_ON THE ORIGIN OF METAMORPHOSES._
The question still remains, Why do insects pass through metamorphoses?
Messrs. Kirby and Spence tell us they "can only answer that such is the
will of the Creator;"[39] this, however, is a general confession of
faith, not an explanation of metamorphoses. So indeed they themselves
appear to have felt; for they immediately proceed to make a suggestion.
"Yet one reason," they say, "for this conformation may be hazarded. A
very important part assigned to insects in the economy of nature, as we
shall hereafter show, is that of speedily removing superabundant and
decaying animal and vegetable matter. For such agents an insatiable
voracity is an indispensable qualification, and not less so unusual
powers of multiplication. But these faculties are in a great degree
incompatible; an insect occupied in the work of reproduction could not
continue its voracious feeding. Its life, therefore, after leaving the
egg, is divided into three stages."
But there are some insects--as, for instance, the Aphides--which
certainly are not among the least voracious, and which grow and breed at
the same time. There are also many scavengers among other groups of
animals--such, for instance, as the dog, the pig, and the vulture--which
undergo no metamorphosis.
It is certainly true that, as a general rule, growth and reproduction do
not occur together; and it follows, almost as a necessary consequence,
that in such cases the first must precede the second. But this has no
immediate connection with the occurrence of metamorphoses. The question
is not, why an insect does not generally begin to breed until it has
ceased to grow, but why, in attaining to its perfect form, it passes
through such remarkable changes; why these changes are so sudden and
apparently violent; and why they are so often closed by a state of
immobility--that of the chrysalis or pupa; for undoubtedly the quiescent
and death-like condition of the pupa is one of the most remarkable
phenomena of insect-metamorphoses.
In the first place, it must be observed that many animals which differ
considerably in their mature state, resemble one another more nearly
when young. Thus birds of the same genus, or of closely allied genera,
which, when mature, differ much in colour, are often very similarly
coloured when young. The young of the lion and the puma are often
striped, and the foetal Black whale has teeth, like its ally the Sperm
whale.
In fact, the great majority of animals do go through well-marked
metamorphoses, though in many cases they are passed through within the
egg, and thus do not come within the popular ken. "La larve," says,
Quatrefages, "n'est qu'un embryon à vie indépendante."[40] Those
naturalists who accept in any form the theory of evolution, consider
that "the embryonal state of each species reproduces more or less
completely the form and structure of its less modified progenitors."[41]
"Each organism," says Herbert Spencer,[42] "exhibits within a short
space of time a series of changes which, when supposed to occupy a
period indefinitely great, and to go on in various ways instead of one
way, give us a tolerably clear conception of organic evolution in
general."
The naturalists of the older school do not, as Darwin and Fritz Müller
have already pointed out, dispute these facts, though they explain them
in a different manner--generally by the existence of a supposed tendency
to diverge from an original type. Thus Johannes Müller says, "The idea
of development is not that of mere increase of size, but that of
progress from what is not yet distinguished, but which potentially
contains the distinction in itself, to the actually distinct. It is
clear that the less an organ is developed, so much the more does it
approach the type, and that during its development it acquires more and
more peculiarities. The types discovered by comparative anatomy and
developmental history must therefore agree." And again, "What is true in
this idea is, that every embryo at first bears only the type of its
section, from which the type of the class, order, &c., is only
afterwards developed." Agassiz also observes that "the embryos of
different animals resemble each other the more the younger they are."
There are, no doubt, cases in which the earlier states are rapidly
passed through, or but obscurely indicated; yet we may almost state it
as a general proposition, that either before or after birth animals
undergo metamorphoses. The state of development of the young animal at
birth varies immensely. The kangaroo (_Macropus major_), which attains a
height of seven feet ten inches, does not when born exceed one inch and
two lines in length; the chick leaves the egg in a much more advanced
condition than the thrush; and so, among insects, the young cricket is
much more highly developed, when it leaves the egg, than the larva of
the fly or of the bee; and, as I have already mentioned, differences
occur even within the limit of one species, though not of course to
anything like the same extent.
