Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/316

Rh 296 AEACHNIDA [SPIDERS. Cornell University at Ithaca, United States (I.e., supra), with Nephila plumipes (Koch), a large epeirid abundant in South Carolina. His experiments appear to have been unconsciously, though surprisingly, similar, and with a similar result, to those of Termeyer ; but the question as to the mercantile importance of spiders silk appears to have hitherto elicited only an unfavourable answer. Dr Wilder, however, is still sanguine upon the point (vide B. G. Wilder, I.e., and also in Proc. Boston. N. H. Soc., October 1865, with other references there noted, as well as in The Galaxy, July 1869, pp. 101, 112). In Zoologist, 1858, p. 5922, a correspondent, speaking of the strength of the silk threads of Nephila clavipes, says that small birds are sometimes entangled in its webs, and that the ladies of Bermuda use the threads for sewing purposes. See also Phil. Trans. 1668. Before leaving this part of the subject, we must notice the office of the calamistrum, mentioned before in speak ing of the armature of the legs of spiders. This instru ment is found in the females of various genera and families; it consists usually of a closely set double row of curved spine-like bristles on the upper sides of the metatarsi of the fourth pair of legs, forming a kind of comb, whence Mr Blackwall, by whom it was first discovered and its use perceived, gave it the name it bears. