WATER-SPIDERS
Here is the place to insert a short account of some near relations of
_Agelena_ which we shall certainly not meet in our walk, but of which
the mode of life is too interesting to be altogether passed over in
silence.
We have seen that the class Crustacea (crabs, shrimps, etc.) is the
great division of the Arthropoda entirely adapted to an aquatic life,
breathing, by means of gills, the air which is dissolved in the water.
Insects and spiders are air-breathing, and properly belong to the land;
yet there are many insects which pass their early stages—often the
greater portion of their life—in the water, and some which are very
fairly at home there when adult. Such insects often have gills when
young, and are therefore at that period true water animals, like the
Crustacea.
The Arachnida—that division of the Arthropoda to which the spiders
belong—include a few groups which permanently inhabit the sea, and
could not live on land. There are even some weird creatures called
Sea-spiders (Pycnogonids), but these do not concern us, for they are
very far removed from the true spiders which are the subject of our
investigations.
Now the true spiders are always air-breathing, and if they venture
into the water at all they must frequently come up to the surface
to breathe, or else they must store up a reservoir of air beneath
the surface of the water if they are to avoid death by drowning.
Nevertheless some of them have been hardy enough to encroach on the
domain of the Crustacea. Not a few are able to run freely on the
surface of the water and even to dive occasionally for the purpose
of seizing one of its denizens, but the number of those which have
succeeded in really adapting themselves to aquatic life is very
limited, and is, as far as we know, restricted to two small groups,
both of them members of the Agelenidae.
Among the coral reefs of the Indian and Pacific oceans, and also off
the southern coast of Africa there are found spiders of the genus
_Desis_ which spend almost all their time under the surface of the sea,
from which they only emerge at low tide. They construct very closely
woven tents, impermeable to sea-water, which imprison air at low tide,
generally choosing for the purpose some cavity which has been excavated
by one of the burrowing molluscs. Beyond this we really know very
little about them, and there is much difference of opinion as to the
mode in which they obtain their food. Some writers state that they only
leave their shelters at low tide to chase small crustaceans, and that
when placed in vessels containing sea water they are quite helpless
and soon drown. On the other hand one observer found that a species of
_Desis_ was quite at home in a sea-water tank, in which it swam freely
and even attacked and fed upon a small fish. Possibly different species
of the genus behave in different ways, some being more truly aquatic
than others, though it is certain that the troubled waters of a coral
sea are not a very promising field for sub-aqueous operations. We know
a great deal more of the mode of life of those Agelenids which have
taken to living in fresh water. Indeed the subject of the water-spider,
_Argyroneta aquatica_, is so hackneyed that in dealing with it we shall
probably be telling the reader much of what he knows already, but that
possibility must be risked.
There is, then, in many of our lakes, ponds and slow-flowing rivers
with a weedy bed, a spider which has entirely taken to a water life,
and for which it is useless to search on land. It is a docile captive,
and consequently a favourite subject for transference to an aquarium,
where its habits can be observed at leisure. Its first care is to
construct beneath the water a small dome-shaped web, open below, and it
generally selects the under surface of the leaf of a water weed for the
purpose of anchorage, though a ready-made shelter is often furnished by
the empty shell of some fresh-water mollusc. Its next proceeding is to
fill this retreat with air in a very ingenious manner.
While swimming about in the water the spider has a most striking
appearance, its abdomen almost resembling a globe of quicksilver.
This is because the body is enveloped in a bubble of air, retained
largely by the long hairs with which it is clothed. Thus it carries its
atmosphere about with it, and as often as not it swims with its back
downwards, which has the effect of bringing the bulk of the air-bubble
towards its ventral surface, where the breathing pores are situated.
Now when the dome-shaped web is ready to be filled with air the spider
rises to the surface, lifts its abdomen above it, and brings it down
with a flop, thus imprisoning an extra large air-bubble which it
embraces with its hind-legs by way of holding it more securely, and
then, swimming rapidly down by means of its other legs to the web it
discharges its load of air beneath the downwardly directed mouth of the
dome.
By a frequent repetition of this process the dome is at length filled
and converted into a veritable diving-bell, in which the spider can
exist quite comfortably until the supply of oxygen in the imprisoned
air is exhausted and has to be renewed. From this base it issues forth
to feed upon fresh-water insects and crustaceans, sometimes even
attacking small fishes.
The proceedings of the male _Argyroneta_ in the mating season are
very curious. He seeks out the tent of a female and sets up his own
establishment—generally somewhat smaller—close at hand, filling it
with air in the approved manner. He then builds a sort of corridor
uniting the two domes, and when this is complete he bites through
the female dome, thus uniting the two air reservoirs by means of a
connecting tube. Not seldom it happens that the female is in no mood
for dalliance, and a battle royal ensues, with disastrous results to
both domiciles and the tube that connects them. The male, however,
is in this case well able to hold his own, for he is larger than
the female, a phenomenon elsewhere unknown in the spider realm.
_Argyroneta_ lives for some years, and makes two diving-bells each
year—one near the surface in summer and one at a greater depth in
winter. It was thought at first that one was constructed especially for
receiving the eggs and the other as a habitation, but the egg-cocoon
may be found in either, for there are two broods in the course of the
year. The winter dome is of very dense silk, glossy in appearance, and
giving the effect of a uniform sheet of silky material rather than a
fabric. Moreover its mouth is closed, and the spider remains inactive
within. It is this winter domicile that is most frequently found in the
shells of molluscs. The egg-cocoon is also dome-shaped, having a convex
upper and a flat under surface. The newly hatched young inhabit their
mother’s tent for a time and then set forth in the water to seek their
living and set up establishments on their own account.
There is only one known species of _Argyroneta_, widely distributed
in the temperate regions of Europe and Asia. The female is about
half-an-inch long, of no particular beauty out of the water, its colour
being reddish-brown, and its body and legs very hairy. There are,
however, a few New Zealand spiders rather closely allied to it and of
very similar habits.