HYPERMETAMORPHOSIS
By a Machiavellian stratagem the primary larva of the Oil-beetle or
the Sitaris has penetrated the Anthophora's cell; it has settled on
the egg, which is its first food and its life-raft in one. What
becomes of it once the egg is exhausted?
Let us, to begin with, go back to the larva of the Sitaris. By the end
of a week the Anthophora's egg has been drained dry by the parasite
and is reduced to the envelope, a shallow skiff which preserves the
tiny creature from the deadly contact of the honey. It is on this
skiff that the first transformation takes place, whereafter the larva,
which is now organized to live in a glutinous environment, drops off
the raft into the pool of honey and leaves its empty skin, split along
the back, clinging to the pellicle of the egg. At this stage we see
floating motionless on the honey a milk-white atom, oval, flat and a
twelfth of an inch long. This is the larva of the Sitaris in its new
form. With the aid of a lens we can distinguish the fluctuations of
the digestive canal, which is gorging itself with honey; and along the
circumference of the flat, elliptical back we perceive a double row of
breathing-pores which, thanks to their position, cannot be choked by
the viscous liquid. Before describing the larva in detail we will wait
for it to attain its full development, which cannot take long, for the
provisions are rapidly diminishing.
The rapidity however is not to be compared with that with which the
gluttonous larvae of the Anthophora consume their food. Thus, on
visiting the dwellings of the Anthophorae for the last time, on the
25th of June, I found that the Bee's larvae had all finished their
rations and attained their full development, whereas those of the
Sitares, still immersed in the honey, were, for the most part, only
half the size which they must finally attain. This is yet another
reason why the Sitares should destroy an egg which, were it to
develop, would produce a voracious larva, capable of starving them in
a very short time. When rearing the larvae myself in test-tubes, I
have found that the Sitares take thirty-five to forty days to finish
their mess of honey and that the larvae of the Anthophora spend less
than a fortnight over the same meal.
It is in the first half of July that the Sitaris-grubs reach their
full dimensions. At this period the cell usurped by the parasite
contains nothing beyond a full-fed larva and, in a corner, a heap of
reddish droppings. This larva is soft and white, about half an inch in
length and a quarter of an inch wide at its broadest part. Seen from
above as it floats on the honey, it is elliptical in form, tapering
gradually towards the front and more suddenly towards the rear. Its
ventral surface is highly convex; its dorsal surface, on the contrary,
is almost flat. When the larva is floating on the liquid honey, it is
as it were steadied by the excessive development of the ventral
surface immersed in the honey, which enables it to acquire an
equilibrium that is of the greatest importance to its welfare. In
fact, the breathing-holes, arranged without means of protection on
either edge of the almost flat back, are level with the viscous liquid
and would be choked by that sticky glue at the least false movement,
if a suitably ballasted hold did not prevent the larva from heeling
over. Never was corpulent abdomen of greater use: thanks to this
plumpness of the belly the larva is protected from asphyxia.
Its segments number thirteen, including the head. This head is pale,
soft, like the rest of the body, and very small compared with the rest
of the creature. The antennae are excessively short and consist of two
cylindrical joints. I have vainly looked for the eyes with a powerful
magnifying-glass. In its former state, the larva, subject to strange
migrations, obviously needs the sense of sight and is provided with
four ocelli. In its present state, of what use would eyes be to it at
the bottom of a clay cell, where the most absolute darkness prevails?
The labrum is prominent, is not distinctly divided from the head, is
curved in front and edged with pale and very fine bristles. The
mandibles are small, reddish toward the tips, blunt and hollowed out
spoonwise on the inner side. Below the mandibles is a fleshy part
crowned with two very tiny nipples. This is the lower lip with its two
palpi. It is flanked right and left by two other parts, likewise
fleshy, adhering closely to the lip and bearing at the tip a
rudimentary palp consisting of two or three very tiny joints. These
two parts are the future jaws. All this apparatus of lips and jaws is
completely immobile and in a rudimentary condition which is difficult
to describe. They are budding organs, still faint and embryonic. The
labrum and the complicated lamina formed by the lip and the jaws leave
between them a narrow slit in which the mandibles work.
The legs are merely vestiges, for, though they consist of three tiny
cylindrical joints, they are barely a fiftieth of an inch in length.
