_ON THE NATURE OF METAMORPHOSES._
In the preceding chapters we have considered the life history of insects
after they have quitted the egg; but it is obvious that to treat the
subject in a satisfactory manner we must take the development as a
whole, from the commencement of the changes in the egg, up to the
maturity of the animal, and not suffer ourselves to be confused by the
fact that insects leave the egg in very different stages of embryonal
development. For though all young insects when they quit the egg are
termed "larvæ," whatever their form may be (the case of the so-called
Pupipara not constituting a true exception), still it must be remembered
that some of these larvæ are much more advanced than others. It is
evident that the larva of a fly, as regards its stage of development,
corresponds in reality neither with that of a moth nor with that of a
grasshopper. The maggots of flies, in which the appendages of the head
are rudimentary, belong to a lower grade than the grubs of bees, &c.,
which have antennæ, mandibles, maxillæ, labrum, labium, and, in fact,
all the mouth parts of a perfect insect.
The caterpillars of Lepidoptera are generally classed with the vermiform
larva of Diptera and Hymenoptera, and contrasted with those of
Orthoptera, Hemiptera, &c.; but, in truth, the possession of thoracic
legs places them, together with the similar larvæ of the Tenthredinidæ,
on a decidedly higher level. Thus, then, the period of growth (that in
which the animal eats and increases in size) occupies sometimes one
stage in the development of an insect, sometimes another; sometimes, as
for instance in the case of _Chloëon_, it continues through more than
one; or, in other words, growth is accompanied by development. But, in
fact, the question is even more complicated than this. It is not only
that the larvæ of insects at their birth offer the most various grades
of development, from the grub of a fly to the young of a grasshopper or
a cricket; but that, if we were to classify larvæ according to their
development, we should have to deal, not with a simple case of
gradations only, but with a series of gradations, which would be
different according to the organ which we took as our test.
Apart, however, from the adaptive changes to which special reference was
made in the previous chapter, the differences which larvæ present are
those of gradation, not of direction. The development of a grasshopper
does not pursue a different course from that of a butterfly, but the
embryo attains a higher state before quitting the egg in the former than
in the latter: while in most Hymenoptera, as for instance in Bees,
Wasps, Ants, &c., the young are hatched without thoracic appendages; in
the Orthoptera, on the contrary, the legs are fully developed before the
young animal quits the egg.
Prof. Owen,[16] indeed, goes so far as to say that the Orthoptera and
other Homomorphous insects are, "at one stage of their development,
apodal and acephalous larvæ, like the maggot of the fly; but instead of
quitting the egg in this stage, they are quickly transformed into
another, in which the head and rudimental thoracic feet are developed to
the degree which characterizes the hexapod larvæ of the _Carabi_ and
_Petalocera_."
[Illustration: FIG. 30, Egg of _Phryganea_ (Mystacides)--_A_^1,
mandibular segment; _C_^1 to _C_^5, maxillary, labial, and three
thoracic segments; _D_, abdomen (after Zaddach). 31, Egg of _Phryganea_
somewhat more advanced--_b_, mandibles; _c_, maxillæ; _cfs_, rudiments
of the three pairs of legs.]
I quite believe that this may have been true of such larvæ at an early
geological period, but the fact now appears to be, so far at least as
can be judged from the observations yet recorded, that the legs of those
larvæ which leave the egg with these appendages generally make their
appearance before the body-walls have closed, or the internal organs
have approached to completion. Indeed, when the legs first appear, they
are merely short projections, which it is not always easy to distinguish
from the segments themselves. It must, however, be admitted, that the
observations are neither so numerous, nor in most cases so full, as
could be wished.
Fig. 30 represents an egg of a May-fly (_Phryganea_), as represented by
Zaddach in his excellent memoir,[17] just before the appearance of the
appendages. It will be seen that a great part of the yolk is still
undifferentiated, that the side walls are incomplete, the back quite
open, and the segments merely indicated by undulations. This stage is
rapidly passed through, and Zaddach only once met with an egg in this
condition; in every other specimen which had indications of segments,
the rudiments of the legs had also made their appearance, as in Fig. 31,
which, however, as will be seen, does not in other respects show much
advance on Fig. 30.
