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 important change was for these early rudiments to
become sunk into the body, so that the cuticle of the second, and,
later, of the third and succeeding instars, showed no outward sign of
their presence. This suggestion is confirmed by Heymons' (1896, 1907)
observation of the occasional appearance of outward wing-rudiments on
the thoracic segments of a mealworm, the larva of the beetle _Tenebrio
molitor_, and by F. Silvestri's discovery (1905) of a 'pro-nymph' stage
with short external wing-rudiments between the second larval and the
pupal instars of the small ground-beetle _Lebia scapularis_. Whatever
may be the exact explanation of these abnormalities, they show that in
the life-story of the higher insects outward wing-rudiments may even yet
appear before the pupal stage, confirming our belief that such
appearance is an ancestral character. The inward growth of these