THE GLOW-WORM
I
HIS SURGICAL INSTRUMENT
Few insects enjoy more fame than the Glow-worm, the curious little
animal who celebrates the joy of life by lighting a lantern at its
tail-end. We all know it, at least by name, even if we have not seen it
roaming through the grass, like a spark fallen from the full moon. The
Greeks of old called it the Bright-tailed, and modern science gives it
the name Lampyris.
As a matter of fact the Lampyris is not a worm at all, not even in
general appearance. He has six short legs, which he well knows how to
use, for he is a real gad-about. The male, when he is full-grown has
wing-cases, like the true Beetle that he is. The female is an
unattractive creature who knows nothing of the delights of flying and
all her life remains in the larva, or incomplete form. Even at this
stage the word “worm” is out of place. We French use the phrase “naked
as a worm” to express the lack of any kind of protection. Now the
Lampyris is clothed, that is to say he wears an outer skin that serves
as a defence; and he is, moreover, rather richly coloured. He is dark
brown, with pale pink on the chest; and each segment, or division, of
his body is ornamented at the edge with two spots of fairly bright red.
A costume like this was never worn by a worm!
Nevertheless we will continue to call him the Glow-worm, since it is by
that name that he is best known to the world.
The two most interesting peculiarities about the Glow-worm are, first,
the way he secures his food, and secondly, the lantern at his tail.
A famous Frenchman, a master of the science of food, once said:
“Show me what you eat, and I will tell you what you are.”
A similar question should be addressed to every insect whose habits we
propose to study; for the information supplied by food is the chief of
all the documents of animal life. Well, in spite of his innocent
appearance, the Glow-worm is an eater of flesh, a hunter of game; and
he carries on his hunting with rare villainy. His regular prey is the
Snail. This fact has long been known; but what is not so well known is
his curious method of attack, of which I have seen no other example
anywhere.
Before he begins to feed on his victim he gives it an anæsthetic—he
makes it unconscious, as a person is made unconscious with chloroform
before a surgical operation. His food, as a rule, is a certain small
Snail hardly the size of a cherry, which collects in clusters during
the hot weather, on the stiff stubble and other dry stalks by the
roadside, and there remains motionless, in profound meditation,
throughout the scorching summer days. In some such place as this I have
often seen the Glow-worm feasting on his unconscious prey, which he had
just paralysed on its shaky support.
But he frequents other places too. At the edge of cool, damp ditches,
where the vegetation is varied, many Snails are to be found; and in
such spots as these the Glow-worm can kill his victim on the ground. I
can reproduce these conditions at home, and can there follow the
operator’s performance down to the smallest detail.
I will try to describe the strange sight. I place a little grass in a
wide glass jar. In this I install a few Glow-worms and a supply of
Snails of a suitable size, neither too large nor too small. One must be
patient and wait, and above all keep a careful watch, for the events
take place unexpectedly and do not last long.
For a moment the Glow-worm examines his prey, which, according to its
habit, is completely hidden in the shell, except for the edge of the
“mantle,” which projects slightly. Then the hunter draws his weapon. It
is a very simple weapon, but it cannot be seen without a
magnifying-glass. It consists of two mandibles, bent back into a hook,
very sharp and as thin as a hair. Through the microscope one can see a
slender groove running down the hook. And that is all.
The insect repeatedly taps the Snail’s mantle with its instrument. It
all happens with such gentleness as to suggest kisses rather than
bites. As children, teasing one another, we used to talk of “tweaks” to
express a slight squeeze of the finger-tips, something more like
tickling than a serious pinch. Let us use that word. In conversation
with animals, language loses nothing by remaining simple. The Glow-worm
gives tweaks to the Snail.
He doles them out methodically, without hurrying, and takes a brief
rest after each of them, as though to find out what effect has been
produced. The number of tweaks is not great: half a dozen at most,
which are enough to make the Snail motionless, and to rob him of all
feeling. That other pinches are administered later, at the time of
eating, seems very likely, but I cannot say anything for certain on
that subject. The first few, however—there are never many—are enough to
prevent the Snail from feeling anything, thanks to the promptitude of
the Glow-worm, who, at lightning speed, darts some kind of poison into
his victim by means of his grooved hooks.