Again in _Aphis_, the embryology of which has been so well worked out by
Huxley,[18] the case is very similar, although the legs are somewhat
later in making their appearance. When the young was 1/140th of an inch
in length, he found the cephalic portion of the embryo beginning, he
says, "to extend upwards again over the anterior face of the germ, so as
to constitute its anterior and a small part of its superior wall. This
portion is divided by a median fissure into two lobes, which play an
important part in the development of the head, and will be termed the
'procephalic lobes.' I have already made use of this term for the
corresponding parts in the embryos of Crustacea. The rudimentary thorax
presents traces of a division into three segments; and the dorso-lateral
margins of the cephalic blastoderm, behind the procephalic lobes, have a
sinuous margin. It is in embryos between this and 1/100th of an inch in
length, that the rudiments of the appendages make their appearance; and
by the growth of the cephalic, thoracic, and abdominal blastoderm,
curious changes are effected in the relative position of those regions."
In _Chrysopa oculata_, one of the Hemerobiidæ, Packard has described[19]
and figured a stage in which the body segments have made their
appearance, but in which he says "there are no indications of limbs. The
primitive band is fully formed, the protozorites being distinctly
marked, the transverse impressed lines indicating the primitive segments
being distinct, and the median furrow easily discerned." Here also,
again, the dorsal walls are incomplete, and the internal organs as yet
unformed.
In certain Dragon-flies (_Calepteryx_), and _Hemiptera_ (_Hydrometra_), the
legs, according to Brandt,[20] appear at a still earlier stage.
According to the observations of Kölliker,[21] it would appear that in
the Coleopterous genus _Donacia_ the segments and appendages appear
simultaneously.
Kölliker himself, however, frankly admits that "meæ de hoc insecto
observationes satis sunt manca," and it is possible that he may never
have met with an embryo in the state immediately preceding the
appearance of the legs; especially as it appears from the observations
of Kowalevski that in _Hydrophilus_ the appendages do not make their
appearance until after the segments.[22]
On the whole, as far as we can judge from the observations as yet
recorded, it seems that in Homomorphous insects the ventral wall is
developed and divided into segments, before the appearance of the legs;
but that the latter are formed almost simultaneously with the cephalic
appendages, and before either the dorsal walls of the body or the
internal organs.
[Illustration: FIG. 32.--Egg of _Pholcus opilionides_ (after Claparède).]
As it is interesting, from this point of view, to compare the
development of other Articulata with that of insects, I give a figure
(Fig. 32), representing an early stage in the development of a spider
(_Pholcus_) after Claparède,[23] who says, "C'est à ce moment qu'a lieu
la formation des _protozonites_ ou segments primordiaux du corps de
l'embryon. Le rudiment ventral s'épaissit suivant six zônes disposées
transversalement entre le capuchon anal et le capuchon céphalique."
[Illustration: FIG. 33.--Embryo of _Julus_ (after Newport).]
Among Centipedes the development of _Julus_ has been described by
Newport.[24] The first period, from the deposition of the egg to the
gradual bursting of the shell, and exposure of the embryo within it,
which, however, remains for some time longer in connection with the
shell, lasts for twenty-five days. The segments of the body, originally
six in number, make their appearance on the twentieth day after the
deposition of the egg, at which time there were no traces of legs. The
larva, when it leaves the egg, is a soft, white, legless grub (Fig. 33),
consisting of a head and seven segments, the head being somewhat firmer
in texture than the rest of the body. It exhibits rudimentary antennæ,
but the legs are still only represented by very slight papilliform
processes on the undersides of the segments to which they belong.
