NATURE’S PARASITES—PUSS-MOTH CATERPILLAR AND
ICHNEUMON-FLY—CATERPILLAR DEFENCES—WASPS AND THEIR VICTIMS—A
SPIDER CAUGHT—ANTS THAT ARE OGRES—OSPREY AND EAGLE—GULLS AND
SKUAS—PEEWIT AND BLACK-HEADED GULL.
In the sea-anemone affixing itself to the shell of the hermit-crab, who
becomes its friendly and interested landlord, we have seen one of the
more pleasing instances of association between two or more different
species of animals. There are many others, such as that between the shark
and the pilot-fish, the honey-guide and the ratel, the rhinoceros and its
little bird, etc., etc., which we can dwell upon with equal pleasure.
Some, however—and, unfortunately, they are much more numerous—are of a
darker character, repelling us almost as much by the picture which they
present of nature’s unbending cruelty as they arouse our admiration by
their wonderful ingenuity and adaptation of means to ends. The most
salient examples of this kind of living together—partnership we can
hardly call it—are to be found, perhaps, in the insect world. Parasitism
is the proper word for it, and the most salient, or at least the most
repulsive, examples are furnished perhaps by the hymenoptera—that genus
of insects in which the bees, wasps, and ants are included. Thus
almost all caterpillars—perhaps all—are victimised by some species of
ichneumon-fly—a wasp-like creature that seeks it out, pierces its soft
body with a long ovipositor, with which it is provided for the purpose,
and lays a number of eggs inside it. Having done this, it goes away, and
the caterpillar goes on feeding. It is, however, doomed, and destined
never to enter into the moth or butterfly state of existence. In due time
the eggs are hatched by the warmth of its own body, and on this body to
which they are so highly indebted, the young ichneumons, now in their
own caterpillar state, begin with unconscious ingratitude to prey. They
feast upon it day and night, but the creature, ordained by the iron laws
of nature to suffer in this way, is long-lived, and though sickening from
day to day, has often sufficient strength to become full-fed, and make
its cocoon, and pass into the chrysalis, or pupal, state. How long it
lives after that it is difficult to say. Probably some vitality remains
as long, or almost as long, as any part of itself does. All that we
know is that after a longer or shorter interval a score or so of ugly,
evil-looking ichneumon-flies issue from the dry shell of the chrysalis,
instead of the innocent and radiant creature that would otherwise have
done so.
It is curious—gratifying, too, if one allows oneself to give way to a
natural, though unreasonable feeling—to learn that some of these very
ichneumons themselves become, in a similar manner, the victims of others
of their own species. Thus from two corpses, on the slowly dying bodies
of both of which it has directly, or indirectly, fed, the third life in
death emerges, like some ghoul from a double tomb. Could the caterpillar
know that the being which so remorselessly preyed upon its tissues and
juices had a similar parasite within its own body, doing it to death in
the same horrid way, how relieved and almost happy it might feel! But
Nature, though she often brings in her revenges, seldom grants to her
suffering children the proverbial sweetness of revenge. Caterpillars,
however, do not always submit to the machinations of the ichneumon-fly
without a struggle, and in some cases they may be successful—how often
it is not easy to say—in guarding themselves against their attacks.
The puss-moth, for instance, which is especially liable to them, is
furnished, no doubt for that very reason, with a special weapon for its
defence. The end of its body is forked, and each fork is prolonged into
a sort of tail, from which a red filament can be extruded and waved
about at the will of the creature. In this way, and by its violent
contortions, it may sometimes succeed in whipping off, as it were, the
ichneumon that is attacking it, but it has another and more efficacious
means of defending itself. An aperture in the skin behind the head
communicates with a gland containing a clear fluid, forty per cent. of
which is formic acid, and the rest water. This the caterpillar can eject
with great force, and so pungent is it that a few drops falling on an
unwary ichneumon-fly are sufficient to incapacitate, if not actually
to kill it. Lizards, indeed, and monkeys, as has been experimentally
ascertained, are affected by this powerful irritant, nor, as far as
we know, is there any other animal secretion which contains so large
a proportion of strong acid. It is probable that the caterpillar of
the puss-moth is not the only one which has this power of spurting a
noxious fluid over its enemies, whilst many are protected in other
ways: some by their hairiness, others by being coloured like the leaves
they eat, or resembling, when at rest, a twig of the plant on which
they sit immovable. By these latter means they certainly avoid being
eaten by birds, and there seems no reason why they should not sometimes
deceive the ichneumon-fly also. But in spite of all defences, whether
consciously or unconsciously brought into play, a large proportion of
most caterpillars yield to destiny, and are slowly eaten alive by the
special parasite which Nature has provided for them.
