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) or just before the imago has to emerge (caddis-flies); working
its way out of the ground (crane-flies) or coming half-way out of its
cocoon (many moths). The pupa of the higher insects almost certainly
corresponds with the may-fly's sub-imago, and the facts just recalled as
to remnants of pupal activity suggest that in the ancestors of
endopterygote insects what is now the pupal instar was represented by an
active nymphal or sub-imaginal stage, possibly indeed by more than one
stage, as Packard and other writers have stated that pupae of bees and
wasps undergo two or three moults before the final exposure of the
imago. Such an early pupal instar has been defined as a 'pro-nymph' or a
'semi-pupa.' Examples have been given of the exceptional passive
condition of the penultimate instar in Exopterygota. The instars
preceding this presumably had originally outward wing-rudiments in all
insect life-histories, and the endopterygote condition was attained by
the postponement of the outward appearance of these to successively
later stages. The leg and wing rudiments of the male coccid (pp. 20-1)
beneath the cuticle of the second instar are strictly comparable to
imaginal buds, and these are present in one instar of what is generally
regarded as an exopterygote life-history. The first instar in all
insects has no visible wing-rudiments, but when they grow outwardly from
the body, they necessarily become covered with cuticle, so that they
must be visible after the first moult. There is no supreme difficulty in
supposing that the imp