PUPAE AND THEIR MODIFICATIONS
The pupal stage is characteristic of the life-story of those insects
whose larvae have wing-rudiments in the form of inpushed imaginal discs,
and in all these insects there is, as we have seen, considerable
divergence in form between larva and imago. In the pupa the wings and
other characteristically adult structures are, for the first time,
visible outwardly; it is the instar which marks the great crisis in
transformation. The pupa rests, as a rule, in a quiescent condition, and
during the early period of this stage the needful internal changes, the
breaking down of many larval tissues, and their replacement by imaginal
organs, go on. Both outwardly and inwardly therefore, the insect
undergoes, at the pupal stage, a reconstruction necessitated by the
differences in form and often in habit, between the larva and the winged
adult; and the greater these differences, the more profound must be the
changes that mark the pupal stage.
From the prominence of imaginal structures in the pupa, it is at once
seen that the pupa of any insect must resemble the adult more nearly
than it resembles the larva. But in different groups of insects we find
different degrees of likeness between pupa and imago. In a beetle pupa
(see fig. 16 _c_), the appendages--feelers, jaws, legs, wings--stand out
from the body as do those of the perfect insect. This type is called a
_free_ pupa. The pupal cuticle has to be shed for the emergence of the
imago, but the pupa is already a somewhat reduced model of the final
instar, with abbreviated wings and doubled-up legs. A free pupa is
characteristic of the Coleoptera, Neuroptera, Trichoptera, Hymenoptera
and many Diptera. In some cases the pupa requires to be specially
adapted for a peculiar mode of life; for example, a special arrangement
of breathing organs may be necessary for life under water, and there
must needs be temporary pupal structures, not represented in the imago.
On the other hand, in the pupae of most Lepidoptera and of some Diptera,
there is more or less coalescence between the cuticle of the appendages
and the cuticle of the body generally, so that the appendages do not
stand out, but being, as it were, glued down to the body, are somewhat
masked (see fig. 1 _e_ and fig. 23). Consequently the _obtect_ pupa, as
this type is called, does not resemble its imago as fully as a free pupa
does. The outline of the wings for example in a butterfly's pupa can in
some cases be traced only with difficulty. T.A. Chapman has shown (1893)
that the completely obtect pupa characterises the more highly developed
families of Lepidoptera, while in the more primitive families the pupa
is incompletely obtect. If the pupa of a butterfly or moth be lifted and
held in the hand, a bending or wriggling motion of the abdomen can be
observed. In the incompletely obtect pupa, this motion is evident in a
greater number of segments than in the completely obtect, the number
concerned varying from five to two in different families. In the
nymphalid butterflies, the pupa is often called a 'chrysalis' on
account of the golden hue displayed by the cuticle, and the term
'chrysalis' is sometimes bestowed indiscriminately on any kind of pupa.
It has been shown by Poulton (1892) and others, that the colour of a
butterfly pupa is to some extent affected by the surroundings of the
caterpillar just before its last moult.
Reference has been made (p. 58) to the power of spinning silk possessed
by many larvae; often the principal use of this silk is to form some
protection for the pupa, the larva before its last moult constructing a
_cocoon_ within which the pupa may rest safely. Many larvae bury
themselves in the earth, and the pupa lies in an earthen chamber, the
lining particles of soil fastened together by fine silken threads.
Larvae that feed in wood, like the caterpillar of the Goat-moth (Cossus)
make a cocoon of splinters spun together, while hairy caterpillars, such
as those of the Tiger-moths, work some of their hairs in with the silk
to make a firm cocoon (fig. 17 _b_). On the other hand, 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 lining of the hind-gut, for the pupa to be
supported while it jerks its cremaster out of the larval cuticle and
works it into the meshes of the silken pad. The moult is thus completed
and the pupa hangs securely all the time. In the numerous cases where
the pupa is enclosed in a cocoon, the cremaster serves to fix the pupa
to the surrounding silk. Chapman (1893) has drawn attention to the fact
that among the more highly organised moths the pupa remains in the
cocoon, the emergence being entirely left to the imago, while the pupae
of the more primitive moths work their way partly out of the cocoon
before the final moult begins. In the latter case, the cremaster is
anchored by a strand of silk which allows a certain degree of emergence,
and the pupa has rows of spines on its abdominal segments, of which a
greater number retain the power of mutual motion than in those pupae
which do not come out of their cocoons.