The creature is unable to make use of them, not only in the liquid
honey upon which it lives, but even on a solid surface. If we take the
larva from the cell and place it on a hard substance, to observe it
more readily, we see that the inordinate protuberance of the abdomen,
by lifting the thorax from the ground, prevents the legs from finding
a support. Lying on its side, the only possible position because of
its conformation, the larva remains motionless or only makes a few
lazy, wriggling movements of the abdomen, without ever stirring its
feeble limbs, which for that matter could not assist it in any way. In
short, the tiny creature of the first stage, so active and alert, is
succeeded by a ventripotent grub, deprived of movement by its very
obesity. Who would recognize in this clumsy, flabby, blind, hideously
pot-bellied creature, with nothing but a sort of stumps for legs, the
elegant pigmy of but a little while back, armour-clad, slender and
provided with highly perfected organs for performing its perilous
journeys?
Lastly, we count nine pairs of stigmata: one pair on the mesothorax
and the rest on the first eight segments of the abdomen. The last
pair, that on the eighth abdominal segment, consists of stigmata so
small that to detect them we have to gather their position by that in
the succeeding states of the larva and to pass a very patient
magnifying-glass along the direction of the other pairs. These are as
yet but vestigial stigmata. The others are fairly large, with pale,
round, flat edges.
If in its first form the Sitaris-larva is organized for action, to
obtain possession of the coveted cell, in its second form it is
organized solely to digest the provisions acquired. Let us take a
glance at its internal structure and in particular at its digestive
apparatus. Here is a strange thing: this apparatus, in which the hoard
of honey amassed by the Anthophora is to be engulfed, is similar in
every respect to that of the adult Sitaris, who possibly never takes
food. We find in both the same very short oesophagus, the same
chylific ventricle, empty in the perfect insect, distended in the
larva with an abundant orange-coloured pulp; in both the same
gall-bladders, four in number, connected with the rectum by one of
their extremities. Like the perfect insect, the larva is devoid of
salivary glands or any other similar apparatus. Its nervous system
comprises eleven ganglia, not counting the oesophageal collar, whereas
in the perfect insect there are only seven: three for the thorax, of
which the last two are contiguous, and four for the abdomen.
When its rations are finished the larva remains a few days in a
motionless condition, ejecting from time to time a few reddish
droppings until the digestive canal is completely cleared of its
orange-coloured pulp. Then the creature contracts itself, huddles
itself together; and before long we see coming detached from its body
a transparent, slightly crumpled and extremely fine pellicle, forming
a closed bag, in which the successive transformations will take place
henceforth. On this epidermal bag, this sort of transparent leather
bottle, formed by the larva's skin detached all of a piece, without a
slit of any kind, we can distinguish the several well-preserved
external organs: the head, with its antennae, mandibles, paws and
palpi; the thoracic segments, with their vestiges of legs; the
abdomen, with its chain of breathing-holes still connected one to
another by tracheal threads.
Then beneath this pellicle, which is so delicate that it can hardly
bear the most cautious touch, we see a soft, white mass taking shape,
a mass which in a few hours acquires a firm, horny consistency and a
vivid yellow hue. The transformation is now complete. Let us tear the
fine gauze bag enclosing the organism which has just come into being
and direct our investigation to this third form of the Sitaris-larva.
It is an inert, segmented body, with an oval outline, a horny
consistency, just like that of pupae and chrysalids, and a
bright-yellow colour, which we can best describe by likening it to
that of a lemon-drop. Its upper surface forms a double inclined plane
with a very blunt ridge; its lower surface is at first flat, but, as
the result of evaporation, becomes more concave daily, leaving a
projecting rim all around its oval outline. Lastly, its two
extremities or poles are slightly flattened. The major axis of the
lower surface averages half an inch in length and the minor axis a
quarter of an inch.
At the cephalic pole of this body is a sort of mask, modelled roughly
on the head of the larva, and at the opposite pole a small circular
disk deeply wrinkled at the centre. The three segments that come after
the head bear each a pair of very minute knobs, hardly visible without
the lens: these are, to the legs of the larva in its previous form,
what the cephalic mask is to the head of the same larva. They are not
organs, but indications, landmarks placed at the points where these
organs will appear later. On either side we count nine stigmata, set
as before on the mesothorax and the first eight abdominal segments.
The first eight breathing-holes are dark brown and stand out plainly
against the yellow colour of the body. They consist of small, shiny,
conical knobs, perforated at the top with a round hole. The ninth
stigma, though fashioned like the others, is ever so much smaller; it
cannot be distinguished without the lens.
The anomaly, already so manifest in the change from the first form to
the second, becomes even more so here; and we do not know what name to
give to an organism without a standard of comparison, not only in the
order of Beetles, but in the whole class of insects. While, on the one
hand, this organism offers many points of resemblance to the pupae of
the Flies in its horny consistency, in the complete immobility of its
various segments, in the all but absolute absence of relief which
would enable one to distinguish the parts of the perfect insect;
while, on the other hand, it approximates to the chrysalids, because
the creature, to attain this condition, has to shed its skin, as the
caterpillars do, it differs from the pupa because it has for covering
not the surface skin, which has become horny, but rather one of the
inner skins of the larva; and it differs from the chrysalids by the
absence of mouldings which in the latter betray the appendages of the
perfect insect. Lastly, it differs yet more profoundly from the pupa
and the chrysalis because from both these organisms the perfect insect
springs straightway, whereas that which follows what we are
considering is simply a larva like that which went before. I shall
suggest, to denote this curious organism, the term _pseudochrysalis_;
and I shall reserve the names _primary larva_, _secondary larva_ and
_tertiary larva_ to denote, in a couple of words, each of the three
forms under which the Sitares possess all the characteristics of
larvae.
Although the Sitaris, on assuming the form of the pseudochrysalis, is
transfigured outwardly to the point of baffling the science of
entomological phases, this is not so inwardly. I have at every season
of the year examined the viscera of the pseudochrysalids, which
generally remain stationary for a whole year, and I have never
observed other forms among their organs than those which we find in
the secondary larva. The nervous system has undergone no change. The
digestive apparatus is absolutely void and, because of its emptiness,
appears only as a thin cord, sunk, lost amid the adipose sacs. The
stercoral intestine has more substance; its outlines are better
defined. The four gall-bladders are always perfectly distinct. The
adipose tissue is more abundant than ever: it forms by itself the
whole contents of the pseudochrysalis, for in the matter of volume the
insignificant threads of the nervous system and the digestive
apparatus count for nothing. It is the reserve upon which life must
draw for its future labours.
A few Sitares remain hardly a month in the pseudochrysalis stage. The
other phases are achieved in the course of August; and at the
beginning of September the insect attains the perfect state. But as a
rule the development is slower; the pseudochrysalis goes through the
winter; and it is not, at the earliest, until June in the second year
that the final transformations take place. Let us pass in silence over
this long period of repose, during which the Sitaris, in the form of a
pseudochrysalis, slumbers at the bottom of its cell, in a sleep as
lethargic as that of a germ in its egg, and come to the months of June
and July in the following year, the period of what we might call a
second hatching.
The pseudochrysalis is still enclosed in the delicate pouch formed of
the skin of the secondary larva. Outside, nothing fresh has happened;
but important changes have taken place inside. I have said that the
pseudochrysalis displayed an upper surface arched like a hog's back
and a lower surface at first flat and then more and more concave. The
sides of the double inclined plane of the upper or dorsal surface also
share in this depression occasioned by the evaporation of the fluid
constituents; and a time comes when these sides are so depressed that
a section of the pseudochrysalis through a plane perpendicular to its
axis would be represented by a curvilinear triangle with blunted
corners and inwardly convex sides. This is the appearance displayed by
the pseudochrysalis during the winter and spring.
But in June it has lost this withered appearance; it represents a
perfect balloon, an ellipsoid of which the sections perpendicular to
the major axis are circles. Something has also come to pass of greater
importance than this expansion, which may be compared with that which
we obtain by blowing into a wrinkled bladder. The horny integuments of
the pseudochrysalis have become detached from their contents, all of a
piece, without a break, just as happened the year before with the skin
of the secondary larva; and they thus form a fresh vesicular envelope,
free from any adhesion to the contents and itself enclosed in the
pouch formed of the secondary larva's skin. Of these two bags without
outlet, one of which is enclosed within the other, the outer is
transparent, flexible, colourless and extremely delicate; the second
is brittle, almost as delicate as the first, but much less translucent
because of its yellow colouring, which makes it resemble a thin flake
of amber. On this second sac are found the stigmatic warts, the
thoracic studs and so forth, which we noted on the pseudochrysalis.
Lastly, within its cavity we catch a glimpse of something the shape of
which at once recalls to mind the secondary larva.