There is no doubt at all that the Snail is made insensible to pain. If,
when the Glow-worm has dealt some four or five of his twitches, I take
away the victim and prick it with a fine needle, there is not a quiver
in the wounded flesh, there is not the smallest sign of life. Moreover,
I occasionally chance to see Snails attacked by the Lampyris while they
are creeping along the ground, the foot slowly crawling, the tentacles
swollen to their full extent. A few disordered movements betray a brief
excitement on the part of the Snail, and then everything ceases: the
foot no longer crawls, the front-part loses its graceful curve, the
tentacles become limp and give way under their own weight, dangling
feebly like a broken stick. The Snail, to all appearance, is dead.
He is not, however, really dead. I can bring him to life again. When he
has been for two or three days in a condition that is neither life nor
death I give him a shower-bath. In about a couple of days my prisoner,
so lately injured by the Glow-worm’s treachery, is restored to his
usual state. He revives, he recovers movement and sensibility. He is
affected by the touch of a needle; he shifts his place, crawls, puts
out his tentacles, as though nothing unusual had occurred. The general
torpor, a sort of deep drunkenness, has vanished outright. The dead
returns to life.
Human science did not invent the art of making a person insensible to
pain, which is one of the triumphs of surgery. Far back in the
centuries the Glow-worm, and apparently others too, was practising it.
The surgeon makes us breathe the fumes of ether or chloroform: the
insect darts forth from his fangs very tiny doses of a special poison.
When we consider the harmless and peaceful nature of the Snail it seems
curious that the Glow-worm should require this remarkable talent. But I
think I know the reason.
When the Snail is on the ground, creeping, or even shrunk into his
shell, the attack never presents any difficulty. The shell possesses no
lid and leaves the hermit’s fore-part to a great extent exposed. But it
very often happens that he is in a raised position, clinging to the tip
of a grass-stalk, or perhaps to the smooth surface of a stone. This
support to which he fastens himself serves very well as a protection;
it acts as a lid, supposing that the shell fits closely on the stone or
stalk. But if the least bit of the Snail be left uncovered the slender
hooks of the Glow-worm can find their way in through the gap, and in a
moment the victim is made unconscious, and can be eaten in comfort.
Now, a Snail perched on top of a stalk is very easily upset. The
slightest struggle, the most feeble wriggle on his part, would dislodge
him; he would fall to the ground, and the Glow-worm would be left
without food. It is necessary for the Snail to be made instantly
unconscious of pain, or he would escape; and it must be done with a
touch so delicate that it does not shake him from his stalk. And that,
I think, is why the Glow-worm possesses his strange surgical
instrument.
II
HIS ROSETTE
The Glow-worm not only makes his victim insensible while he is poised
on the side of a dry grass-stalk, but he eats him in the same dangerous
position. And his preparations for his meal are by no means simple.
What is his manner of consuming it? Does he really eat, that is to say,
does he divide his food into pieces, does he carve it into minute
particles, which are afterwards ground by a chewing-apparatus? I think
not. I never see a trace of solid nourishment on my captives’ mouths.
The Glow-worm does not eat in the strict sense of the word; he merely
drinks. He feeds on a thin gruel, into which he transforms his prey.
Like the flesh-eating grub of the Fly, he can digest his food before he
swallows it; he turns his prey into liquid before feeding on it.
This is how things happen. A Snail has been made insensible by a
Glow-worm, who is nearly always alone, even when the prize is a large
one like the Common Snail. Soon a number of guests hasten up—two,
three, or more—and, without any quarrel with the real owner, all alike
fall to. A couple of days later, if I turn the shell so that the
opening is downwards, the contents flow out like soup from a saucepan.
By the time the meal is finished only insignificant remains are left.
