Spiders :: CHAPTER V

CHAPTER V

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.

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.

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