Still worse, perhaps, though very similar, is the fate which various
insects—caterpillars, grasshoppers, etc.—as well as spiders, experience
at the hands of several species of wasps. These wasps are not social,
like the ones we are familiar with, and make no nest other than a cell in
which to place their eggs, together with the nourishment which the young,
when hatched, will require. This nourishment is the creatures aforesaid.
In the best-known instance the female wasp first makes a long tunnel in
the earth, with three or four separate chambers or cells at the end of
it, in which she deposits her eggs. She then seizes an insect, which, if
it is a grasshopper, or something equally active, struggles violently to
escape, and being often as large as, or larger, than the wasp herself,
the contest may be a long one. Invariably, however, the wasp—or _sphex_,
to give her her generic name—is victorious. She could, indeed, sting at
once, if she were so minded, but this she does not do. She reserves her
fire, so to speak, till, after more or less violent exertion, she has
succeeded in throwing her victim on its back. Then she stings it in two
particular spots, the throat, namely, and between the thorax and abdomen.
Instantly the struggles of the wretched creature cease: a ganglion, or
nerve centre, has, with each sting, been pierced—it is paralysed, but not
by any means dead. To kill it, indeed, is far from the intention of the
_sphex_ herself, and, kind and thoughtful mother, she does not allow the
meat which she provides for her offspring to go bad. It will last, as she
has managed it, for the whole time that it is wanted. All now is quite
satisfactory. She can be grateful for mercies vouchsafed. She rests for
a little, then seizing the helpless, living food by a leg or a wing, she
drags it—for it is usually too heavy for her to fly with—to the mouth of
her nursery larder. Here she leaves it for a little, while she enters to
see that all is well. Re-emerging, she seizes it again, drags it down the
tunnel, deposits it in one of the chambers, plasters it up, and leaves it
for a while. In due time she returns with, and inters, another paralytic,
and, having thus successively filled all the four chambers, she closes
the mouth of the tunnel and flies merrily away—doubtless
“With the gratifying feeling that her duty has been done.”
Of course, after a certain number of days, the eggs are hatched, each
young _sphex_ caterpillar immediately falls to, and the grasshoppers
that have been previously buried alive are now eaten alive—two kinds
of deaths which are equally unpleasant, and each of which lasts a long
time. However, they are paralysed—insensible, we may hope, therefore, to
such pain as insects feel. Whether the paralysis is mental as well as
corporeal it is impossible to say, but it may, in any case, be doubted
whether grasshoppers can suffer through the mind. Assuming that they
cannot, the inability, though it would involve another of an opposite
nature, must yet here be considered advantageous, since that sort of
pleasure which arises out of a just sense of the beautiful contrivances
and adaptations of nature, must always, we may suppose, be beyond the
capacity of an insect; and even were it not, it is the _sphex_ in
this case whose mind would, in all probability, be most open to such
reflections.
The habits of the _sphex_ have been studied in Europe by “that inimitable
observer,” as Darwin calls him, M. Fabre, from whose writings the
foregoing account has been compiled. In India its place is taken by a
very large wasp of a uniform steely-blue colour and a most venomous
aspect. This kind makes a clay nest, of about the size and shape of a
very large Brazil-nut, on the outside of some perpendicular surface, and
often chooses for this purpose the walls of houses or bungaloes. I have
watched it time after time flying in first with little glistening round
balls of moist clay, and afterwards with curled up balls of caterpillars
of about the same colour, but larger. With these she filled up one large
cell, the entrance to which she then closed with more mud. It was an
interesting sight, but, to enjoy it thoroughly, one ought to be an
optimist.