In oviparous animals the condition of the young at birth depends much on
the size of the egg: where the egg is large, the abundant supply of
nourishment enables the embryo to attain a high stage of development;
where the egg is small, and the yolk consequently scanty, the embryo
requires an additional supply of food before it can do so. In the former
case the embryo is more likely to survive; but when the eggs are large,
they cannot be numerous, and a multiplicity of germs may be therefore in
some circumstances a great advantage. Even in the same species the
development of the egg presents certain differences.[43]
The metamorphoses of insects depend then primarily on the fact that the
young quit the egg at a more or less early stage of development; and
that consequently the external forces, acting upon them in this state,
are very different from those by which they are affected when they
arrive at maturity.
Hence it follows that, while in many instances mature forms, differing
greatly from one another, arise from very similar larvæ, in other cases,
as we have seen, among some the parasitic Hymenoptera, insects agreeing
closely with one another, are produced from larvæ which are very unlike.
The same phenomenon occurs in other groups. Thus, while in many cases
very dissimilar jelly-fishes arise from almost identical Hydroids, we
have also the reverse of the proposition in the fact that in some
species, Hydroids of an entirely distinct character produce very similar
Medusæ.[44]
We may now pass to the second part of our subject: the apparent
suddenness and abruptness of the changes which insects undergo during
metamorphosis. But before doing so I must repeat that these changes are
not always, even apparently, sudden and great. The development of an
Orthopterous insect, say a grasshopper, from its leaving the egg to
maturity, is so gradual that the ordinary nomenclature of entomological
works (larva state and pupa state) does not apply to it; and even in the
case of Lepidoptera, the change from the caterpillar to the chrysalis
and from this to the butterfly is in reality less rapid than might at
first sight be supposed; the internal organs are metamorphosed very
gradually, and even the sudden and striking change in external form is
very deceptive, consisting merely of a throwing off of the outer
skin--the drawing aside, as it were of a curtain and the revelation of a
form which, far from being new, has been in preparation for days;
sometimes even for months.
Swammerdam, indeed, supposed (and his view was adopted by Kirby and
Spence) that the larva contained within itself "the germ of the future
butterfly, enclosed in what will be the case of the pupa, which is
itself included in three or more skins, one over the other, that will
successively cover the larva." This was a mistake; but it is true that,
if a larva be examined shortly before it is full grown, the future pupa
may be traced within it. In the same manner, if we examine a pupa which
is about to disclose the butterfly, we find the future insect, soft
indeed and imperfect, but still easily recognizable, lying more or less
loosely within the pupa-skin.
One important difference between an insect and a vertebrate animal is,
that whereas in the latter--as, for instance, in ourselves--the muscles
are attached to an internal bony skeleton, in insects no such skeleton
exists. They have no bones, and their muscles are attached to the skin;
whence the necessity for the hard and horny dermal investment of
insects, so different from the softness and suppleness of our own skin.
The chitine, or horny substance, of which the outside of an insect
consists, is formed by a layer of cells lying beneath it, and, once
secreted, cannot be altered. From this the result is, that without a
change of skin, a change of form is impossible. In some cases, as for
instance in _Chloëon_, each change of skin is accompanied by a change of
form, and thus the perfect insect is gradually evolved. In others, as in
caterpillars, several changes of skin take place without any material
alteration of form, and the change, instead of being spread over many,
is confined to the last two moults.
One explanation of this difference between the larvæ which change their
form with every change of skin, and those which do not, is, I believe,
to be found in the structure of the mouth. That of the caterpillar is
provided with a pair of strong jaws, fitted to eat leaves; and the
digestive organs are adapted for this kind of food. On the contrary, the
mouth of the butterfly is suctorial; it has a long proboscis,
beautifully adapted to suck the nectar from flowers, but which would be
quite useless, and indeed only an embarrassment to the larva. The
digestive organs also of the butterfly are adapted for the assimilation,
not of leaves, but of honey. Now it is evident that if the mouth-parts
of the larva were slowly metamorphosed into those of the perfect insect,
through a number of small changes, the insect would in the meantime be
unable to feed, and liable to perish of starvation in the midst of
plenty. In the Orthoptera, and among those insects in which the changes
are gradual, the mouth of the so-called larva resembles that of the
perfect insect, and the principal difference consists in the presence of
wings.
