and, those
caterpillars known as 'silkworms' make a dense cocoon of pure silk,
consisting of two layers, the outer of coarse and the inner of fine
threads. Silken cocoons very similar in appearance are spun by the
larvae of small Ichneumon-flies. Many pupae lie in a loose cocoon formed
of a few interlacing threads, as for example the conspicuous black and
yellow banded pupa of the Magpie-moth (_Abraxas grossulariata_) and the
pupae of various leaf-beetles. Others again spin together the edges of
leaves with connecting silken threads. The grubs of bees and wasps which
are reared in the comb-chambers of their nests seal up the opening of
the chamber with a lid, partly silk (fig. 18 _co_) and partly excretion,
when ready to pass into the pupal state. An additional external
'capping' may be also supplied by the workers.

The pupae of butterflies are especially interesting, as illustrating the
extreme reduction of the silken cocoon. The pupa of a 'swallowtail'
(Papilionid) or a 'white' (Pierid) butterfly (fig. 23) may be found
attached to a twig of its food-plant or to a wall, in an upright
position, its tail fastened to a pad of silk and a slender silken girdle
encircling its thorax. The pupa of a 'Tortoiseshell' or 'Admiral'
(Nymphalid) butterfly hangs head downwards from a twig, supported only
by the tail-pad of silk, which, useless as a shelter, serves only for
attachment. The pupa is fastened to this pad by a spiny hook or process,
the _cremaster_ (fig. 23 _cr_), on the last abdominal segment. The
cremaster is a characteristic structure in the pupa of a moth or
butterfly. C.V. Riley (1880) and W. Hatchett-Jackson (1890) have shown
that it corresponds with a spiny area, the suranal plate, which lies
above the opening of the caterpillar's intestine. The means by which the
suspended pupa of a nymphalid butterfly attaches its cremaster to the
silken pad which the larva has spun in preparation for pupation, is
worthy of brief attention. The caterpillar, hanging head downwards, is
attached to the silken pad by its hindmost pair of pro-legs or claspers
and by the suranal plate, and the cuticle is slowly worked off from
before backwards, so as to expose the pupa. Were the process of moulting
to be simply completed while the insect hangs by the claspers, the pupa
would of course fall to the ground. But there is enough adhesion between
the pupal and larval cuticles at the hinder end of the body, especially
by means of the everted