TRANSFORMATIONS,--OUTWARD AND INWARD
We are now in a position to study in some detail the transformation of
those insects whose life-story corresponds more or less closely with
that of the butterfly, sketched in the opening pages of this little
book. In the case of some of the insects reviewed in the last three
chapters, the may-flies and cicads for example, a marked difference
between the larva and the imago has been noticed; in others, as the
coccids, we find a resting instar before the winged condition is
assumed, suggesting the pupal stage in the butterfly's life-story.
The various insect orders whose members exhibit no marked divergence
between larva and imago (the Orthoptera for example) are often said to
undergo no transformation, to be 'Ametabola.' Those with life-stories
such as the dragon-flies' are said to undergo partial transformation,
and are termed 'Hemimetabola.' Moths, caddis-flies, beetles, two-winged
flies, saw-flies, ants, wasps, bees, and the great majority of insects,
having the same type of life-story as the butterfly, are said to undergo
complete transformation and are classed as 'Metabola' or 'Holometabola.'
Wherein lies the fundamental difference between these Holometabola on
the one hand and the Hemimetabola and Ametabola on the other? It is not
that the larva differs from the imago or that there is a passive stage
in the life-history; these conditions are observable among insects with
a 'partial' transformation as we have seen, though the resting instar
that simulates the butterfly pupa is certainly exceptional. It has been
pointed out by Sharp (1899) that the most important indication of the
difference between the two modes of development is furnished by the
position of the wing-rudiments. In all Ametabola and Hemimetabola these
are visible externally long before the penultimate instar has been
reached; in the Holometabola they are not seen until the pupal stage.
Attention has already been drawn to the contrast in outward form between
a butterfly and its caterpillar. As in the case of dragon-fly or
may-fly, the larval period is essentially a time for feeding and growth,
and during this period the larval cuticle is cast four or five, in some
species even seven or eight times. After each moult some changes in
detail may be observable, for example in the proportions of the
body-segments or their outgrowths, in the colour or the closeness of the
hairy or spiny armature. But in all main features the caterpillar
retains throughout its life the characteristic form in which it left
the egg. From the tiny, newly-hatched larva to the full-fed caterpillar,
possibly several inches in length, there is all along the same crawling,
somewhat worm-like body, destitute of any outward trace of wings. When
however the last larval cuticle has split open lengthwise along the
back, and has been worked off by vigorous wriggling motions of the
insect, the pupa thus revealed shows the wing-rudiments conspicuous at
the sides of the body, and lying neatly alongside these are to be seen
the forms of feelers, legs, and maxillae of the imago prefigured in the
cuticle of the pupa (fig. 1 _e_). The pupa thus resembles the imago much
more closely than it resembles the larva; even in the proportions of the
body a relative shortening is to be noticed, and the imago of any insect
with complete transformation is reduced in length as compared with the
full-fed larva. Now these wings and other structures characteristic of
the imago, appear in the pupa which is revealed by the shedding of the
last larval cuticle. From these facts we infer that the wing-rudiments
must be present in the larva, hidden beneath the cuticle; and until the
last larval instar, not beneath the cuticle only, but growing in
such-wise that they are hidden by the epidermis. For if they were
growing outwardly the new cuticle would be formed over them, so that
they would be apparent after the next moult. But it is clear that only
in the pupa, forming beneath the cuticle of the last larval instar, can
they grow outwards.
Anatomical study of the caterpillar at various stages verifies the
conclusions just drawn from superficial observation. A hundred and fifty
years ago P. Lyonet in his monumental work (1762) on the caterpillar of
the Goat Moth (Cossus) detected, in the second and third thoracic
segments, four little white masses buried in the fat-body, and, while
doubtful as to their real meaning, he suggested that their number and
position might well give rise to the suspicion that they were rudiments
of the wings of the moth. But it was a century later that A. Weismann in
his classical studies (1864) on the development of common flies, showed
the presence in the maggot of definite rudiments of wings, and other
organs of the adult--rudiments to which he gave the name of _imaginal
discs_. We will recur later to these transformations of the Diptera. For
the present, we pursue our survey of changes in the life-history of the
Lepidoptera and can take to guide us the excellent researches of J.
Gonin (1894).
Careful study of the imaginal discs of the wings in a caterpillar (fig.
