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 Börner 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 wings. How the wings arose, what function their precursors
performed before they became capable of supporting flight, we can hardly
even guess.

Our study of the life-story of insects, therefore, while it has taught
us something of what is going on around us to-day, and has given us