As already mentioned, it is possible that at one time the vermiform
state of the Homomorphous insects--which, as we have seen, is now so
short, and passed through at so early a stage of development--was more
important, more prolonged, and accompanied by a more complete condition
of the internal organs. The compression, and even disappearance of those
embryonal stages which are no longer adapted to the mode of life--which
do not benefit the animal--is a phenomenon not without a parallel in
other parts of the animal or even of the vegetable kingdom. Just as in
language long compound words have a tendency to concision, and single
letters sometimes linger on, indicating the history of a word, like the
"l" in "alms," or the "b" in "debt," long after they have ceased to
influence the sound; so in embryology useless stages, interesting as
illustrations of past history, but without direct advantage under
present conditions, are rapidly passed through, and even, as it would
appear, in some cases altogether omitted.
[Illustration: FIG. 34.--Colony of _Bougainvillea fruticosa_, natural
size, to the underside of a piece of floating timber (after Allman).]
For instance, among the Hydroida, in the great majority of cases, the
egg produces a body more or less resembling the common _Hydra_ of our
ponds, and known technically as the "trophosome," which develops into
the well-known Medusæ or jelly-fishes. The group, however, for which
Prof. Allman has proposed the term Monopsea,[25] and of which the genus
_Ægina_ may be taken as the type, is, as he says, distinguished by the
absence of a hydriform stage, "the ovum becoming developed through
direct metamorphosis into a medusiform body, just as in the other orders
it is developed into a hydriform body." Fig. 34 represents, after
Allman, a colony of _Bougainvillea fruticosa_ of the natural size. It is
a British species, which is found growing on buoys, floating timber,
&c., and, says Allman,[26] "when in health and vigour, offers a
spectacle unsurpassed in interest by any other species--every branchlet
crowned by its graceful hydranth and budding with Medusæ in all stages
of development (Fig. 35), some still in the condition of minute buds, in
which no trace of the definite Medusa-form can yet be detected; others,
in which the outlines of the Medusa can be distinctly traced within the
transparent _ectothèque_ (external layer); others, again, just casting
off this thin outer pellicle, and others completely freed from it,
struggling with convulsive efforts to break loose from the colony, and
finally launched forth in the full enjoyment of their freedom into the
surrounding water. I know of no form in which so many of the
characteristic features of a typical hydroid are more finely expressed
than in this beautiful species."
[Illustration: FIG. 35.--Portion of colony of _Bougainvillea fruticosa_,
more magnified.]
[Illustration: FIG. 36.--The Medusa form of the same species.]
Fig. 36 represents the Medusa form of this species, and the development
thus described may be regarded as typical of the Hydroida; yet, as
already mentioned, the Æginidæ do not present us with any stage
corresponding to the fixed condition of _Bougainvillea_, but, on the
contrary, are developed into Medusæ direct from the egg.
On the other hand, there are groups in which the Medusiform stage
becomes less and less important.
[Illustration: FIG. 37, Larva of Prawn, Nauplius stage (after F.
Müller). 38, Larva of Prawn, more advanced, Zoëa stage.]
The great majority of the higher Crustacea go through well-marked
metamorphoses. Figs. 37 and 38 represent two stages in the development
of the prawn. In the first (Fig. 37), representing the young animal as
it quits the egg, the body is more or less oval and unsegmented; there
is a median frontal eye, and three pairs of natatory feet, the first
pair simple, while the two posterior are two-branched. Very similar
larvæ occur in various other groups of Crustacea. They were at first
regarded as mature forms, and O. F. Müller gave them the name of
Nauplius. So also, the second or Zoëa form (Fig. 38) was at first
supposed to be a mature animal, until its true nature was discovered by
Vaughan Thompson.
The Zoëa form of larva differs from the perfect prawn or crab in the
absence of the middle portion of the body and its appendages. The
mandibles have no palpi, the maxillipeds or foot-jaws are used as feet,
whereas in the mature form they serve as jaws. Branchiæ are either
wanting or rudimentary, respiration being principally effected through
the walls of the carapace. The abdomen and tail are destitute of
articulate appendages. The development of Zoëa into the perfect animal
has been well described by Mr. Spence Bate[27] in the case of the common
crab (_Carcinus mænas_).