10) made by examining microscopically sections cut through them, shows
that the epidermis is pushed in to form a little pouch (_C, p_) and that
into this grows the actual wing-rudiment. Consequently the whitish disk
which seems to lie within the body-wall of the larva, is really a
double fold of the epidermis, the outer fold forming the pouch, the
inner the actual wing-bud. Into the cavity of the latter pass branches
from the air-tube system. In its earliest stage, the wing-bud is simply
an ingrowing mass of cells (fig. 10 _A_) which subsequently becomes an
inpushed pouch (_B_). Until the last stage of larval life the wing-bud
remains hidden in its pouch, and no cuticle is formed over it. When the
pupal stage draws near the bud grows out of its sheath, and projecting
from the general surface of the epidermis becomes covered with cuticle
to be revealed, as we have seen, after the last larval moult, as the
pupal wing. Thus all through the life of the humble, crawling
caterpillar, 'it doth not yet appear what it shall be,' but there are
being prepared, hidden and unseen, the wondrous organs of flight, which
in due time will equip the insect for the glorious aerial existence that
awaits it.
[Illustration: Fig. 10. A, B, C, Sections through epidermis and cuticle,
showing three stages in growth of the imaginal disc (_w_) of a wing in
the caterpillar of a White Butterfly (_Pieris_). _ep_, epidermis; _cu_,
cuticle; _t_, air-tube, whence branches pass into the developing wing.
In C, _cu'_ represents the new cuticle forming beneath the old one, and
(_p_) the pouch within which the wing-disc (_w_) lies. Highly magnified.
After Gonin, _Bull. Soc. Vaud._ XXX.]
As mentioned above, this hidden growth of the wing-rudiments, in
butterflies, beetles, flies, bees, and the great majority of the winged
insects, has been emphasised by Sharp (1899) as a character contrasting
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
that the imaginal discs of the legs (fig. 11--1, 2, 3) grow out from
deep dermal inpushings. Simple at first, these outgrowths by partial
splitting, become differentiated into thigh and shin.
[Illustration: Fig. 11. Front region of Maggot of Blow-fly
(_Calliphora_) showing diagrammatically the imaginal discs, which are
shaded. _e_, eye; _f_, feeler; _W_, fore-wing; _w_, hind-wing; 1, 2, 3,
legs. _H_ is the 'cephalic vesicle,' which becomes everted at the close
of the metamorphosis, so as to bring the feelers and eyes to the front,
the brain (_B_) moving forwards at the same time. After Van Rees, _Zool.
Jahrb._ 1894, and Lowne's _Blow-fly_.]
Similarly the feelers and jaws of the butterfly are developed from
imaginal discs, and this fact explains how it comes to pass that they
differ so widely from the corresponding structures in the caterpillar.
The larval feelers (fig. 3 _At_) are short and stumpy, those of the
butterfly long and many-jointed. The maxilla of the larva (fig. 3 _Mx_)
consists of a base carrying two short jointed processes; in the
butterfly a certain portion of the maxilla, the hood or galea, is
modified into a long, flexible grooved process, capable of forming with
its fellow the trunk through which the insect sucks its liquid food
(fig. 2). Nothing but some such provision as that of the imaginal discs
could render possible the wonderful replacement of the caterpillar's
jaws, biting solid food, into those of the butterfly sipping nectar from
flowers.
A curious segmental displacement of the imaginal discs with regard to
the larva is noticeable in some Diptera. In the larva of the
harlequin-midge (Chironomus) as described by Miall and Hammond (1900)
the brain is situated in the thorax, and the imaginal discs for the
head, eyes, and feelers of the adult lie in close association with it,
though they arise from inpushings of the larval head. These rudiments do
not appear until the last larval stage has been reached. In the gnats
Culex and Corethra, on the other hand, the imaginal discs for the
head-appendages retain their normal position within the larval head, and
appear in an early stage of larval life. Among the flies of the
bluebottle group (Muscidae) the brain (fig. 11 _B_) is situated, as in
Chironomus, in the thoracic region of the legless maggot, which is the
larva of an insect of this family, and the imaginal discs for eyes and
feelers (fig. 11 _e_, _f_) lie just in front of it. Here, the imaginal
buds of the legs (fig. 11--1, 2, 3) and wings (fig. 11 _W_, _w_) are
deeply inpushed, retaining their connection with the skin only by means
of a thread of cells. As the larva is legless and headless its outer
form is not affected by the discs and it is not surprising to learn that
they appear early. It has indeed been suggested that the pharyngeal
region of the larva, in connection with which the imaginal head-discs
are developed, should be regarded, though it lies in the thorax, as an
inpushed anterior section of the larval head. In any case this region is
pushed out during the formation of the pupa within the final larval
cuticle, so that the imaginal head with its contained brain, its
compound eyes, and its complex feelers, takes its rightful place at the
front end of the insect.
The mention of the brain suggests a few brief remarks on the changes in
the internal organs during insect transformation. There are no imaginal
discs for the nervous system; the brain, nerve-cords and ganglia of the
butterfly or bluebottle are the direct outcome of those of the
caterpillar or maggot. More than seventy years ago, Newport (1839)
traced the rapid but continuous changes, which, during the early pupal
period, convert the elongate nerve-cord of the caterpillar with its
relatively far-separated ganglia into the shortened, condensed
nerve-cord of the Tortoise-shell butterfly (_Vanessa urticae_) with
several of the ganglia coalesced. In many Diptera, on the other hand,
the nervous system of the larva is more concentrated than that of the
imago.