And indeed, if we tear the double envelope which protects this
mystery, we recognize, not without astonishment, that we have before
our eyes a new larva similar to the secondary. After one of the
strangest transformations, the creature has gone back to its second
form. To describe the new larva is unnecessary, for it differs from
the former in only a few slight details. In both there is the same
head, with its various appendages barely outlined; the same vestiges
of legs, the same stumps transparent as crystal. The tertiary larva
differs from the secondary only by its abdomen, which is less fat,
owing to the absolute emptiness of the digestive apparatus; by a
double chain of fleshy cushions extending along each side; by the rim
of the stigmata, crystalline and slightly projecting, but less so than
in the pseudochrysalis; by the ninth pair of breathing-holes, hitherto
rudimentary but now almost as large as the rest; lastly by the
mandibles ending in a very sharp point. Evicted from its twofold
sheath, the tertiary larva makes only very lazy movements of
contraction and dilation, without being able to advance, without even
being able to maintain its normal position, because of the weakness of
its legs. It usually remains motionless, lying on its side, or else
displays its drowsy activity merely by feeble, wormlike movements.
By dint of these alternate contractions and dilations, indolent though
they be, the larva nevertheless contrives to turn right round in the
sort of shell with which the pseudochrysalidal integuments provide it,
when by accident it finds itself placed head downwards; and this
operation is all the more difficult inasmuch as the larva almost
exactly fills the cavity of the shell. The creature contracts, bends
its head under its belly and slides its front half over its hinder
half by wormlike movements so slow that the lens can hardly detect
them. In less than a quarter of an hour the larva, at first turned
upside down, finds itself again head uppermost. I admire this
gymnastic feat, but have some difficulty in understanding it, so small
is the space which the larva, when at rest in its cell, leaves
unoccupied, compared with that which we should be justified in
expecting from the possibility of such a reversal. The larva does not
long enjoy the privilege which enables it to resume inside its cell,
when this is moved from its original position, the attitude which it
prefers, that is to say, with its head up.
Two days, at most, after its first appearance it relapses into an
inertia as complete as that of the pseudochrysalis. On removing it
from its amber shell, we see that its faculty of contracting or
dilating at will is so completely paralysed that the stimulus of a
needle is unable to provoke it, though the integuments have retained
all their flexibility and though no perceptible change has occurred in
the organization. The irritability, therefore, which in the
pseudochrysalis is suspended for a whole year, reawakens for a moment,
to relapse instantly into the deepest torpor. This torpor will be
partly dispelled only at the moment of the passing into the nymphal
stage, to return immediately afterwards and last until the insect
attains the perfect state.
Further, on holding larvae of the third form, or nymphs enclosed in
their cells, in an inverted position, in glass tubes, we never see
them regain an erect position, however long we continue the
experiment. The perfect insect itself, during the time that it is
enclosed in the shell, cannot regain it, for lack of the requisite
flexibility. This total absence of movement in the tertiary larva,
when a few days old, and also in the nymph, together with the
smallness of the space left free in the shell, would necessarily lead
to the conviction, if we had not witnessed the first moments of the
tertiary larva, that it is absolutely impossible for the creature to
turn right round.
And now see to what curious inferences this lack of observations made
at the due moment may lead us. We collect some pseudochrysalids and
heap them in a glass jar in all possible positions. The favourable
season arrives; and with very legitimate astonishment we find that, in
a large number of shells, the larva or nymph occupies an inverted
position, that is to say, the head is turned towards the anal
extremity of the shell. In vain we watch these reversed bodies for any
indications of movement; in vain we place the shells in every
imaginable position, to see if the creature will turn round; in vain,
once more, we ask ourselves where the free space is which this turning
would demand. The illusion is complete: I have been taken in by it
myself; and for two years I indulged in the wildest conjectures to
account for this lack of correspondence between the shell and its
contents, to explain, in short, a fact which is inexplicable once the
propitious moment has passed.
On the natural site, in the cells of the Anthophora, this apparent
anomaly never occurs, because the secondary larva, when on the point
of transformation into the pseudochrysalis, is always careful to place
its head uppermost, according as the axis of the cell more or less
nearly approaches the vertical. But, when the pseudochrysalids are
placed higgledy-piggledy in a box or jar, all those which are upside
down will later contain inverted larvae or nymphs.
After four changes of form so profound as those which I have
described, one might reasonably expect to find some modifications of
the internal organization. Nevertheless, nothing is changed; the
nervous system is the same in the tertiary larva as in the earlier
phases; the reproductive organs do not yet show; and there is no need
to mention the digestive apparatus, which remains invariable even in
the perfect insect.
