THE ADVENTURES OF A GRUB
I
THE YOUNG SITARIS
The high banks of sandy clay in the country round about Carpentras are
the favourite haunts of a host of Bees and Wasps, those lovers of a
sunny aspect and of soil that is easy to dig in. Here, in the month of
May, two Bees, both of them Mason-bees, builders of subterranean cells,
are especially abundant. One of them builds at the entrance of her
dwelling an advanced fortification, an earthly cylinder, wrought in
open work and curved, of the width and length of a man’s finger. When
it is peopled with many Bees one stands amazed at the elaborate
ornamentation formed by all these hanging fingers of clay.
The other Bee, who is very much more frequently seen and is called
Anthophora pilipes, leaves the opening of her corridor bare. The chinks
between the stones in old walls and abandoned hovels, or exposed
surfaces of sand stone or marl, are found suitable for her labours; but
the favourite spots, those to which the greatest number of swarms
resort, are straight stretches of ground exposed to the south, such as
occur in the cuttings of deeply-sunken roads. Here, over areas many
yards in width, the wall is drilled with a multitude of holes, which
give to the earthy mass the look of some enormous sponge. These round
holes might have been made with a gimlet, so regular are they. Each is
the entrance to a winding corridor, which runs to the depth of four or
five inches. The cells are at the far end. If we wish to watch the
labours of the industrious Bee we must visit her workshop during the
latter half of May. Then—but at a respectful distance—we may see, in
all its bewildering activity, the tumultuous, buzzing swarm, busied
with the building and provisioning of the cells.
But it has been most often during the months of August and September,
the happy months of the summer holidays, that I have visited the banks
inhabited by the Anthophora. At this season all is silent near the
nests: the work has long been completed: and numbers of Spiders’ webs
line the crevices or plunge their silken tubes into the Bees’
corridors. That is no reason, however, for hastily abandoning the city
that was once so full of life and bustle, and now appears deserted. A
few inches below the surface, thousands of grubs are imprisoned in
their cells of clay, resting until the coming spring. Surely these
grubs, which are paralysed and incapable of self-defence, must be a
temptation—fat little morsels as they are—to some kind of parasite,
some kind of insect stranger in search of prey. The matter is worth
inquiring into.
Two facts are at once noticeable. Some dismal-looking Flies, half black
and half white, are flying indolently from gallery to gallery,
evidently with the object of laying their eggs there. Many of them are
hanging dry and lifeless in the Spiders’ webs. At other places the
entire surface of a bank is hung with the dried corpses of a certain
Beetle, called the Sitaris. Among the corpses, however, are a few live
Beetles, both male and female. The female Beetle invariably disappears
into the Bees’ dwelling. Without a doubt she, too, lays her eggs there.
If we give a few blows of the pick to the surface of the bank we shall
find out something more about these things. During the early days of
August this is what we shall see: the cells forming the top layer are
unlike those at a greater depth. The difference is owing to the fact
that the same establishment is used by two kinds of Bee, the Anthophora
and the Osmia.
The Anthophoræ are the actual pioneers. The work of boring the
galleries is wholly theirs, and their cells are right at the end. If
they, for any reason, leave the outer cells, the Osmia comes in and
takes possession of them. She divides the corridors into unequal and
inartistic cells by means of rough earthen partitions, her only idea of
masonry.
The cells of the Anthophora are faultlessly regular and perfectly
finished. They are works of art, cut out of the very substance of the
earth, well out of reach of all ordinary enemies; and for this reason
the larva of this Bee has no means of spinning a cocoon. It lies naked
in the cell, whose inner surface is polished like stucco.
In the Osmia’s cells, however, means of defence are required, because
they are at the surface of the soil, are roughly made, and are badly
protected by their thin partitions. So the Osmia’s grubs enclose
themselves in a very strong cocoon, which preserves them both from the
rough sides of their shapeless cells and from the jaws of various
enemies who prowl about the galleries. It is easy, then, in a bank
inhabited by these two Bees, to recognise the cells belonging to each.