The matter is obvious. By repeated tiny bites, similar to the tweaks
which we saw administered at the beginning, the flesh of the Snail is
converted into a gruel on which the various guests nourish themselves
each in his own way, each working at the broth by means of some special
pepsine (or digestive fluid), and each taking his own mouthfuls of it.
The use of this method shows that the Glow-worm’s mouth must be very
feebly armed, apart from the two fangs which sting the patient and
inject the poison. No doubt these fangs at the same time inject some
other substance which turns the solid flesh into liquid, in such a
thorough way that every morsel is turned to account.
And this is done with exquisite delicacy, though sometimes in a
position that is anything but steady. The Snails imprisoned in my
apparatus sometimes crawl up to the top, which is closed with a glass
pane. To this pane they fix themselves with a speck of the sticky
substance they carry with them; but, as they are miserly in their use
of this substance, the merest shake is enough to loosen the shell and
send it to the bottom of the jar.
Now it is not unusual for the Glow-worm to hoist himself to the top,
with the help of a certain climbing-organ that makes up for the
weakness of his legs. He selects his prey, makes a careful inspection
of it to find a slit, nibbles it a little, makes it insensible, and
then, without delay, proceeds to prepare the gruel which he will go on
eating for days on end.
When he has finished his meal the shell is found to be absolutely
empty. And yet this shell, which was fixed to the glass only by the
slight smear of stickiness, has not come loose, nor even shifted its
position in the smallest degree. Without any protest from the hermit
who has been gradually converted into broth, it has been drained dry on
the very spot at which the first attack was made. These small details
show us how promptly the anæsthetic bite takes effect, and how very
skilfully the Glow-worm treats his Snail.
To do all this, poised high in air on a sheet of glass or a grass-stem,
the Glow-worm must have some special limb or organ to keep him from
slipping. It is plain that his short clumsy legs are not enough.
Through the magnifying-glass we can see that he does indeed possess a
special organ of this kind. Beneath his body, towards the tail, there
is a white spot. The glass shows that this is composed of about a dozen
short, fleshy little tubes, or stumpy fingers, which are sometimes
gathered into a cluster, sometimes spread into a rosette. This bunch of
little fingers helps the Glow-worm to stick to a smooth surface, and
also to climb. If he wishes to fix himself to a pane of glass or a
stalk he opens his rosette, and spreads it wide on the support, to
which it clings by its own natural stickiness. And by opening and
shutting alternately it helps him to creep along and to climb.
The little fingers that form this rosette are not jointed, but are able
to move in all directions. Indeed they are more like tubes than
fingers, for they cannot seize anything, they can only hold on by their
stickiness. They are very useful, however, for they have a third
purpose, besides their powers of clinging and climbing. They are used
as a sponge and brush. At a moment of rest, after a meal, the Glow-worm
passes and repasses this brush over his head and sides and his whole
body, a performance made possible by the flexibility of his spine. This
is done point by point, from one end of the body to the other, with a
scrupulous care that proves the great interest he takes in the
operation. At first one may wonder why he should dust and polish
himself so carefully. But no doubt, by the time he has turned the Snail
into gruel inside the shell and has then spent several days in eating
the result of his labours, a wash and brush-up is not amiss.
III
HIS LAMP
If the Glow-worm possessed no other talent than that of chloroforming
his prey by means of a few tweaks as gentle as kisses, he would be
unknown to the world in general. But he also knows how to light himself
like a lantern. He shines; which is an excellent manner of becoming
famous.
In the case of the female Glow-worm the lighting-apparatus occupies the
last three divisions of the body. On each of the first two it takes the
form, on the under surface, of a wide belt of light; on the third
division or segment the bright part is much smaller, and consists only
of two spots, which shine through the back, and are visible both above
and below the animal. From these belts and spots there comes a glorious
white light, delicately tinged with blue.
The male Glow-worm carries only the smaller of these lamps, the two
spots on the end segment, which are possessed by the entire tribe.