The duration of the tertiary larva is a bare four or five weeks, which
is also about the duration of the second. In July, when the secondary
larva passes into the pseudochrysalid stage, the tertiary larva passes
into the nymphal stage, still inside the double vesicular envelope.
Its skin splits along the back in front; and with the assistance of a
few feeble contractions, which reappear at this juncture, it is thrust
behind in the shape of a little ball. There is therefore nothing here
that differs from what happens in the other Beetles.
Nor does the nymph which succeeds this tertiary larva present any
peculiarity: it is the perfect insect in swaddling-bands, yellowish
white, with its various external members, clear as crystal, displayed
under the abdomen. A few weeks elapse, during which the nymph partly
dons the livery of the adult state; and, in about a month, the insect
moults for a last time, in the usual manner, in order to attain its
final form. The wing-cases are now of a uniform yellowish white, as
are the wings, the abdomen and the greater part of the legs; very
nearly all the rest of the body is of a glossy black. In the space of
twenty-four hours, the wing-cases assume their half-black, half-russet
colouring; the wings grow darker; and the legs finish turning black.
This done, the adult organism is completed. However, the Sitaris
remains still a fortnight in the intact shell, ejecting at intervals
white droppings of uric acid, which it pushes back together with the
shreds of its last two sloughs, those of the tertiary larva and of the
nymph. Lastly, about the middle of August, it tears the double bag
that contains it, pierces the lid of the Anthophora's cell, enters a
corridor and appears outside in quest of the other sex.
I have told how, while digging in search of the Sitaris, I found two
cells belonging to _Meloe cicatricosus_. One contained an Anthophora's
egg; with this egg was a yellow Louse, the primary larva of the Meloe.
The history of this tiny creature we know. The second cell also was
full of honey. On the sticky liquid floated a little white larva,
about a sixth of an inch in length and very different from the other
little white larvae belonging to Sitares. The rapid fluctuations of
the abdomen showed that it was eagerly drinking the strong-scented
nectar collected by the Bee. This larva was the young Meloe in the
second period of its development.
I was not able to preserve these two precious cells, which I had
opened wide to examine the contents. On my return from Carpentras, I
found that their honey had been spilt by the motion of the carriage
and that their inhabitants were dead. On the 25th of June, a fresh
visit to the nests of the Anthophorae furnished me with two larvae
like the foregoing, but much larger. One of them was on the point of
finishing its store of honey, the other still had nearly half left.
The first was put in a place of safety with a thousand precautions,
the second was at once immersed in alcohol.
These larvae are blind, soft, fleshy, yellowish white, covered with a
fine down visible only under the lens, curved into a fish-hook like
the larvae of the Lamellicorns, to which they bear a certain
resemblance in their general configuration. The segments, including
the head, number thirteen, of which nine are provided with
breathing-holes with a pale, oval rim. These are the mesothorax and
the first eight abdominal segments. As in the Sitaris-larvae, the last
pair of stigmata, that of the eighth segment of the abdomen, is less
developed than the rest.
The head is horny, of a light brown colour. The epistoma is edged with
brown. The labrum is prominent, white and trapezoidal. The mandibles
are black, strong, short, obtuse, only slightly curved, sharp-edged
and furnished each with a broad tooth on the inner side. The maxillary
and labial palpi are brown and shaped like very small studs with two
or three joints to them. The antennae, inserted just at the base of
the mandibles, are brown, and consist of three sections: the first is
thick and globular; the two others are much smaller in diameter and
cylindrical. The legs are short, but fairly strong, able to serve the
creature for crawling or digging; they end in a strong black claw. The
length of the larva when fully developed is one inch.
As far as I can judge from the dissection of the specimen preserved in
alcohol, whose viscera were affected by being kept too long in that
liquid, the nervous system consists of eleven ganglia, not counting
the oesophageal collar; and the digestive apparatus does not differ
perceptibly from that of an adult Oil-beetle.