The Anthophora’s cells contain a naked grub: those of the Osmia contain
a grub enclosed in a cocoon.
Now each of these two Bees has its own especial parasite, or uninvited
guest. The parasite of the Osmia is the black-and-white Fly who is to
be seen so often at the entrance to the galleries, intent on laying her
eggs within them. The parasite of the Anthophora is the Sitaris, the
Beetle whose corpses appear in such quantities on the surface of the
bank.
If the layer of Osmia-cells be removed from the nest we can observe the
cells of the Anthophora. Some will be occupied by larvæ, some by the
perfect insect, and some—indeed many—will contain a singular egg-shaped
shell, divided into segments with projecting breathing-pores. This
shell is extremely thin and fragile; it is amber-coloured, and so
transparent that one can distinguish quite plainly through its sides a
full-grown Sitaris, struggling as though to set herself at liberty.
What is this curious shell, which does not appear to be a Beetle’s
shell at all? And how can this parasite reach a cell which seems to be
inaccessible because of its position, and in which the most careful
examination under the magnifying-glass reveals no sign of violence?
Three years of close observation enabled me to answer these questions,
and to add one of its most astonishing chapters to the story of insect
life. Here is the result of my inquiries.
The Sitaris in the full-grown state lives only for a day or two, and
its whole life is passed at the entrance to the Anthophora’s galleries.
It has no concern but the reproduction of the species. It is provided
with the usual digestive organs, but I have grave reasons to doubt
whether it actually takes any nourishment whatever. The female’s only
thought is to lay her eggs. This done, she dies. The male, after
cowering in a crevice for a day or two, also perishes. This is the
origin of all those corpses swinging in the Spiders’ web, with which
the neighbourhood of the Anthophora’s dwelling is upholstered.
At first sight one would expect that the Sitaris, when laying her eggs,
would go from cell to cell, confiding an egg to each of the Bee-grubs.
But when, in the course of my observations, I searched the Bees’
galleries, I invariably found the eggs of the Sitaris gathered in a
heap inside the entrance, at a distance of an inch or two from the
opening. They are white, oval, and very small, and they stick together
slightly. As for their number, I do not believe I am exaggerating when
I estimate it at two thousand at least.
Thus, contrary to what one was to some extent entitled to suppose, the
eggs are not laid in the cells of the Bee; they are simply dumped in a
heap inside the doorway of her dwelling. Nay more, the mother does not
make any protective structure for them; she takes no pains to shield
them from the rigours of winter; she does not even attempt to stop up
the entrance-lobby in which she has placed them, and so protect them
from the thousand enemies that threaten them. For as long as the frosts
of winter have not arrived these open galleries are trodden by Spiders
and other plunderers, for whom the eggs would make an agreeable meal.
The better to observe them, I placed a number of the eggs in boxes; and
when they hatched out about the end of September I imagined they would
at once start off in search of an Anthophora-cell. I was entirely
wrong. The young grubs—little black creatures no more than the
twenty-fifth of an inch long—did not move away, though provided with
vigorous legs. They remained higgledy-piggledy, mixed up with the skins
of the eggs whence they came. In vain I placed within their reach lumps
of earth containing open Bee-cells: nothing would tempt them to move.
If I forcibly removed a few from the common heap they at once hurried
back to it in order to hide themselves among the rest.
At last, to assure myself that the Sitaris-grubs, in the free state, do
not disperse after they are hatched, I went in the winter to Carpentras
and inspected the banks inhabited by the Anthophoræ. There, as in my
boxes, I found the grubs all piled up in heaps, all mixed up with the
skins of the eggs.
I was no nearer answering the question: how does the Sitaris get into
the Bees’ cells, and into a shell that does not belong to it?
