he thorax are relatively short, and as many as five
segments of the abdomen may carry short cylindrical limbs or pro-legs,
which assist the clinging habits and worm-like locomotion of the
caterpillar. No trace of wings is visible externally. The caterpillar,
therefore, differs markedly from its parent in its outward structure, in
its mode of progression, and in its manner of feeding; for while the
butterfly sucks nectar or other liquid food, the caterpillar bites up
and devours solid vegetable substances, such as the leaves of herbs or
trees. It is well-known that between the close of its larval life and
its attainment of perfection as a butterfly, the insect spends a
period as a _pupa_ (fig. 1 _e_) unable to move from place to place, and
taking no food.

[1] The term _larva_ is applied to any young animal which differs
markedly from its parent.

[Illustration: Fig. 3. Head of Caterpillar of Goat-moth (_Cossus_) seen
from behind. _At_, feeler; _Mn_, mandible; _Mx_, maxilla; _Lm_, labium,
spinneret projecting beyond it. Magnified. After Lyonet from Miall and
Denny's _Cockroach_.]

Such, in brief, is the course of the most familiar of insect
life-stories. For the student of the animal world as a whole, this
familiar transformation raises some startling problems, which have been
suggestively treated by F. Brauer (1869), L.C. Miall (1895), J. Lubbock
(1874), R. Heymons (1907), P. Deegener (1909) and other writers[2]. To
appreciate these problems is the first step towards learning the true
meaning of the transformation.

[2] The dates in brackets after authors' names will facilitate reference
to the Bibliography (pp. 124-8).

The butterfly's egg is absolutely and relatively of large size, and
contains a considerable amount of yolk. As a rule we find that young
animals hatched from such eggs resemble their parents rather closely and
pass through no marked changes during their lives. A chicken, a
crocodile, a dogfish, a cuttlefish, and a spider afford well-known
examples of this rule. Land-animals, generally, produce young which are
miniature copies of themselves, for example horses, dogs, and other
mammals, snails and slugs, scorpions and earthworms. On the other hand,
metamorphosis among animals is associated with eggs of small size, with
aquatic habit, and with relatively low zoological rank. The young of a
starfish, for example, has hardly a character in common with its parent,
while a marine segmented worm and an oy