THE ENEMIES OF SPIDERS
When one comes to consider the multitudinous risks to which a spider is
exposed during the whole course of its life it seems at first a little
surprising that the whole tribe has not long ago been exterminated.
Spiders continue to flourish, however, and it is very clear that
however careless Nature may be of the individual she is extremely
solicitous about the race.
The infant mortality among these creatures must be appalling. There is
first their cannibalistic propensity to be reckoned with. Newly hatched
spiders while still within the cocoon seldom attack each other, but as
soon as ever each sets up for itself, no quarter is given. It often
happens that members of a brood of sedentary spiders spin their first
snares in close contiguity, and if food is scarce they eat one another
without compunction. It is said that a few individuals of a brood may
be reared to maturity on no other food than their sisters and brothers!
The case of the survivor of the “Nancy Bell” in the Bab Ballads would
be exceedingly commonplace in the aranead world. We have seen, too,
how, on occasion, _Atypus_ will devour her young if they do not leave
the nest with due expedition. Then if the weather conditions chance to
be unfavourable just at the period of departure from the cocoon broods
are liable to perish wholesale, washed away and destroyed by deluges of
rain; myriads, too, must be carried out to sea in the course of their
ballooning operations, and never come safely to land.
But the mortality is probably even greater at a still earlier stage,
for hosts of spiders’ eggs never hatch at all, and this for two
reasons. In the first place, the silk of spiders is a favourite
material with many birds for the lining of their nests, and many of
them use the cocoons for this purpose. Secondly, there are numerous
Ichneumon flies which attack and parasitise spiders’ cocoons, piercing
them with their ovipositors and laying their eggs inside. The eggs of
the Ichneumon fly hatch first and feed upon the eggs of the spider.
Two such flies are known to attack the cocoons of the garden-spider,
and not a single spider will emerge from a cocoon thus parasitised.
The spiders whose cocoons are most subject to these attacks belong,
as might perhaps be expected, to the sedentary groups, and the most
elaborate but unavailing precautions are often taken to render them
Ichneumon-proof. The cocoons of the peripatetic wolf-spiders have never
been observed to be parasitised.
Even if a spider has survived these early perils there are still many
dangers ahead. During its period of growth it has to moult some eight
or nine times, and the operation is at least as dangerous as, say, an
attack of measles to the human infant. For some time beforehand feeding
ceases, and the animal becomes inert and apparently dead, but presently
the integument splits, and out struggles the spider, pale and soft, and
leaving behind it not only the outer skin but the lining of most of
its alimentary canal and of its breathing tubes. Sometimes, as we have
said, it fails to extricate itself and dies; quite often it emerges
with the loss of a limb, which will reappear—reduced in size—at
the next moult. It is necessary to go into retreat for a time after
moulting, till strength has returned and the integument has hardened.
But the dangers of moulting, though not negligible, are insignificant
beside others to which the spider is exposed during its later stages,
nor is a prolonged dearth of food necessarily fatal, for, as we have
seen, a spider can fast for an astonishing time and yet retain its
health if it has a fair supply of water. But there are terrible
enemies at hand from which it has little or no protection. Birds,
of course, come first, for to most insectivorous birds spiders are
acceptable morsels. I have seen a hedge sparrow going conscientiously
over a trellis work and picking out all the spiders from the nooks and
corners. Then insectivorous mammals make no distinction between the
Insecta and the Arachnida, and often eat spiders with avidity, as also
will toads and lizards.
Moreover, Ichneumon flies do not confine their attention to cocoons,
but often attack well-grown spiders. They invariably lay their eggs on
one spot—at the very front of the abdomen, near the cephalothorax,
where the spider is powerless to dislodge them. The egg hatches out
to a grub which is a veritable “old man of the sea” on the spider’s
back, and there it remains until it causes the death of its victim by
feeding on the contents of the abdomen. Four such Ichneumon flies have
been found to attack the garden-spider, and no kind of spider seems
exempt. How they contrive to deposit their eggs in the proper place
without great danger of themselves falling a prey to their victims
is a mystery. To venture into a garden-spider’s web for the purpose
would seem a fool-hardy proceeding. The actual deposition of the egg
has seldom been witnessed, but in one of the few cases that have come
under observation the spider made little resistance and appeared quite
demoralised. It was hanging from a thread, down which the Ichneumon fly
was seen to crawl. When it reached the spider the latter dropped an
inch lower on two or three occasions but then remained passive, and the
parasite on nearing it, turned round, backed down the line, and with
great care and deliberation attached an egg at the usual spot.
But no enemies of spiders are more terrible than some of the solitary
wasps, and gruesome indeed is the fate of any creature that falls
into their clutches. The social wasps often capture spiders to feed
their young but in their case the proceeding is summary and without
any finesse. They merely catch a spider, sting it to death, cut it
to pieces with their jaws, and feed it into the mouths of their
expectant grubs. The treatment is brutal enough, but at all events it
is expeditious. Now the solitary “digger” wasps never see their young.
They make cells, either by burrowing in the ground or by agglomerating
particles of mud or gravel, and in each cell is placed an egg together
with sufficient food to last the grub which hatches out for the whole
of its larval existence. The mother will not be at hand—as is the
social worker-wasp—to supply new food as required, and it is therefore
necessary so to arrange matters that the food provided may retain its
fresh condition for at least a fortnight. On the other hand the victims
must be deprived of all power of motion, otherwise the egg will stand
a great chance of being displaced and crushed, and even if it hatches
it will be unable to commence its meal upon the struggling spider.
Now in the whole range of animal instinct there is nothing more
remarkable than the manner in which the solitary wasps have learnt to
solve this problem. The solution lies in so stinging the victim that
it is paralysed but not killed, and though quite unable to move, it
neither shrivels nor decays, but remains perfectly sound and edible for
two or three weeks. To accomplish this result the wasp acts as though
it possessed a knowledge of the minute anatomy of its victim, and
knew to a hair’s breadth the position of the principal nerve ganglia
which control its actions. Into these it unerringly thrusts its sting.
But even accuracy of aim is not everything; there must be the finest
discrimination in the severity of the wound. A slight excess, and the
animal is killed; too timid a thrust will not destroy movement. When
the delicate operation has been successfully performed, the paralysed
spider is dragged into the cell, placed on its back, and an egg
carefully deposited at the base of its abdomen, after which the cell
is sealed up. Some wasps, instead of providing a single large spider,
store their cells with a number of smaller victims, all rendered limp
and motionless.
In attacking a spider the first action of one of these wasps is to
remove it from its natural environment. A garden-spider in its web, or
a burrowing spider in its tunnel are more or less formidable, but if
the one can be thrown down, or the other dragged forth into the open,
they are well-nigh defenceless. Therefore in attacking an Epeirid the
wasp first darts at it, seizes a leg, and attempts to jerk it out of
the web. If unsuccessful, the spider will now be on its guard, and the
wasp leaves it and tries the same manœuvre on another individual. Taken
by surprise, it is instantly thrown to the ground, and can then offer
no effectual resistance. Even the large “bird-eaters” fall victims to
these terrible foes.