The larger of the two larvae of the 25th of June, placed in a
test-tube with what remained of its provisions, assumed a new form
during the first week of the following month. Its skin split along the
front dorsal half and, after being pushed half back, left partly
uncovered a pseudochrysalis bearing the closest analogy with that of
the Sitares. Newport did not see the larva of the Oil-beetle in its
second form, that which it displays when it is eating the mess of
honey hoarded by the Bees, but he did see its moulted skin
half-covering the pseudochrysalis which I have just mentioned. From
the sturdy mandibles and the legs armed with a powerful claw which he
observed on this moulted skin, Newport assumed that, instead of
remaining in the same Anthophora-cell, the larva, which is capable of
burrowing, passes from one cell to another in search of additional
nourishment. This suspicion seems to me to be well-founded, for the
size which the larva finally attains exceeds the proportions which the
small quantity of honey enclosed in a single cell would lead us to
expect.
Let us go back to the pseudochrysalis. It is, as in the Sitares, an
inert body, of a horny consistency, amber-coloured and divided into
thirteen segments, including the head. Its length is 20
millimetres.[1] It is slightly curved into an arc, highly convex on
the dorsal surface, almost flat on the ventral surface and edged with
a projecting fillet which marks the division between the two. The head
is only a sort of mask on which certain features are vaguely carved in
still relief, corresponding with the future parts of the head. On the
thoracic segments are three pairs of tubercles, corresponding with the
legs of the recent larva and the future insect. Lastly, there are nine
pairs of stigmata, one pair on the mesothorax and the eight following
pairs on the first eight segments of the abdomen. The last pair is
rather smaller than the rest, a peculiarity which we have already
noted in the larva which precedes the pseudochrysalis.
[Footnote 1: .787 inch.--_Translator's Note_.]
On comparing the pseudochrysalids of the Oil-beetles and Sitares, we
observe a most striking similarity between the two. The same structure
occurs in both, down to the smallest details. We find on either side
the same cephalic masks, the same tubercles occupying the place of the
legs, the same distribution and the same number of stigmata and,
lastly, the same colour, the same rigidity of the integuments. The
only points of difference are in the general appearance, which is not
the same in the two pseudochrysalids, and in the covering formed by
the cast skin of the late larva. In the Sitares, in fact, this cast
skin constitutes a closed bag, a pouch completely enveloping the
pseudochrysalis; in the Oil-beetles, on the contrary, it is split down
the back and pushed to the rear and, consequently, only half-covers
the pseudochrysalis.
The post-mortem examination of the only pseudochrysalis in my
possession showed me that, similarly to that which happens in the
Sitares, no change occurred in the organization of the viscera,
notwithstanding the profound transformations which take place
externally. In the midst of innumerable little sacs of adipose tissue
is buried a thin thread in which we easily recognize the essential
features of the digestive apparatus, both of the preceding larval form
and of the perfect insect. As for the medullary cord of the abdomen,
it consists, as in the larva, of eight ganglia. In the perfect insect
it comprises only four.
I could not say positively how long the Oil-beetle remains in the
pseudochrysalid form; but, if we consider the very complete analogy
between the evolution of the Oil-beetles and that of the Sitares,
there is reason to believe that a few pseudochrysalids complete their
transformation in the same year, while others, in greater numbers,
remain stationary for a whole year and do not attain the state of the
perfect insect until the following spring. This is also the opinion
expressed by Newport.
Be this as it may, I found at the end of August one of these
pseudochrysalids which had already attained the nymphal stage. It is
with the help of this precious capture that I shall be able to finish
the story of the Oil-beetle's development. The horny integuments of
the pseudochrysalis are split along a fissure which includes the whole
ventral surface and the whole of the head and runs up the back of the
thorax. This cast skin, which is stiff and keeps its shape, is
half-enclosed, as was the pseudochrysalis, in the skin shed by the
secondary larva. Lastly, through the fissure, which divides it almost
in two, a Meloe-nymph half-emerges; so that, to all appearances, the
pseudochrysalis has been followed immediately by the nymph, which does
not happen with the Sitares, which pass from the first of these two
states to the second only by assuming an intermediary form closely
resembling that of the larva which eats the store of honey.
But these appearances are deceptive, for, on removing the nymph from
the split sheath formed by the integuments of the pseudochrysalis, we
find, at the bottom of this sheath, a third cast skin, the last of
those which the creature has so far rejected. This skin is even now
adhering to the nymph by a few tracheal filaments. If we soften it in
water, we easily recognize that it possesses an organization almost
identical with that which preceded the pseudochrysalis. In the latter
case only, the mandibles and the legs are not so robust. Thus, after
passing through the pseudochrysalid stage, the Oil-beetles for some
time resume the preceding form, almost without modification.
