ortant 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
wing-rudiments may well have been correlated with a difference in form
between the newly-hatched insect and its parent. As this difference
persisted until a constantly later stage, and the pre-imaginal instar
became necessarily a stage for reconstruction, the present condition of
complete metamorphosis in the more highly organised orders was finally
attained.

To explain satisfactorily these complex life-stories is however
admittedly a difficult task. The acquisition of wings is, as we have
seen, a dominating feature in them all, but if we try to go yet a step
farther back and speculate on the origin of wings in the most primitive
exopterygote insects, the task becomes still more difficult. Many years
ago Gegenbaur (1878) was struck by the correspondence of insect wings to
the tracheal gills of may-fly larvae, which are carried on the abdominal
segments somewhat as wings are on the thoracic segments. But Boerner has
recently (1909) brought forward evidence that these abdominal gills
really correspond serially with legs. Moreover Gegenbaur's theory
suggests that the ancestral insects were aquatic, whereas the presence
of tubes for breathing atmospheric air in well-nigh all members of the
class, and the fact that aquatic adaptations, respiratory and otherwise,
in insect-larvae are secondary force the student to regard the ancestral
insects as terrestrial. It is indeed highly probable that insects had a
common origin with aquatic Crustacea, but all the evidence points to the
ancestors of insects having become breathers of atmospheric air before
they acquired win