II
THE FIRST ADVENTURE
The appearance of the young Sitaris showed me at once that its habits
must be peculiar. It could not, I saw, be called on to move on an
ordinary surface. The spot where this larva has to live evidently
exposes it to the risk of many dangerous falls, since, in order to
prevent them, it is equipped with a pair of powerful mandibles, curved
and sharp; robust legs which end in a long and very mobile claw; a
variety of bristles and probes; and a couple of strong spikes with
sharp, hard points—an elaborate mechanism, like a sort of ploughshare,
capable of biting into the most highly polished surface. Nor is this
all. It is further provided with a sticky liquid, sufficiently adhesive
to hold it in position without the help of other appliances. In vain I
racked my brains to guess what the substance might be, so shifting, so
uncertain, and so perilous, which the young Sitaris is destined to
inhabit. I waited with eager impatience for the return of the warm
weather.
At the end of April the young grubs imprisoned in my cages, hitherto
lying motionless and hidden in the spongy heap of egg-skins, suddenly
began to move. They scattered, and ran about in all directions through
the boxes and jars in which they have passed the winter. Their hurried
movements and untiring energy showed they were in search of something,
and the natural thing for them to seek was food. For these grubs were
hatched at the end of September, and since then, that is to say for
seven long months, they had taken no nourishment, although they were by
no means in a state of torpor. From the moment of their hatching they
are doomed, though full of life, to an absolute fast lasting for seven
months; and when I saw their excitement I naturally supposed that an
imperious hunger had set them bustling in that fashion.
The food they desired could only be the contents of the Anthophora’s
cells, since at a later stage the Sitaris is found in those cells. Now
these contents are limited to honey and Bee-grubs.
I offered them some cells containing larvæ: I even slipped the Sitares
into the cells, and did all sorts of things to tempt their appetite. My
efforts were fruitless. Then I tried honey. In hunting for cells
provisioned with honey I lost a good part of the month of May. Having
found them I removed the Bee-grub from some of them, and laid the
Sitaris-grub on the surface of the honey. Never did experiment break
down so completely! Far from eating the honey, the grubs became
entangled in the sticky mass and perished in it, suffocated. “I have
offered you larvæ, cells, honey!” I cried in despair. “Then what do you
want, you fiendish little creatures?”
Well, in the end I found out what they wanted. They wanted the
Anthophora herself to carry them into the cells!
When April comes, as I said before, the heap of grubs at the entrance
to the Bees’ cells begins to show signs of activity. A few days later
they are no longer there. Strange as it may appear, they are all
careering about the country, sometimes at a great distance, clinging
like grim death to the fleece of a Bee!
When the Anthophoræ pass by the entrance to their cells, on their way
either in or out, the young Sitaris-grub, who is lying in wait there,
attaches himself to one of the Bees. He wriggles into the fur and
clutches it so firmly that he need not fear a fall during the long
journeys of the insect that carries him. By thus attaching himself to
the Bee the Sitaris intends to get himself carried, at the right
moment, into a cell supplied with honey.
One might at first sight believe that these adventurous grubs derive
food for a time from the Bee’s body. But not at all. The young Sitares,
embedded in the fleece, at right angles to the body of the Anthophora,
head inwards, tail outwards, do not stir from the spot they have
selected, a point near the Bee’s shoulders. We do not see them
wandering from spot to spot, exploring the Bee’s body, seeking the part
where the skin is most delicate, as they would certainly do if they
were really feeding on the insect. On the contrary, they are always
fixed on the toughest and hardest part of the Bee’s body, a little
below the insertion of the wings, or sometimes on the head; and they
remain absolutely motionless, clinging to a single hair. It seems to me
undeniable that the young Sitares settle on the Bee merely to make her
carry them into the cells that she will soon be building.