In the above-mentioned instances the cell which serves as cradle and
tomb combined is made by the provident mother. Some wasps, however, have
learnt to save time and trouble by walling the victim up in a cell of
its own manufacture, or, at least, of its own choosing. This happens to
a certain spider in South America, which sits in a little hole in the
ground waiting for insects either to pass or come in. The wasp, which is
blue like the other, but smaller than our common one, goes about from
hole to hole, and when it finds one occupied by a spider, goes a little
way into it, and then rushes out, hotly pursued by the owner. When on the
point of being overtaken it suddenly turns, grapples with the spider,
stings it, drags it back, paralysed, into its own hole, lays an egg by
it, and departs, having previously blocked up the entrance with earth.
The entrance of the wasp into the spider’s hole, with its retreat,
some time afterwards, in feigned alarm, so as to draw the spider out,
is certainly an act of great intelligence. The intelligence, however,
is surpassed, or exhibited in a more entertaining manner, in the case
of another wasp which has been seen to creep noiselessly round to the
entrance of a spider’s nest, and then wriggle one of its antennæ in front
of the opening. Upon this, the owner of the nest, a very large spider,
came out, and was at once stung to death by the wasp. The latter then
wriggled an antennæ again, and upon no notice being taken, entered the
nest and killed all the young spiders, which he then carried off at his
leisure.
Other wasps lay their eggs in the nests of humble bees, and the young
growing up there prey upon the honey and comb. Amongst ants, again, some
of the smaller species are parasitic upon the larger ones, an enforced
association which may be very much to the disadvantage of the latter.
Especially is this the case where an ant whose Latin name is _Solenopsis
fugax_—if it has an English one I do not know it—is the unbidden guest.
Lord Avebury tells us that it “makes its chambers and galleries in the
walls of the nests of larger species, and is the bitter enemy of its
hosts. The latter cannot get at them because they are too large to enter
the galleries. The little _solenopses_, therefore, are quite safe, and,
as it appears, make incursions into the nurseries of the larger ant,
and carry off the larvæ as food. It is as if we had small dwarfs, about
eighteen inches to two feet long, harbouring in the walls of our houses,
and every now and then carrying off some of our children into their
horrid dens.” The insect world is particularly rich in these parasitic
relations, but space will not allow me to enlarge upon them further.
Turning to birds, we meet with instances not less interesting, whilst
very much less painful, since here the victimised species is only robbed
by the other, and not so frequently as to prevent its making a living.
The osprey, for instance, which preys almost exclusively on fish, which
it hooks with its claws out of the water, is forced, though itself a
large bird, to give up much of its booty to the still more powerful
white-headed eagle. The latter sits on some rocky crag or peak “that
beetles o’er its base into the sea,” and watches with a greedy eye the
“inferior fiend,” as, far below, it hovers on broad wings above its
destined prey. At once the wings are closed, and the spray dashes over
them as the bird precipitates itself upon a gleaming light amidst the
waves. For a moment it is almost hidden in the foam and swirl, the next
it emerges out of it, and mounts with powerful beats into the air, its
head stretched shorewards, and its bent claws struck deep into the body
of a large fish, beneath the weight of which it labours. Slowly at
first, but gaining strength and speed as it ascends, it heads towards
the cliff’s face. Already it can see the crag on which its eyrie hangs,
when, like a thunderbolt, and with the shriek or laugh of a demon, the
lonely watcher, who has marked it all, hurls itself downwards on spoiler
and spoil. With a quick turn the startled bird avoids the furious rush,
but almost at the same moment another maniac laugh, answering the first,
drowns its own note of anger and despair, as the mate of the eagle that
has commenced the attack swoops towards it from a neighbouring pinnacle.
