Two stout bridge-lines thus constructed will form the upper and lower boundaries of the net. The lateral limits are easily formed by cross lines between them at a convenient distance apart. The spider chooses a point, say, on the upper bridge-line, fixes its thread there, and carries it round to the lower line, where it is hauled taut and firmly attached. Two such cross lines give, with the bridge-lines, an irregular four-sided figure within which to stretch the snare, and now the work is perfectly straightforward, and can proceed without interruption.
Attention is first paid to the radii of the circular web. The first radii are formed by drawing cross lines within the framework in the same manner as before, but the spider carefully attaches these where they intersect by a small flossy mass of silk, and this central point or hub becomes the basis of its subsequent operations. It is a simple matter to add new radial lines by walking from the centre along one of those already formed and fixing the thread to some new point of the circumference. They are not laid down in any invariable order, but with a kind of alternation which has the general effect of keeping the strain on every side fairly equal. Almost every time the spider reaches the centre it slowly revolves, uniting the radii afresh at their point of junction, and increasing the strength and complexity of the hub. It also occasionally digresses so far as to stretch the whole structure by bracing the framework at additional points, so that it loses its four-sided form and becomes polygonal. We have now a number of spokes connecting a central hub with an irregular circumference.
The hub is next surrounded by what Dr. M‘Cook calls a “notched zone,” consisting of a few turns of spiral thread which serve to bind more firmly the spokes of the wheel. The most important part of the work is still to be performed. The lines hitherto laid down are perfectly dry and free from viscidity, so that an entangled insect would easily be able to free itself. A viscid spiral line remains to be spun, and the snare will be complete. The precise method of laying this down will vary somewhat according to the species, but, to refer again to the large Garden-spider, the proceeding is as follows:—Commencing at a point somewhat outside the notched zone, the creature rapidly works in a spiral thread of ordinary silk with the successive turns rather far apart. This forms a kind of scaffolding, by clinging to which the spider can put in the viscid spiral, which it commences at the circumference.
Its action now becomes exceedingly careful and deliberate, though by no means slow, and so great is its absorption in the work that it may be observed quite closely with a hand-lens without fear of interrupting it. The proceeding consists in drawing out from its spinnerets with one (or both) of its hind legs successive lengths of a highly elastic line, which it stretches just at the moment of fixing it to a radius, and then lets go with a snap. There is no hesitation or pause for consideration, but there is a peculiar deliberateness in drawing out each length of the thread which, together with stretching and sudden release, require explanation. Now, it has already been mentioned that the framework and radii of the snare are not at all moist or adhesive. This, however, is not the case with the spiral, upon which the spider chiefly relies in capturing its prey. A close examination of it—even with the naked eye—will show it to be beaded over with little viscid globules which, under a low magnifying power, are seen to be arranged with remarkable regularity.
Fig. [190].—A, B, C, D, Stages in the formation of the viscid globules of the web.
A very convenient method of investigation is to carry off a newly-constructed web—or, better still, one not quite finished—on a piece of plate glass, to which it will adhere by reason of the viscid spiral, and on which it may be examined at leisure. Immersion in a staining fluid will colour the viscid spiral, and show its structure in a striking manner. It will appear to consist of a thread strung with beads of two sizes, occurring with pretty uniform alternation, though two of the larger beads are often separated by two or more of the smaller.
Until recently it was supposed that the deposition of these beads upon the spiral line was a subsequent operation, and, in view of their vast numbers and regularity, the circumstance naturally excited great wonder and admiration. Blackwall[[273]] estimated that, in a fourteen-inch net of Epeira cornuta, there were at least 120,000 viscid globules, and yet he found that its construction occupied only about forty minutes! The feat, from his point of view, must be allowed to be rather startling.
As a matter of fact, the thread, on being slowly drawn out, is uniformly coated with viscid matter which afterwards arranges itself into beads, the change being assisted by the sudden liberation of the stretched line.
Boys[[274]] has shown their formation to be quite mechanical, and has obtained an exact imitation of them by smearing with oil a fine thread ingeniously drawn out from molten quartz. The oil arranged itself into globules exactly resembling the viscid “beads” on the spider’s line. If the web be carried bodily away on a sheet of glass, as above described, while the spider is engaged upon the spiral line, the experimenter will have permanent evidence of the manner in which the globules are formed. The last part of the line will be quite free from them, but uniformly viscid. Tracing it backwards, however, the beads are soon found, at first irregularly, but soon with their usual uniformity. The thread which the spider has thus “limed” for the capture of prey is really two-stranded—the strands not being twisted, but lying side by side, and glued together by their viscid envelope.