The tubular heart also of a winged insect is the directly modified
survival of the larval heart.
Similarly the reproductive organs undergo a gradual, continuous
development throughout an insect's life-story. Their rudiments appear in
the embryo, often at a very early stage; they are recognisable in the
larva, and the matured structures in the imago are the result of their
slow process of growth, the details of which must be reckoned beyond the
scope of this book. For a full summary of the subject the reader is
referred to L.F. Henneguy's work (1904) containing references to much
important modern literature, which cannot be mentioned here.
On the other hand, the digestive system of insects that undergo a
metamorphosis, passes through a profound crisis of dissolution and
rebuilding. This is not surprising when we remember that there is often
a great difference between larva and imago in the nature of the food.
The digestive canal of a caterpillar runs a fairly straight course
through the body and consists of a gullet, stomach (mid-gut),
intestine, and rectum; it is adapted for the digestion of solid food. In
the butterfly there is one outgrowth of the gullet in the head--a
pharyngeal sac adapted for sucking liquids; and another outgrowth at the
hinder end of the gullet (which is much longer than in the larva)--a
crop or food-reservoir lying in the abdomen. The intestine of the
butterfly also is longer than that of the larva, being coiled or
twisted. Towards the end of the last larval stage, the cells of the
inner coat (epithelium) lining the stomach begin to undergo
degeneration, small replacing cells appearing between their bases and
later giving rise to the more delicate epithelium that lines the mid-gut
of the imago. The larval cells are shed into the cavity of the stomach
and become completely broken down. J. Anglas (1902), describing these
microscopic changes in the transformations of wasps and bees, has shown
that the tiny replacing cells can be recognised in sections through the
digestive canal of a very young larva; they may be regarded as
representing imaginal buds of the adult gastric epithelium. In the
transformations of two-winged flies of the bluebottle group, A.
Kowalevsky (1887) has shown that these replacing cells are aggregated in
little masses scattered at different points along the stomach and thus
corresponding rather closely to the imaginal discs of the legs and
wings.
The gullet, crop, and gizzard of an insect, which lie in front of the
stomach, are lined by cells derived from the outer skin (ectoderm) which
is pushed in to form what is called the 'fore-gut.' Similarly the
intestine and rectum, behind the stomach, are lined with ectodermal
cells which arise from the inpushed 'hind-gut.' The larval fore- and
hind-guts are broken down at the end of larval life and their lining is
replaced by fresh tissue derived from two imaginal bands which surround
the cavity of the digestive tube, one at the hinder end of the fore-gut,
and the other at the front end of the hind-gut. The larval salivary
glands in connection with the gullet are also broken down, and fresh
glands are formed for the imago.
A large part of the substance of an insect larva consists of muscular
tissue, surrounding the digestive tube, and forming the great muscles
that move the various parts of the body, and of fat, surrounding the
organs and serving as a store of food-material. Very many of the
muscle-fibres and the fat-cells also become disintegrated during the
late larval and pupal stages, and the corresponding tissues of the adult
are new formations derived from special groups of imaginal cells, though
some muscles may persist from the larva to the adult. Similarly the
complex air-tube or tracheal system of the larva is broken down and a
fresh set of tubes is developed, adapted to the altered body-form of
pupa and imago.
The destruction of larval tissue and the development of replacing organs
from special groups of cells, derived of course from the embryo, and
carrying on the continuity of cell-lineage to the adult, are among the
most remarkable facts connected with the life-story of insects. The
process of tissue-destruction is known as 'histolysis'; the rebuilding
process is called 'histogenesis.' Considerable difference of opinion has
existed as to factors causing histolysis, and for a summary of the
conflicting or complementary theories, the reader is referred to the
work of L.F. Henneguy (1904, pp. 677-684). In the histolysis of the
two-winged flies, wandering amoeboid cells--like the white corpuscles or
leucocytes of vertebrate blood--have been observed destroying the larval
tissues that need to be broken down, as they destroy invading
micro-organisms in the body. But students of the internal changes that
accompany transformation in insects of other orders have often been
unable to observe such devouring activity of these 'phagocytes,' and
attribute the dissolution of the larval tissues to internal chemical
changes. The fact that in all insect transformation a part, and in many
a large part, of the larval organs pass over to the pupa and imago,
suggests that only those structures whose work is done are broken down
through the action of internally formed destructive substances, and one
function of the phagocytes is to act as scavengers by devouring what has
become effete and useless.