Similar considerations throw much light on the nature of the chrysalis
or pupa state--that remarkable period of death-like quiescence which is
one of the most striking characteristics of insect metamorphosis. The
quiescence of the pupa is mainly owing to the rapidity of the changes
going on in it. In that of a butterfly, not only (as has been already
mentioned) are the mouth and the digestive organs undergoing change, but
the muscles are in a similar state of transition. The powerful ones
which move the wings are in process of formation; and even the nervous
system, by which the movements are set on foot and regulated, is in a
state of rapid change.[45]
It must not be forgotten that all insects are inactive for a longer or
shorter space of time after each moult. The slighter the change, as a
general rule, the shorter is the period of inaction. Thus, after the
ordinary moult of a caterpillar, the insect only requires a short rest
until the new skin is hardened. When, however, the change is great, the
period of inaction is correspondingly prolonged. Most pupæ indeed have
some slight powers of motion; those which assume the chrysalis state in
wood or beneath the ground usually come to the surface when about to
assume the perfect state, and the aquatic pupæ of certain Diptera swim
about with much activity. Among the Neuroptera, certain families have
pupæ as quiescent as those of the Lepidoptera: others--as, for
instance, _Raphidia_--are quiescent at first, but at length acquire
sufficient strength to walk, though still enclosed within the pupa-skin:
a power dependent partly on the fact that this skin is very thin. Others
again--as, for instance, dragon-flies--are not quiescent on assuming the
so-called pupa state for any longer time than at their other changes of
skin. The inactivity of the pupa is therefore not a new condition
peculiar to this stage, but a prolongation of the inaction which has
accompanied every previous change of skin.
Nevertheless the metamorphoses of insects have always seemed to me one
of the greatest difficulties of the Darwinian theory. In most cases, the
development of the individual reproduces to a certain extent that of the
race; but the motionless, imbecile pupa cannot represent a mature form.
No one, so far as I know, has yet attempted to explain, in accordance
with Mr. Darwin's views, a life-history in which the mouth is first
mandibulate and then suctorial, as, for example, in a butterfly. A clue
to the difficulty may, I think, be found in the distinction between
developmental and adaptive changes; to which I have called attention in
a previous chapter. The larva of an insect is by no means a mere stage
in the development of the perfect animal. On the contrary, it is subject
to the influence of natural selection, and undergoes changes which have
reference entirely to its own requirements and condition. It is evident,
then, that while the embryonic development of an animal in the egg may
be an epitome of its specific history, this is by no means the case
with species in which the immature forms have a separate and
independent existence. If an animal which, when young, pursues one mode
of life, and lives on one kind of food, subsequently, either from its
own growth in size and strength, or from any change of season, alters
its habits or food, however slightly, it immediately becomes subject to
the action of new forces: natural selection affects it in two different,
and, it may be, very distinct manners, gradually tending to changes
which may become so great as to involve an intermediate period of change
and quiescence.
There are, however, peculiar difficulties in those cases in which, as
among the Lepidoptera, the same species is mandibulate as a larva, and
suctorial as an imago. From this point of view _Campodea_ and the
Collembola (_Podura_, &c.) are peculiarly interesting. There are in
insects three principal types of mouth:--
First, the mandibulate;
Secondly, the suctorial; and
Thirdly, that of _Campodea_ and the Collembola generally,
in which the mandibles and maxillæ are retracted, but have some freedom
of motion, and can be used for biting and chewing soft substances. This
type is, in some respects, intermediate between the other two. Assuming
that certain representatives of such a type were placed under conditions
which made a suctorial mouth advantageous, those individuals in which
the mandibles and maxillæ were best calculated to pierce or prick would
be favoured by natural selection, and their power of lateral motion
would tend to fall into abeyance; while, on the other hand, if
masticatory jaws were an advantage, the opposite process would take
place.
