The snare is now practically complete, and the proprietor takes up her position either in the centre thereof, or in some retreat close at hand, and connected with the hub by special lines diverging somewhat from the plane of the web. Notwithstanding the possession of eight eyes—which, in sedentary spiders, are by no means sharp-sighted—it is mainly by the sense of touch that the spider presently becomes aware that an insect is struggling in the net. She immediately rushes to the spot, and suits her action to the emergency.
If the intruder is small, it is at once seized, enveloped in a band of silken threads drawn out from the spinnerets, and carried off to the retreat, to be feasted on at leisure. If it seems formidable it is approached carefully—especially if armed with a sting—and silk is deftly thrown over it from a safe distance till it is thoroughly entangled, and can be seized in safety by the venomous jaws of its captor. Sometimes the insect is so powerful, or the spider so sated with food, that the latter hastens to set free the intruder by biting away the threads which entangle it before much havoc has been wrought with the net.
The viscid matter on the spiral line dries up after some hours, so that, even if the web has not been destroyed by insects and stress of weather, this portion of it must be frequently renewed. Commencing a new web is, as has been seen, a troublesome matter, and it will readily be understood that the spider prefers, where practicable, to patch up the old one. This is done by biting away torn and ragged portions and inserting new lines in their place.
The part played by the various spinning glands in the construction of the orb-web may be briefly stated.[[275]] The ampullaceal glands furnish the silk for the foundation lines and radii. The spiral has a double ground-line proceeding from the middle spinnerets, but it is not quite certain whether it proceeds from the ampullaceal or the tubuliform glands. The chief function of the latter, in the female, is to furnish silk for the egg-cocoon. The viscid globules are the products of the aggregate glands. The aciniform and piriform glands provide the multitudinous threads by which the spider anchors its various lines and enwraps its prey.
Some Orb-weavers always decorate their snares with patches or tufts of flossy silk. In the snare of the North American Argiope cophinaria the hub is sheeted, and from it extends downwards a zigzag ribbon of silk stretched between two consecutive radii. Vinson[[276]] discovered a remarkable use for similar zigzag bands in the web of the Mauritian spider, Epeira mauritia. It furnished a reserve supply of silk for enveloping partly entangled insects whose struggles were too vigorous to succumb to the rather feeble threads which the spider was able to emit at the moment of capture. The spider was able to overcome a grasshopper much more powerful than itself by dexterously throwing over it with one of its hind legs a portion of the ribbon of silk which it had thus stored up for emergencies.
Fig. [191].—A, Snare of Hyptiotes cavatus; B, enlarged view of the Spider, showing the “slack” of the hauled-in line. (After Emerton.)
Many orb-webs are defective, a sector of the circle being uniformly omitted in the structure. The genus Hyptiotes does not belong to the Epeiridae but to the cribellate Uloboridae, but its defective orb-web is so curious that it deserves a special mention. A single foundation-line is laid down, and from it four radii are drawn and are connected with cross lines, the snare constituting about one-sixth of a circle. From the centre of the incomplete circle a thread proceeds to some more or less distant object, and on this the spider takes up its position, inverted, and hauls in the line till the snare is taut. When the trembling of the line shows the spider that an insect has struck the net, it lets go with its fore-legs, and the web, springing back to its normal position, entangles the intruder more thoroughly by its vibrations. When large insects are in question the spider has been observed to “spring” the net several times in succession. H. cavatus is common in the pine woods of Pennsylvania, but the only English species, H. paradoxus, is extremely rare.
A remarkable spider has been discovered in Texas by M‘Cook, which, after building a horizontal orb-web, converts it subsequently into a dome (Fig. [192]) of exceedingly perfect form. It is named Epeira basilica, and has been the object of careful study by Dr. Marx, who observed the whole process of web-construction. Threads are attached at various points on the upper surface of the horizontal wheel, the central portion of which is gradually pulled up until the height of the dome is nearly equal to the diameter of its base. But the snare of this spider does not consist of the dome alone. A sheet of irregular lines is stretched below, while above there is a maze of threads in the form of a pyramid. Several other Orb-weavers, as, for instance, E. labyrinthea and E. triaranea, supplement their typical webs by an irregular structure of silk, and thus form connecting links, as regards habit, between the group of which we have been speaking and the Theridiidae or Line-weavers, which may now briefly be dealt with.
