TRAP-SNARES AND BALLOONS
There are some interesting variants of the circular snare spun by some
exotic Epeirids. One North American species spins it in a horizontal
position and then raises the centre, and, by an elaborate system of
stay lines from above, converts it into a very accurately shaped dome.
A whole group of orb-weavers habitually decorate a sector of the snare
with bands of flocculent silk, the object of which for a long time
puzzled arachnologists, till it was observed that the spider drew upon
this reserve supply of material to wrap up particularly obstreperous
insects. It is not unusual for a spider of one of the common species to
remove a whole sector of the snare, and by stretching a line from the
centre to a place of retreat along the gap thus formed, to provide an
unencumbered avenue between its home and its post when on duty. For it
must not be forgotten that a spider has to walk warily on its own web,
and must avoid, as far as possible, treading on the adhesive lines, or
delay and damage to the structure are sure to ensue.
As a rule the circular snares of the different British species are of
a very uniform pattern, differing chiefly in the degree of neatness
with which they are constructed, and in certain minor details of the
“hub,” but we have one spider, _Hyptiotes paradoxus_—an exceedingly
rare species, scarcely ever seen beyond the limits of the New
Forest—which makes such a strange snare that it seems a pity to omit
all mention of it. It consists of a sector—about one sixth of the full
circle—comprising about four radii with cross lines. From the point
where the radii meet, a “trap-line” connects the sector to another
point of attachment; on, or rather under, this trap-line, the spider
takes up its position, hauling it in so as to tighten the web and to
leave a slack portion of the line between the points where it holds
on by its front and hind legs. When an insect impinges on the web and
causes it to tremble, the spider immediately lets go with its fore
legs, and the consequent vibration of the web helps to entangle its
prey.
The circular snare is the highest form of spinning work attained by
spiders, and there is little temptation to expend much time in studying
the cruder structures that meet the eye everywhere, but two other types
are worth a brief notice. Examine any garden bush—particularly a
holly bush, of which the rather rigid leaves provide excellent points
of support—and you will find numberless small webs made without any
discoverable method, the lines crossing one another at random in all
directions. These are the snares of some species of _Theridion_, and if
the webs lack interest the spiders themselves are worth looking at, for
they are nearly always quite prettily ornamented.
The other common type of snare is that of _Linyphia_. It is larger and
of more definite design, consisting of a finely-spun hammock stretched
horizontally, and surmounted by a labyrinth of irregular lines. Flies
entangled in the labyrinth fall upon the hammock in their struggles to
escape, and the spider is at hand—always on the under surface of the
hammock—to ensure their capture.
Having noted these three common types of snare, let us leave the garden
and choose a new field for our observations.
If it is an absolutely calm sunny October morning it will be a suitable
occasion for visiting an iron railing, the “knobbier” the better. Early
summer will do, but late autumn is generally more fruitful. Almost
any railing will serve, but the most satisfactory kind is one with
the uprights surmounted by round knobs, and not by spikes. We see at
once that the knobs, and the upper rail are glistening with silken
lines; many spiders have obviously been at work there. Lines streak
the top-rail in all directions, stretch from knob to rail, or from
knob to knob if not too distant, while here and there loose ends or
streamers flutter gently in the slight currents of air. And closer
inspection reveals various small objects moving among this labyrinth
of threads. Most of them are spiders, though insects, and particularly
weevils, are not wanting. No doubt the weevils know their own business,
though the writer has not been taken into their confidence, but the
spiders are the particular object of our investigation. And first of
all note that it is a veritable race-congress of spiders; the most
varied groups are represented. Wolf-spiders (Lycosidae) which under
ordinary circumstances rarely leave the ground are found in company
with crab-spiders (Thomisidae), jumping spiders (Attidae), as well as
Epeiridae and Theridiidae of which we already know something. They
have only one thing in common—they are either small species or small
and immature specimens of larger species. They seem to be scrambling
about in a meaningless sort of way, paying little or no attention to
each other—which is odd, for spiders are terrible cannibals, and as a
general thing it would be exceedingly unsafe for a small spider to rub
shoulders with a larger one of a different species.
The majority of them are very small, more or less black, Theridiid
spiders, the “micros” of the tribe, and their proper home is among
the roots of grass and herbage. Many of these are interesting objects
for the microscope—especially if males—because of the remarkable
protuberances or turrets which rise from their heads and bear their
eyes as on a watch tower.