There is yet a third possibility--namely, that during the first portion
of life, the power of mastication should be an advantage, and during the
second that of suction, or _vice versâ_. A certain kind of food might
abound at one season and fail at another; might be suitable for the
animal at one age and not at another. Now in such cases we should have
two forces acting successively on each individual, and tending to modify
the organization of the mouth in different directions. It cannot be
denied that the innumerable variations in the mouth-parts of insects
have special reference to their mode of life, and are of some advantage
to the species in which they occur. Hence, no believer in natural
selection can doubt the possibility of the three cases above suggested,
the last of which seems to throw some light on the possible origin of
species which are mandibulate in one period of life and not in another.
Granting then the transition from the one condition to the other, this
would no doubt take place contemporaneously with a change of skin. At
such times we know that, even when there is no change in form, the
softness of the organs temporarily precludes the insect from feeding for
a time, as, for instance, in the case of caterpillars. If, however, any
considerable change were involved, this period of fasting must be
prolonged, and would lead to the existence of a third condition, that of
the pupa, intermediate between the other two. Since the acquisition of
wings is a more conspicuous change than any relating to the mouth, we
are apt to associate with it the existence of a pupa-state: but the case
of the Orthoptera (grasshoppers, &c.) is sufficient proof that the
development of wings is perfectly compatible with permanent activity;
the necessity for prolonged rest is in reality much more intimately
connected with the change in the constitution of the mouth, although in
many cases, no doubt, this is accompanied by changes in the legs, and in
the internal organization. An originally mandibulate mouth, however,
like that of a beetle, could not, I think, have been directly modified
into a suctorial organ like that of a butterfly or a gnat, because the
intermediate stages would necessarily be injurious. Neither, on the
other hand, for the same reasons, could the mouth of the Hemiptera be
modified into a mandibulate type like that of the Coleoptera. But in
_Campodea_ and the _Collembola_ we have a type of animal closely
resembling certain larvæ which occur both in the mandibulate and
suctorial series of insects, possessing a mouth neither distinctly
mandibulate nor distinctly suctorial, but constituted on a peculiar
type, capable of modification in either direction by gradual change,
without loss of utility.
In discussing this subject, it is necessary also to take into
consideration the nature and origin of wings. Whence are they derived?
why are there normally two pairs? and why are they attached to the
meso-and meta-thorax? These questions are as difficult as they are
interesting. It has been suggested, and I think with justice, that the
wings of insects originally served for aquatic and respiratory
purposes.
In the larva of _Chloëon_ (Pl. IV., Fig. 1), for instance, which in
other respects so singularly resembles _Campodea_ (Pl. III., Fig. 5),
several of the segments are provided with foliaceous expansions which
serve as respiratory organs. These so-called branchiæ are in constant
agitation, and the muscles which move them in several points resemble
those of true wings. It is true that in _Chloëon_ the vibration of the
branchiæ is scarcely, if at all, utilized for the purpose of locomotion;
the branchiæ are, in fact, placed too far back to act efficiently. The
situation of these branchiæ differs in different groups; indeed, it
seems probable that originally there were a pair on each segment. In
such a case, those branchiæ situated near the centre of the body,
neither too much in front nor too far back, would serve the most
efficiently as propellers: the same causes which determined the position
of the legs would also affect the wings. Thus a division of labour would
be effected; the branchiæ on the thorax would be devoted to locomotion;
those on the abdomen to respiration. This would tend to increase the
development of the thoracic segments, already somewhat enlarged, in
order to receive the muscles of the legs.