But in the meantime the future parasites must hold tight to the fleece
of their hostess, in spite of her rapid flights among the flowers, in
spite of her rubbing against the walls of the galleries when she enters
to take shelter, and in spite, above all, of the brushing which she
must often give herself with her feet, to dust herself and keep spick
and span. We were wondering a little time ago what the dangerous,
shifting thing could be on which the grub would have to establish
itself. That thing is the hair of a Bee who makes a thousand rapid
journeys, now diving into her narrow galleries, now forcing her way
down the tight throat of a flower.
We can now quite understand the use of the two spikes, which close
together and are able to take hold of hair more easily than the most
delicate tweezers. We can see the full value of the sticky liquid that
helps the tiny creature to hold fast; and we can realise that the
elastic probes and bristles on the legs serve to penetrate the Bee’s
down and anchor the grub in position. The more one considers this
arrangement, which seems so useless as the grub drags itself
laboriously over a smooth surface, the more does one marvel at all the
machinery which this fragile creature carries about to save it from
falling during its adventurous rides.
III
THE SECOND ADVENTURE
One 21st of May I went to Carpentras, determined to see, if possible,
the entrance of the Sitaris into the Bee’s cells.
The works were in full swing. In front of a high expanse of earth a
swarm of Bees, stimulated by the sun, was dancing a crazy ballet. From
the tumultuous heart of the cloud rose a monotonous, threatening
murmur, while my bewildered eye tried to follow the movements of the
throng. Quick as a lightning-flash thousands of Anthophoræ were flying
hither and thither in search of booty: thousands of others, also, were
arriving, laden with honey, or with mortar for their building.
At that time I knew comparatively little about these insects. It seemed
to me that any one who ventured into the swarm, or—above all—who laid a
rash hand on the Bees’ dwellings, would instantly be stabbed by a
thousand stings. I had once observed the combs of the Hornet too
closely; and a shiver of fear passed through me.
Yet, to find out what I wished to know, I must needs penetrate that
fearsome swarm; I must stand for whole hours, perhaps all day, watching
the works I intended to upset; lens in hand, I must examine, unmoved
amid the whirl, the things that were happening in the cells. Moreover,
the use of a mask, of gloves, of a covering of any kind, was out of the
question, for my fingers and eyes must be absolutely free. No matter:
even though I should leave the Bee’s nest with my face swollen beyond
recognition, I was determined that day to solve the problem that had
puzzled me too long.
Having caught a few stray Anthophoræ with my net, I satisfied myself
that the Sitaris-larvae were perched, as I expected, on the Bees.
I buttoned my coat tightly and entered the heart of the swarm. With a
few blows of the mattock I secured a lump of earth, and to my great
surprise found myself uninjured. A second expedition, longer than the
first, had the same result: not a Bee touched me with her sting. After
this I remained permanently in front of the nest, removing lumps of
earth, spilling the honey, and crushing the Bees, without arousing
anything worse than a louder hum. For the Anthophora is a pacific
creature. When disturbed in the cells it leaves them hastily and
escapes, sometimes even mortally wounded, without using its venomous
sting except when it is seized and handled.
Thanks to this unexpected lack of spirit in the Mason-bee, I was able
for hours to investigate her cells at my leisure, seated on a stone in
the midst of the murmuring and distracted swarm, without receiving a
single sting, though I took no precautions whatever. Country folk,
happening to pass and seeing me seated thus calmly amid the Bees,
stopped aghast to ask me if I had bewitched them.
In this way I examined the cells. Some were still open, and contained
only a more or less complete store of honey. Others were closely sealed
with an earthen lid. The contents of these varied greatly. Sometimes I
found the larva of a Bee; sometimes another, fatter kind of larva; at
other times honey with an egg floating on the surface. The egg was of a
beautiful white, and was shaped like a cylinder with a slight curve, a
fifth or sixth of an inch in length—the egg of the Anthophora.
In a few cells I found this egg floating all alone on the surface of
the honey: in others, very many others, I saw, lying on the Bee’s egg
as though on a sort of raft, a young Sitaris-grub. Its shape and size
were those of the creature when it is hatched. Here, then, was the
enemy within the gates.