All crabs, as far as we know, with the exception of a species of land
crab (_Gegarcinus_), described by Westwood, pass through a stage more or
less resembling that shown in Fig. 38. On the other hand, the great
group of Edriopthalma, comprising Amphipoda (shore-hoppers, &c.) and
Isopoda (wood-lice, &c.) pass through no such metamorphosis; the
development is direct, as in the Orthoptera. It is true that one
species, _Tanais Dulongii_, though a typical Isopod in form and general
character, is said to retain in some points, and especially in the mode
of respiration, some peculiarities of the Zoëa type; but this is quite
an exceptional case. In _Mysis_, says F. Müller,[28] "there is still a
trace of the Nauplius stage; being transferred back to a period when it
had not to provide for itself, the Nauplius has become degraded into a
mere skin; in _Ligia_ this larva-skin has lost the traces of limbs, and
in _Philoscia_ it is scarcely demonstrable."
The Echinodermata in most cases "go through a very well-marked
metamorphosis, which often has more than one larval stage.... The mass
of more or less differentiated sarcode, of which the larva, or
pseud-embryo, as opposed to the Echinoderm within it, is made up, always
carries upon its exterior certain bilaterally-arranged ciliated bands,
by the action of which the whole organism is moved from place to place;
and it may be strengthened by the super-addition to it of a framework
of calcareous rods."[29] Müller considered that the mouth and pharynx of
the larva were either absorbed or cast off with the calcareous rods, but
were never converted into the corresponding organs of the perfect
Echinoderm. According to A. Agassiz, however, this is not the case, but
on the contrary "the whole larva and all its appendages are gradually
drawn into the body, and appropriated."[30]
Fig. 39 represents the larva of a sea-egg (_Echino cidaris_) after
Müller.[31] The body is transparent, shaped somewhat like a double
easel, but with two long horns in front, which, as well as the posterior
processes, are supported by calcareous rods. This larva swims by means
of minute vibratile hairs, or ciliæ. It has a mouth, stomach, and in
fact a well-defined alimentary canal; but no nerves or other internal
organs have yet been discovered in it. After swimming about in this
condition for a while, it begins to show signs of change. An involution
of the integument takes place on one side of the back, and continues to
deepen till it reaches a mass or store of what is called blastema, or
the raw material of the animal body. This blastema then begins to
change, and gradually assumes the form of the perfect Echinoderm.[32]
[Illustration: FIG. 39.--Larva of _Echino cidaris_, seen from above × 6/10
(after Müller).]
[Illustration: FIG. 40, Larva of _Echinus_, × 100. _A_, front arm; _F_,
arms of the mouth process; _B_, posterior side arm; _E_(1), accessory
arm of the mouth process; _a_, mouth; _a'_, oesophagus; _b_, stomach;
_b'_, intestine; _o_, posterior orifice; _d_, ciliated bands; _f_,
ciliated epaulets; _c_, disc of future _Echinus_ (after Müller).]
Fig. 40 represents a larva, probably of another sea-egg (_Echinus
lividus_), from the Mediterranean, and shows the commencement of the
sea-egg within the body of the larva. The capital letters denote the
different arms: _a_ is the mouth, _a'_ the oesophagus, _b_ the
stomach, _b'_ the intestine, _f_ the ciliated lobes or epaulets, _c_ the
young sea-egg.
The development of the beautiful _Comatula rosacea_ (Fig. 41) has been
described in the "Philosophical Transactions," by Prof. Wyville Thomson
and Dr. Carpenter.[33] The larva quits the egg, as shown in Fig. 42, in
the form of an oval body about 1/30 inch in length, something like a
barrel, surrounded by four bands or hoops of long vibratile hairs or
ciliæ. There is also a tuft of still longer hairs at the narrower
posterior end of the body. Gradually a number of minute calcareous
spines and plates make their appearance (Fig. 43) in the body of this
larva, and at length arrange themselves in a definite order, so as to
form a bent calcareous club or rod with an enlarged head.