That wings may be of use to insects under water is proved by the very
interesting case of _Polynema natans_,[46] which uses its wings for
swimming. This, however, is a rare case, and it is possible that the
principal use of the wings was, primordially, to enable the mature forms
to pass from pond to pond, thus securing fresh habitats and avoiding
in-and-in breeding. If this were so, the development of wings would
gradually have been relegated to a late period of life; and by the
tendency to the inheritance of characters at corresponding ages, which
Mr. Darwin has pointed out,[47] the development of wings would have thus
become associated with the maturity of the insect. Thus the late
acquisition of wings in the Insecta generally seems to be itself an
indication of their descent from a stock which was at one period, if not
originally, aquatic, and which probably resembled the present larvæ of
_Chloëon_ in form, but had thoracic as well as abdominal branchiæ.
Finally, from the subject of metamorphosis we pass naturally to that
most remarkable phenomenon which is known as the "Alternation of
Generations:" for the first systematic view of which we are indebted to
my eminent friend Prof Steenstrup.[48]
I have always felt it very difficult to understand why any species
should have been created in this double character; nor, so far as I am
aware, has any explanation of the fact yet been attempted. Nevertheless
insects offer, in their metamorphoses, a phenomenon not altogether
dissimilar, and give a clue to the manner in which alternation of
generations may have originated.
The caterpillar owes its difference from the butterfly to the
undeveloped state in which it leaves the egg; but its actual form is
mainly due to the influence of the conditions under which it lives. If
the caterpillar, instead of changing into one butterfly, produced
several, we should have an instance of alternation of generations. Until
lately, however, we knew of no such case among insects; each larva
produced one imago, and that not by generation, but by development. It
has long been known, indeed, that there are species in which certain
individuals remain always apterous, while others acquire wings. Many
entomologists, however, regard these abnormal individuals as perfect,
though wingless insects; and therefore I shall found no argument upon
these cases, although they appear to me deserving of more attention than
they have yet received.
Recently, however, Prof. Wagner[49] has discovered that, among certain
small gnats, the larvæ do not directly produce in all cases perfect
insects, but give birth to other larvæ, which undergo metamorphoses of
the usual character, and eventually become gnats. His observations have
been confirmed, as regards this main fact, by other naturalists; and
Grimm has met with a species of _Chironomus_ in which the pupæ lay
eggs.[50]
Here, then, we have a distinct case of alternation of generations, as
characterized by Steenstrup. Probably other cases will be discovered in
which insects undeniably in the larval state will be found fertile. Nay,
it seems to me possible, if not probable, that some larvæ which do not
now breed may, in the course of ages, acquire the power of doing so. If
this idea is correct, it shows how the remarkable phenomenon, known as
alternation of generations, may have originated.
Summing up, then, the preceding argument, we find among insects various
modes of development; from simple growth on the one hand, to well-marked
instances of the so-called alternation of generation on the other. In
the wingless species of Orthoptera there is little external difference,
excepting in size, between the young larva and the perfect insect. The
growth is gradual, and there is nothing which would, in ordinary
language, be called a metamorphosis. In the majority of Orthoptera,
though the presence of wings produces a marked difference between the
larva and the imago, the habits are nearly the same throughout life, and
consequently the action of external circumstances affects the larva in
the same manner as it does the perfect insect.
This is not the case with the Neuroptera. The larvæ do not live under
the same conditions as the perfect insects: external forces accordingly
affect them in a different manner; and we have seen that they pass
through some changes which bear no reference to the form of the perfect
insect: these changes, however, are for the most part very gradual. The
caterpillars of Lepidoptera have even more extensive modifications to
undergo; the mouth of the larva, for instance, being remarkably unlike
that of the perfect insect. A change in this organ, however, could
hardly take place while the insect was growing fast, and consequently
feeding voraciously; nor, even if the change could be thus effected,
would the mouth, in its intermediate stages, be in any way fitted for
biting and chewing leaves. The same reasoning applies also to the
digestive organs. Hence the caterpillar undergoes little, if any,
change, except in size, and the metamorphosis is concentrated, so to
say, into the last two moults. The changes then become so rapid and
extensive, that the intermediate period is necessarily one of
quiescence. In some exceptional cases, as in _Sitaris_ (_ante_, p. 30) we
even find that, the conditions of life not being uniform throughout the
larval period, the larva itself undergoes metamorphoses.
