markedly with the outward and visible growth of the wing-rudiments in
such insects as cockroaches, bugs, and dragon-flies. The divergence
between the two modes of development is certainly very striking, and a
conceivable method of transition from the one to the other is not easy
to explain. Sharp has expressed the divergence by the terms
_Endopterygota_, applied to all the orders of insects with hidden
wing-rudiments (the 'Metabola' or 'Holometabola' of most
classifications) and _Exopterygota_, including all those insects whose
wing-rudiments are visible throughout growth ('Hemimetabola' and
'Ametabola'). Those curious lowly insects, belonging to the two orders
of the Collembola and Thysanura, none of whose members ever develop
wings at all, form a third sub-class, the _Apterygota_ (see
Classificatory Table, p. 122).

Not the wings only, but other structures of the imago, varying in extent
in different orders, are formed from the imaginal discs. For example, de
Réaumur and G. Newport (1839) found that if the thoracic leg of a
late-stage caterpillar were cut off, the corresponding leg of the
resulting butterfly would still be developed, although in a truncated
condition. Gonin has shown that in the Cabbage White butterfly (_Pieris
brassicae_) the legs of the imago are represented, through the greater
part of larval life, only by small groups of cells situated within the
bases of the larval legs. After the third moult these imaginal discs
grow rapidly and the proximal portion of each, destined to develop into
the thigh and shin of the butterfly's leg, sinks into a depression at
the side of the thorax, while the tip of the shin and the
five-segmented foot project into the cavity of the larval leg. Hence we
understand that the amputation of the latter by the old naturalists
truncated only and did not destroy the imaginal limb. In the blow-fly
maggot, Weismann, B.T. Lowne (1890) and J. Van Rees (1888) have shown