All striving now on the osprey’s part is in vain. Like storm-clouds the
two strong robbers gather above him and descend like the jagged lightning
out of them. Their screams sound almost in his ears, their claws have
cut his feathers, when his own reluctantly relax their grip, and the
glittering booty falls. Something falls with it—over it. There is a
rushing wind of wings, an overshadowing darkness in the air, the trail of
light is checked in its descent, and out of that whirlwind of excessive
speed an eagle soars serenely to the sky bearing a fish in its claws.
In their eyrie, or on a ledge of the precipice, the pair of imperial
brigands share their meal, or distribute it to their eaglets. The osprey
tries again, and may, perhaps, catch another fish before they have
finished.
[Illustration: HIGHWAY ROBBERY.
The osprey rose with its prey when the eagle swooped, but by swerving,
the osprey momentarily escaped. The eagle is shewn stopping himself
against the wind, to swoop again with fiendish cries until the osprey
drops his prey in terror.]
Other piratical plunderers are the skuas and some other members of the
gull family. With the former the practice is more habitual, or, rather,
it is pursued more to the exclusion of other habits of feeding. In the
more northern parts of the British Isles—especially in the Orkneys and
Shetlands—the lesser or arctic skua may be seen all day long during the
breeding season, taking toll of the various sea-fowl, as they fly with
fish to feed their young. One might think that when once the fish had
been swallowed there would be an end of the annoyance, and that the
rightful owner must, by the very nature of things, now be safe. Such,
however, is by no means the case. Most birds have no difficulty in
bringing up again what they have swallowed down.[1]
The skua, when it swoops upon a gull, does so with the deliberate
intention of forcing it to disgorge the fish it has swallowed, which
it then, like the eagle, catches in the air before it has touched the
sea. Should it not succeed in doing this, the fisherman asserts that
it will not touch it, but invariably leaves it lying on the water, or
on the land, should it chance to fall there. I have myself seen skuas
act in this manner, but am not so satisfied that it is their invariable
practice. Terns, should there happen to be a colony in the neighbourhood,
are particularly persecuted by these skuas, insomuch that the gulls
derive a distinct benefit from their presence. Puffins and guillemots are
also pursued, and so ingrained is the habit of piracy that the skuas will
sometimes, as it were, play at it, swooping at and chasing one another
in the same manner and with the same wild cries as when they practise
the art in earnest. Of course, under these circumstances neither bird
disgorges to the other, and it is easy to see that neither expects the
other to do so.
Though gulls uniformly suffer at the hands of the skuas, they can be
pirates too amongst each other, and in harbour or where fishing-smacks
are anchored nothing is commoner than to see a bird that has seized on
some offal of fish thrown overboard mobbed by a host of others, till the
morsel reappears again _de profundis_.
Only one British gull, however, as far as I know, has taken up piracy as
a profession, and that is the black-headed one. It is difficult in works
of natural history to find any reference to this interesting fact, but
it seems to be alluded to in one of the common or local names of this
species, viz. the peewit-gull. For here the parasitic relation is between
a sea-bird and a land-bird—the peewit, namely—which to me makes it still
more interesting. At certain times of the year, and in certain parts of
the country, almost every field or piece of land near the sea-shore in
which peewits are feeding is sure to have a few of these gulls scattered
about it. They stand, apparently, doing nothing, but are really keenly
on the look-out, and as soon as a peewit has found anything, come
sweeping down upon it. In the chase which ensues the pirate is not always
successful, but very generally the peewit drops his booty, and the gull
either catches it in the air or picks it up off the ground.
In all the above kinds of robberies the young of the piratical species
are fed more or less frequently with the food carried off by it from the
various victims. This, however, is only incidental to the main habit, so
that there is little in these bird doings to remind us of those horrid
relations between insect and insect, with some examples of which this
chapter opened, wherein one species is wholly sacrificed for the sake of
the young of another. There is, however, a nearer approach to this—since
though the effects are less tragic, the governing cause is the same—in
that instinct which impels some few birds to lay their eggs in the
nests of other species. Here, as the services of the foster parent are
required, it does not itself suffer, but its own young perish to make
place for the stranger. One most familiar example of this more advanced
and complicated kind of parasitism is, of course, the cuckoo, but as the
habits of this bird have been treated of in so many books, I need say
nothing of them here.