Owing to the fact that the organs connected with the reproduction of the
species come to maturity at a late period, larvæ are generally incapable
of breeding. There are, however, some flies which have viviparous larvæ,
and thus offer a typical case of alternation of generations.
Thus, then, we find among insects every gradation, from simple growth to
alternation of generations; and see how, from the single fact of the
very early period of development at which certain animals quit the egg,
we can throw some light on their metamorphoses, and for the still more
remarkable phenomenon that, among many of the lower animals, the species
is represented by two very different forms. We may even conclude, from
the same considerations, that this phenomenon may in the course of ages
become still more common than it is at present. As long, however, as the
external organs arrive at their mature form before the internal
generative organs are fully developed, we have metamorphosis; but if the
reverse is the case, then alternation of generations often results.
The same considerations throw much light on the remarkable circumstance,
that in alternation of generations the reproduction is, as a general
rule, agamic in one form. This results from the fact that reproduction
by distinct sexes requires the perfection both of the external and
internal organs; and if the phenomenon arise, as has just been
suggested, from the fact that the internal organs arrive at maturity
before the external ones, reproduction will result in those species only
which have the power of agamic multiplication.
Moreover, it is evident that we have in the animal kingdom two kinds of
dimorphism.
This term has usually been applied to those cases in which animals or
plants present themselves at maturity under two forms. Ants and Bees
afford us familiar instances among animals; and among plants the
interesting case of the genus _Primula_ has recently been described by
Mr. Darwin. Even more recently he has made known to us the still more
remarkable phenomenon afforded by the genus _Lythrum_, in which there
are three distinct forms, and which therefore offers an instance of
polymorphism.[51]
The other kind of dimorphism or polymorphism differs from the first in
being the result of the differentiating action of external
circumstances, not on the mature, but on the young individual. Such
different forms, therefore, stand towards one another in the relation of
succession. In the first kind the chain of being divides at the
extremity; in the other it is composed of dissimilar links. Many
instances of this second form of dimorphism have been described under
the name of alternation of generations.
The term, however, has met with much opposition, and is clearly
inapplicable to the differences exhibited by insects in various periods
of their life. Strictly speaking, the phenomena are frequently not
alternate, and in the opinion of some eminent naturalists they are not,
strictly speaking, cases of generation at all.[52]
In order, then, to have some name for these remarkable phenomena, and to
distinguish them from those cases in which the _mature_ animal or plant
is represented by two or more different forms, I think it would be
convenient to retain exclusively for these latter the terms dimorphism
and polymorphism; and those cases in which animals or plants pass
through a succession of different forms might be distinguished by the
name of dieidism or polyeidism.
The conclusions, then, which I think we may draw from the preceding
considerations, are:--
1. That the occurrence of metamorphoses arises from the immaturity of
the condition in which some animals quit the egg.
2. That the form of the insect larva depends in great measure on the
conditions in which it lives. The external forces acting upon it are
different from those which affect the mature form; and thus changes are
produced in the young, having reference to its immediate wants, rather
than to its final form.
3. That metamorphoses may therefore be divided into two kinds,
developmental and adaptional or adaptive.
4. That the apparent abruptness of the changes which insects undergo,
arises in great measure from the hardness of their skin, which admits of
no gradual alteration of form, and which is itself necessary in order to
afford sufficient support to the muscles.
5. The immobility of the pupa or chrysalis depends on the rapidity of
the changes going on in it.
6. Although the majority of insects go through three well-marked stages
after leaving the egg, still a large number arrive at maturity through a
greater or smaller number of slight changes.
7. When the external organs arrive at this final form before the organs
of reproduction are matured, these changes are known as metamorphoses;
when, on the contrary, the organs of reproduction are functionally
perfect before the external organs, or when the creature has the power
of budding, then the phenomenon is known as alternation of generations.