[Illustration: Fig. 23. Pupa of White Butterfly (_Pieris_), side view;
_f_, feeler; _w_, wing; _sp_, spiracle; _p_, anal pro-leg; _cr_,
cremaster. Magnified 8 times. In part after Hatchett-Jackson, _Trans.
Linn. Soc._ 1900, and Tutt's _British Butterflies_.]
While the pupa on the whole resembles the imago that is to emerge from
it, there are not a few cases in which a special structure necessary for
some contingency in pupal life is retained or adopted in this stage. A
butterfly pupa, like the imago, has no mandibles, but in the case of the
Caddis-flies (Trichoptera) and two families of small moths, the most
primitive of all Lepidoptera, the pupa, like the larva, has
well-developed mandibles. These enable the caddis pupa to bite its way
out of the shortened larval case in which it has pupated, and then to
swim upwards through the water ready for the caddis-fly's emergence into
the air. Pupae that are submerged require special breathing-organs. In
the previous chapter (p. 77) mention was made of the gnat's aquatic
larva with its tail-spiracles adapted for procuring atmospheric air
through the surface-film. The pupa of the gnat[10] also has 'respiratory
trumpets' serving the same purpose, but these are a pair of processes on
the prothorax, so that the pupa, which is fairly active, hangs from the
surface-film with its abdomen pointing downwards through the water. This
change of position is correlated with the necessity for the imago to
emerge into the air; were the pupa to hang head downwards as the larva
does, the gnat would perforce have to dive into the water. With the
beautifully adapted transfer of the functional spiracles, their position
is appropriately arranged for the gnat's emergence at the surface, and
the empty pupal cuticle floats serving the insect as a raft. On this it
rests securely and the crumpled wings have opportunity to expand and
harden before the insect takes to flight.
[10] See _Frontispiece_, B.
The aquatic pupae of other Diptera, many species of the midges
Chironomus and Simulium for example, breathe dissolved air by means of
tufts of thread-like gills, which arise on either side of the prothorax.
The pupae of Simulium rest in their curious little cup-like dwellings,
attached to submerged stones or plants. The Chironomus pupa is usually
found in an elongate gelatinous case adhering to a stone. From this case
the pupa rises to the surface of the water, that the midge may emerge
into the air. Miall and Hammond (1900) describe the arrangement by
which, when the pupal stage ends, and these gills are no longer
required, their connection with the air-tube system is severed 'without
undue violence.' The walls of the fine air-tubes that pass into the
gills are specially strengthened, but just below the pupal cuticle these
walls are exceedingly thin and delicate. Thus when the pupal cuticle is
cast, they are readily broken there, and the cuticle of the midge
forming beneath has a spiracular opening into the main air-trunk, ready
for use during the insect's aerial life.
Among those Diptera whose larva is the headless maggot a most
remarkable arrangement for protecting the pupa is to be found. The last
larval cuticle, instead of being as usual worked off and cast, after
separation from the underlying structures, becomes hard and firm,
forming a protective case (_puparium_) within which by the processes of
histolysis and histogenesis already described the organs of the pupa and
imago are built up. This puparium (fig. 22 _d_) is usually dark in
colour, often brown and barrel-shaped, and a subcircular lid splits off
from it at the head-end to allow the emergence of the fly[11]. While the
maggot breathes by its tail-spiracles, the functional spiracles of the
puparium (connected with the tracheal system of the enclosed pupa) are
far forward, and these may be situated at the tips of long sometimes
branching processes, which recall the thoracic gills of the aquatic
pupae mentioned a few pages above. Adaptations, various and beautiful,
to special modes of life, are thus seen to characterise pupae as well as
larvae.
[11] The presence of this sub-circular lid characterises Brauer's
suborder Cyclorrhapha. Those Diptera in which the pupal cuticle splits
in the normal, longitudinal manner are included in the Orthorrhapha (see
p. 67).