The nymph comes next. It presents no peculiarities. The only nymph
that I have reared attained the perfect insect state at the end of
September. Under ordinary conditions would the adult Oil-beetle have
emerged from her cell at this period? I do not think so, since the
pairing and egg-laying do not take place until the beginning of
spring. She would no doubt have spent the autumn and the winter in the
Anthophora's dwelling, only leaving it in the spring following. It is
even probable that, as a rule, the development is even slower and that
the Oil-beetles, like the Sitares, for the most part spend the cold
season in the pseudochrysalid state, a state well-adapted to the
winter torpor, and do not achieve their numerous forms until the
return of the warm weather.
The Sitares and Meloes belong to the same family, that of the
Meloidae.[2] Their strange transformations must probably extend
throughout the group; indeed, I had the good fortune to discover a
third example, which I have not hitherto been able to study in all its
details after twenty-five years of investigation. On six occasions, no
oftener, during this long period I have set eyes on the
pseudochrysalis which I am about to describe. Thrice I obtained it
from old Chalicodoma-nests built upon a stone, nests which I at first
attributed to the Chalicodoma of the Walls and which I now refer with
greater probability to the Chalicodoma of the Sheds. I once extracted
it from the galleries bored by some wood-eating larva in the trunk of
a dead wild pear-tree, galleries afterwards utilized for the cells of
an Osmia, I do not know which. Lastly, I found a pair of them in
between the row of cocoons of the Three-pronged Osmia (_O.
tridentata_, DUF.), who provides a home for her larvae in a channel
dug in the dry bramble stems. The insect in question therefore is a
parasite of the Osmiae. When I extract it from the old
Chalicodoma-nests, I have to attribute it not to this Bee but to one
of the Osmiae (_O. tricornis_ and _O. Latreillii_) who, when making
their nests, utilize the old galleries of the Mason-bee.
[Footnote 2: Later classifiers place both in the family of the
Cantharidae.--_Translator's Note_.]
The most nearly complete instances that I have seen furnishes me with
the following data: the pseudochrysalis is very closely enveloped in
the skin of the secondary larva, a skin consisting of fine transparent
pellicle, without any rent whatever. This is the pouch of the Sitaris,
save that it lies in immediate contact with the body enclosed. On this
jacket we distinguish three pairs of tiny legs, reduced to short
vestiges, to stumps. The head is in place, showing quite perceptibly
the fine mandibles and the other parts of the mouth. There is no trace
of eyes. Each side has a white edging of shrivelled tracheae, running
from one stigmatic orifice to another.
Next comes the pseudochrysalis, horny, currant-red, cylindrical,
cone-shaped at both ends, slightly convex on the dorsal surface and
concave on the ventral surface. It is covered with delicate, prominent
spots, sprinkled very close together; it takes a lens to show them. It
is 1 centimetre long and 4 millimetres wide.[3] We can distinguish a
large knob of a head, on which the mouth is vaguely outlined; three
pairs of little shiny brown specks, which are the hardly perceptible
vestiges of the legs; and on each side a row of eight black specks,
which are the stigmatic orifices. The first speck stands by itself, in
front; the seven others, divided from the first by an empty space,
form a continuous row. Lastly, at the opposite end is a little pit,
the sign of the anal pore.
[Footnote 3: .393 x .156 inch.--_Translator's Note_.]
Of the six pseudochrysalids which a lucky accident placed at my
disposal, four were dead; the other two were furnished by _Zonitis
mutica_. This justified my forecast, which from the first, with
analogy for my guide, made me attribute these curious organizations to
the genus Zonitis. The meloidal parasite of the Osmiae, therefore, is
recognized. We have still to make the acquaintance of the primary
larva, which gets itself carried by the Osmia into the cell full of
honey, and the tertiary larva, the one which, at a given moment, must
be found contained in the pseudochrysalis, a larva which will be
succeeded by the nymph.
Let us recapitulate the strange metamorphoses which I have sketched.
Every Beetle-larva, before attaining the nymphal stage, undergoes a
greater or smaller number of moults, of changes of skin; but these
moults, which are intended to favour the development of the larva by
ridding it of covering that has become too tight for it, in no way
alter its external shape. After any moult that it may have undergone,
the larva retains the same characteristics. If it begin by being
tough, it will not become tender; if it be equipped with legs, it will
not be deprived of them later; if it be provided with ocelli, it will
not become blind. It is true that the diet of these non-variable
larvae remains the same throughout their duration, as do the
conditions under which they are destined to live.