When and how did it get in? In none of the cells was I able to detect
any chink by which it could have entered: they were all sealed quite
tightly. The parasite must have established itself in the
honey-warehouse before the warehouse was closed. On the other hand, the
open cells, full of honey but as yet without an egg, never contain a
Sitaris. The grub must therefore gain admittance either while the Bee
is laying the egg, or else afterwards, while she is busy plastering up
the door. My experiments have convinced me that the Sitaris enters the
cell in the very second when the egg is laid on the surface of the
honey.
If I take a cell full of honey, with an egg floating in it, and place
it in a glass tube with some Sitaris-grubs, they very rarely venture
inside it. They cannot reach the raft in safety: the honey that
surrounds it is too dangerous. If one of them by chance approaches the
honey it tries to escape as soon as it sees the sticky nature of the
stuff under its feet. It often ends by falling back into the cell,
where it dies of suffocation. It is therefore certain that the grub
does not leave the fleece of the Bee when the latter is in her cell or
near it, in order to make a rush for the honey; for this honey would
inevitably cause its death, if it so much as touched the surface.
We must remember that the young Sitaris which is found in a closed cell
is always placed on the egg of the Bee. This egg not only serves as a
raft for the tiny creature floating on a very treacherous lake, but
also provides it with its first meal. To get at this egg, in the centre
of the lake of honey, to reach this raft which is also its first food,
the young grub must somehow contrive to avoid the fatal touch of the
honey.
There is only one way in which this can be done. The clever grub, at
the very moment when the Bee is laying her egg, slips off the Bee and
on to the egg, and with it reaches the surface of the honey. The egg is
too small to hold more than one grub, and that is why we never find
more than one Sitaris in a cell. Such a performance on the part of a
grub seems extraordinarily inspired—but then the study of insects
constantly gives us examples of such inspiration.
When dropping her egg upon the honey, then, the Anthophora at the same
time drops into her cell the mortal enemy of her race. She carefully
plasters the lid which closes the entrance to the cell, and all is
done. A second cell is built beside it, probably to suffer the same
fate; and so on until all the parasites sheltered by her fleece are
comfortably housed. Let us leave the unhappy mother to continue her
fruitless task, and turn our attention to the young larva which has so
cleverly secured for itself board and lodging.
Let us suppose that we remove the lid from a cell in which the egg,
recently laid, supports a Sitaris-grub. The egg is intact and in
perfect condition. But now the work of destruction begins. The grub, a
tiny black speck which we see running over the white surface of the
egg, at last stops and balances itself firmly on its six legs; then,
seizing the delicate skin of the egg with the sharp hooks of its
mandibles, it tugs at it violently till it breaks and spills the
contents. These contents the grub eagerly drinks up. Thus the first
stroke of the parasite’s mandibles is aimed at the destruction of the
Bee’s egg.
This is a very wise precaution on the part of the Sitaris-grub! It will
have to feed on the honey in the cell: the Bee’s grub which would come
out of the egg would also require the honey: there is not enough for
two. So—quick!—a bite at the egg, and the difficulty is removed.
Moreover, another reason for the destruction of the egg is that special
tastes compel the young Sitaris to make its first meals of it. The tiny
creature begins by greedily drinking the juices which the torn wrapper
of the egg allows to escape. For several days it continues to rip the
envelope gradually open, and to feed on the liquid that trickles from
it. Meanwhile it never touches the honey that surrounds it. The Bee’s
egg is absolutely necessary to the Sitaris-grub, not merely as a boat,
but also as nourishment.
At the end of a week the egg is nothing but a dry skin. The first meal
is finished. The Sitaris-grub, which is now twice as large as before,
splits open along the back, and through this slit the second form of
this singular Beetle falls on the surface of the honey. Its cast skin
remains on the raft, and will presently disappear with it beneath the
waves of honey.
Here ends the history of the first form adopted by the Sitaris.