These luminous spots appear upon the young grub, and continue
throughout life unchanged. And they are always visible both on the
upper and lower surface, whereas the two large belts peculiar to the
female shine only below the body.
I have examined the shining belt under the microscope. On the skin a
sort of whitewash is spread, formed of some very fine grain-like
substance, which is the source of the light. Close beside it is a
curious air-tube, with a short wide stem leading to a kind of bushy
tuft of delicate branches. These branches spread over the sheet of
shining matter, and sometimes dip into it.
It is plain to me that the brightness is produced by the
breathing-organs of the Glow-worm. There are certain substances which,
when mixed with air, become luminous or even burst into flame. Such
substances are called combustible, and the act of their producing light
or flame by mingling with the air is called oxidisation. The lamp of
the Glow-worm is the result of oxidisation. The substance that looks
like whitewash is the matter that is oxidised, and the air is supplied
by the tube connected with the Glow-worm’s breathing-organs. But as to
the nature of the shining substance, no one as yet knows anything.
We are better informed as regards another question. We know that the
Glow-worm has complete control of the light he carries. He can turn it
up or down, or out, as he pleases.
If the flow of air through the tube be increased, the light becomes
more intense: if the same air-tube, influenced by the will of the
animal, stops the passage of air, the light grows fainter or even goes
out.
Excitement produces an effect upon the air-tube. I am speaking now of
the modest fairy-lamp, the spots on the last segment of the Glow-worm’s
body. These are suddenly and almost completely put out by any kind of
flurry. When I am hunting for young Glow-worms I can plainly see them
glimmering on the blades of grass; but should the least false step
disturb a neighbouring twig, the light goes out at once and the insect
becomes invisible.
The gorgeous belts of the females, however, are very little, if at all,
affected by even the most violent surprise. I fire a gun, for instance,
beside a wire-gauze cage in which I am rearing a menagerie of female
Glow-worms in the open air. The explosion produces no result: the
illumination continues, as bright and placid as before. I take a spray,
and rain down a slight shower of cold water upon the flock. Not one of
my animals puts out its light; at the very most there is a brief pause
in the radiance, and then only in some cases. I send a puff of smoke
from my pipe into the cage. This time the pause is more marked. There
are even some lamps put out, but they are soon relit. Calm returns, and
the light is as bright as ever. I take some of the captives in my
fingers and tease them a little. Yet the illumination is not much
dimmed, if I do not press too hard with my thumb. Nothing short of very
serious reasons would make the insect put out its signals altogether.
All things considered, there is not a doubt but that the Glow-worm
himself manages his lighting-apparatus, extinguishing and rekindling it
at will; but there is one circumstance over which the insect has no
control. If I cut off a strip of the skin, showing one of the luminous
belts, and place it in a glass tube, it will shine away merrily, though
not quite as brilliantly as on the living body. The presence of life is
unnecessary, because the luminous skin is in direct contact with the
air, and the flow of oxygen through the air-tube is therefore not
required. In aerated water the skin shines as brightly as in the free
air, but the light is extinguished in water that has been deprived of
its air by boiling. There could be no better proof that the Glow-worm’s
light is the effect of oxidisation.
The light is white, calm, and soft to the eyes, and suggests a spark
dropped by the full moon. In spite of its splendour it is very feeble.
If we move a Glow-worm along a line of print, in perfect darkness, we
can easily make out the letters one by one, and even words when they
are not too long; but nothing is visible beyond this very narrow zone.
A lantern of this kind soon tires the reader’s patience.
These brilliant creatures know nothing at all of family affection. They
lay their eggs anywhere, or rather strew them at random, either on the
earth or on a blade of grass. Then they pay no further attention to
them.
From start to finish the Glow-worm shines. Even the eggs are luminous,
and so are the grubs. At the approach of cold weather the latter go
down into the ground, but not very far. If I dig them up I find them
with their little stern-lights still shining. Even below the soil they
keep their lanterns bravely alight.