But suppose that this diet varies, that the environment in which they
are called upon to live changes, that the circumstances accompanying
their development are liable to great changes: it then becomes evident
that the moult may and even must adapt the organization of the larva
to these new conditions of existence. The primary larva of the Sitaris
lives on the body of the Anthophora. Its perilous peregrinations
demand agility of movement, long-sighted eyes and masterly
balancing-appliances; it has, in fact, a slender shape, ocelli, legs
and special organs adapted to averting a fall. Once inside the Bee's
cell, it has to destroy the egg; its sharp mandibles, curved into
hooks, will fulfil this office. This done, there is a change of diet:
after the Anthophora's egg the larva proceeds to consume the ration of
honey. The environment in which it has to live also changes: instead
of balancing itself on a hair of the Anthophora, it has now to float
on a sticky fluid; instead of living in broad daylight, it has to
remain plunged in the profoundest darkness. Its sharp mandibles must
therefore become hollowed into a spoon that they may scoop up the
honey; its legs, its cirri, its balancing-appliances must disappear as
useless and even harmful, since all these organs can only involve the
larva in serious danger, by causing it to stick in the honey; its
slender shape, its horny integuments, its ocelli, being no longer
necessary in a dark cell where movement is impossible, where there are
no rough encounters to be feared, may likewise give place to complete
blindness, to soft integuments, to a heavy, slothful form. This
transfiguration, which everything shows to be indispensable to the
life of the larva, is effected by a simple moult.
We do not so plainly perceive the necessity of the subsequent forms,
which are so abnormal that nothing like them is known in all the rest
of the insect class. The larva which is fed on honey first adopts a
false chrysalid appearance and afterwards goes back to its earlier
form, though the necessity for these transformations escapes us
entirely. Here I am obliged to record the facts and to leave the task
of interpreting them to the future. The larva of the Meloidae,
therefore, undergo four moults before attaining the nymphal state; and
after each moult their characteristics alter most profoundly. During
all these external changes, the internal organization remains
unchangingly the same; and it is only at the moment of the nymph's
appearance that the nervous system becomes concentrated and that the
reproductive organs are developed, absolutely as in the other Beetles.
Thus, to the ordinary metamorphoses which make a Beetle pass
successively through the stages of larva, nymph and perfect insect,
the Meloidae add others which repeatedly transform the larva's
exterior, without introducing any modification of its viscera. This
mode of development, which preludes the customary entomological forms
by the multiple transfigurations of the larva, certainly deserves a
special name: I suggest that of _hypermetamorphosis_.
Let us now recapitulate the more prominent facts of this essay.
The Sitares, the Meloes, the Zonites and apparently other Meloidae,
possibly all of them, are in their earliest infancy parasites of the
harvesting Bees.
The larva of the Meloidae, before reaching the nymphal state, passes
through four forms, which I call the _primary larva_, the _secondary
larva_, the _pseudochrysalis_ and the _tertiary larva_. The passage
from one of these forms to the next is effected by a simple moult,
without any changes in the viscera.
The primary larva is leathery and settles on the Bee's body. Its
object is to get itself carried into a cell filled with honey. On
reaching the cell, it devours the Bee's egg; and its part is played.
The secondary larva is soft and differs completely from the primary
larva in its external characteristics. It feeds upon the honey
contained in the usurped cell.
The pseudochrysalis is a body deprived of all movement and clad in
horny integuments which may be compared with those of the pupae and
chrysalids. On these integuments we see a cephalic mask without
distinct or movable parts, six tubercles indicating the legs and nine
pairs of breathing-holes. In the Sitares the pseudochrysalis is
enclosed in a sort of sealed pouch and in the Zonites in a
tight-fitting bag formed of the skin of the secondary larva. In the
Meloes it is simply half-sheathed in the split skin of the secondary
larva.
The tertiary larva reproduces almost exactly the peculiarities of the
second; it is enclosed, in the Sitares and probably also the Zonites,
in a double vesicular envelope formed of the skin of the secondary
larva and the slough of the pseudochrysalis. In the Meloes, it is
half-enclosed in the split integuments of the pseudochrysalis, even as
these, in their turn, are half-enclosed in the skin of the secondary
larva.
From the tertiary larva onwards the metamorphoses follow their
habitual course, that is to say, this larva becomes a nymph; and this
nymph the perfect insect.