be secondarily wingless.  These orders are designated by Sharp ‘Anapterygota.’  And from the analogy of the periodic loss and recovery of wings in various generations of the same species, he has concluded that the gap between the exopterygote and the endopterygote method of development may have been bridged by an anapterygote condition; that the ancestors of those insects with complete transformations were the wingless descendants of primitive insects which grew their wings from visible external rudiments, and that in later times re-acquiring wings, they developed these organs in a new way, from inwardly directed rudiments or imaginal buds.

This theory of Sharp’s is original, daring, and ingenious, but the loss and re-acquisition of wings which it presupposes is difficult to imagine in large groups during a prolonged evolutionary history, while the sudden appearance of a totally new mode of wing-growth in the offspring of wingless insects would be an extreme example of discontinuity in development.

On the whole the most probable suggestion which can be made as to the origin of ‘complete’ transformation in insects is that the instar in which wings were first visible externally became later and later in the course of the evolution of the more highly organised groups.  In this way a gradual transition from the exopterygote to the endopterygote type of life-story is at least conceivable.  It will be remembered that a may-fly (p. 33) undergoes a moult after acquiring functional wings, emerging into the air as a ‘sub-imago.’  In not a few endopterygote insects, the pupa shows more or less activity, swimming through water intermittently (gnats)