wherewith they endow their possessors, are evidently beneficial to the
race in giving power of extending the range during the breeding period
and thus ensuring a wide distribution of the eggs. In no case are wings
fully developed until the closing stage of the insect's life, they are
always acquired after hatching or birth. We have already noticed (p. 40)
how Sharp (1899) has laid stress on the essential difference between the
exopterygote and endopterygote insects, the wing-rudiments of the former
growing outwards throughout life while those of the latter remain hidden
until the pupal instar. Sharp considers that there is some difficulty 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