[Illustration: FIG. 41.--_Comatula rosacea_ (after Forbes).]
[Illustration: FIG. 42, Larva of Comatula rosacea (after Thomson). 43,
Larva of _Comatula rosacea_, more advanced. 44, Larva of Comatula rosacea,
in the Pentacrinus state.]
As this process continues, the little creature gradually loses its power
of swimming, and, sinking to the bottom, looses the bands of ciliæ, and
attaches itself by its base to some stone or other solid substance, the
knob of the club being free. The calcareous framework increases in size,
and the expanded head forms itself into a cup, round which from five to
fifteen delicate tentacles, as shown in Fig. 44, make their appearance.
In this stage the young animal resembles one of the stalked Crinoids, a
family of Echinoderms very abundant in earlier geological periods, but
which has almost disappeared, being, as we see, now represented by the
young states of existing more advanced, free, species. This attached,
plant-like condition of _Comatula_ was indeed at first supposed to be a
mature form, and was named Pentacrinus; but we now know that it is only
a stage in the development of _Comatula_. The so-called Pentacrinus
increases considerably in size, and after various gradual changes, which
time does not now permit me to describe, quits the stalk, and becomes a
free _Comatula_.
The metamorphoses of the Starfishes are also very remarkable. Sars
discovered, in the year 1835, a curious little creature about an inch in
length, which he named _Bipinnaria asterigera_ (Figs. 45-47), and which
he then supposed to be allied to the ciliograde Medusæ. Subsequent
observations, however, made in 1844, suggested to him that it was the
larva of a Starfish, and in 1847 MM. Koren and Danielssen satisfied
themselves that this was the case.
Figs. 45 and 46 represent the front and side view of a Bipinnaria found
by Müller[34] near Marseilles. _a_ is the mouth, _b_ the oesophagus,
_c_ the stomach, _c_' the intestine. Fig. 47 represents a somewhat older
specimen, in which the Starfish (_k_) is already beginning to make its
appearance.
[Illustration: FIG. 45, Larva of Starfish (Bipinnaria), × 100 (after
Müller). 46, Larva of Starfish (Bipinnaria), × 100, seen from the
side--_a_, mouth; _b_, oesophagus; _c_, stomach; _c'_, intestine. 47,
Larva of another Bipinnaria, showing the commencement of the
Starfish--_g_, canal of the ciliated sac; _i_, rudiments of tentacles;
_d_, ciliated band.]
But while certain Starfishes thus go through metamorphoses similar in
character, and not less remarkable than those of sea-eggs, there are
others--as, for instance, the genus _Asteracanthion_--in which development
may be said to be direct--the organs and appendages special to the
Pseud-embryo being in abeyance; while in another genus, _Pteraster_, they
are reduced to a mere investing membrane.[35]
Among the Ophiurans also we find two well-marked types of development.
Some passing through metamorphoses, while others, as for instance
_Ophiopholis bellis_, "is developed very much after the method of
_Asteracanthion Mülleri_, without passing through the Plutean
stage."[36]
Even in the same species of Echinoderm the degree of development
attained by the larva differs to a certain extent according to the
temperature, the supply of food, &c. Thus in _Comatula_, specimens which
are liberally supplied with sea-water, and kept warm, hurry as it were
through their early stages, and the free larva becomes distorted by the
growing Pentacrinus (see Fig. 43), almost before it has attained its
perfect form. On the other hand, under less favourable conditions, if
the temperature is low and food less abundant, the early stages are
prolonged, the larva is longer lived, and reaches a much higher degree
of independent development. Similar differences occur in the development
of other animals, as for instance, in the Hydroids,[37] and among the
insects themselves, in Flies;[38] and it is obvious that these facts
throw much light on the nature and origin of the metamorphoses of
insects, which subject we shall now proceed to consider.