These spiders are clearly not “out” for food; they have left their
ordinary beat for quite another purpose, and we shall probably not
have to wait long before discovering it. Some one of the group ceases
its apparently purposeless wandering, and, turning its head in the
direction of what slight wind there is, raises its fore-body to the
full extent of its straightened legs, and elevates its abdomen to the
utmost. Now watch closely—using a handglass if you have one—and you
will see streamers of silk proceeding from its spinnerets. They are
shot out for a short distance and then the air current draws them out
further till they often extend for several feet, though their extreme
fineness makes it almost impossible to form an accurate judgment of
their length. Meanwhile the spider has not merely been standing on its
“toes,”—it has been firmly gripping the silken lines on the railings
with its claws. Soon it feels the pull of the streaming threads,
and when the tension is sufficient it lets go with all its claws
simultaneously, vaults into the air and sails away. Sometimes a start
is made prematurely and the insufficient buoyancy of the streamers
causes the spider to descend almost at once, and a new start is made.
[Illustration: Fig. 4. Young spider preparing for an aerial voyage.]
This, then, is the habitual method by which new broods of spiders
distribute themselves, especially the sedentary kinds which would
otherwise soon become over crowded in the neighbourhood of the parent
nest. And we really need not have sought out a railing at all except
for its very great convenience of observation. The same thing is going
on everywhere. It largely accounts for the astonishing carpet of silk
that the dew reveals to us on lawns and meadows at such times of the
year. Young spiders have been busy from early dawn crawling over the
grass, climbing the higher blades, and setting sail, and the whole
field is covered with their lines. Railings come in handy as furnishing
an elevated starting point, but any shrub or bush will do, and young
spiders have been seen setting sail from the parent web itself.
McCook has given some interesting notes of his own observations on
aeronautic spiders. He followed an Attid spider fifty feet till it was
carried upward out of sight in a current of air. A Lycosid disappeared
in the same way after being followed—at a run—for a hundred feet. The
largest Epeirid he ever saw taking flight was “the size of a marrowfat
pea, say one-fourth of an inch long. After having floated over a field
and above a hedge-row, it crossed a road and anchored upon the top of
a young tree.” But perhaps his most interesting observation was on the
ability of spiders to control in some measure the duration of their
flight by reefing their sails if they wish to descend, for he saw a
ballooning spider collecting some of the streamers into a ball of silk
which accumulated near its mouth as it gradually sank to earth.
The phenomenon known as “gossamer” has puzzled people for centuries,
and English poetical literature is full of allusions to it. Chaucer
classes it with “ebbe and floud” as an unsolved riddle, and Spenser,
Quarles and Thomson all make mention of it, generally embodying the
popular belief that it somehow had its origin in dew. “Scorchèd deaw”
Spenser calls it, while Thomson’s expression is “dew evaporate.” The
phenomenon in question is the occasional appearance of vast numbers of
silken flakes which fill the air, and which in some recorded instances
extend over many square miles and to a height of several hundred feet.
Our observations will have given a clue to its origin which is entirely
attributable to spiders, and in large measure to their ballooning
habit, though no doubt reinforced by a large quantity of silk spun
for other purposes and caught up into the air by the breeze. For a
vivid account of such a shower the reader is referred to Letter LXV of
White’s _Natural History of Selborne_, and Darwin in his _Naturalist’s
Voyage_ (Chap. VIII) records a case of the “gossamer spider” descending
in multitudes on the “Beagle” when sixty miles from land.
In the ballooning habit we have the probable explanation of the
wide distribution of certain species of spiders which seem at first
exceedingly ill adapted for covering large distances. The Huntsman
Spider, _Heteropoda venatorius_, is practically cosmopolitan in
tropical and sub-tropical regions and the usual view has been that
ships have conveyed it from port to port. McCook, however, gives
several reasons for believing that the trade winds have much more to do
with the matter, and this may well be the case, though both agencies
have doubtless been at work.
Very likely it was not obvious to the reader why he was recommended to
select a particularly calm, sunny autumn day for his study of spider
aeronautics; a strong steady breeze might well appear more suitable for
the purpose. Yet he would find these operations at a standstill on a
windy day, and the best possible conditions are a still warm morning
after a spell of cooler weather. The lightest air-currents serve to
float the delicate silken threads, and, what is more important, the
increase of temperature causes an upward draught which rapidly carries
the spider to a useful height where it sails gently away instead of
being swept roughly over the surface of the ground.