example can be readily made between the parts of the caterpillar and the
butterfly, whose story was sketched in the first chapter of this book,
widely different though caterpillar and butterfly may appear at a
superficial glance. And the survey of variety in form, food, and habit
of insect larvae given in Chapter VI enforces surely the conclusion that
the larva is eminently plastic, adaptable, capable of changing so as to
suit the most diverse surroundings. In a most suggestive recent
discussion on the transformation of insects P. Deegener (1909) has
claimed that the larva must be regarded as the more modified stage,
because while all the adult's structures are represented in the larva,
even if only as imaginal buds, there are commonly present in the larva
special adaptive organs not found in the imago, for example the pro-legs
of caterpillars or the skin-gills of midge-grubs. The correspondence of
parts in butterfly and caterpillar just referred to, may still be
traced, though less easily, in bluebottle and maggot. The latter is an
extreme example of degenerative evolution, and its contrast with the
elaborately organised two-winged fly marks the greatest divergence
observable between the larva and imago. With this divergence the resting
pupal stage, during which more or less dissolution and reconstruction of
organs goes on, becomes a necessity, and it has already been pointed out
how the amount of this reconstruction is greatest where the divergence
between the larval and perfect stages is most marked. Whatever
differences of opinion may prevail on points of detail, the general
explanation of insect metamorphosis as the result of divergent evolution
in the two active stages of the life-story must assuredly be accepted.
No other explanation accords with the increasing degree of divergence to
be observed as we pass from the lower to the higher insect orders.

The successive incidents of the life-story of most insects are largely
connected with the acquisition of wings. Wings, and the power of flight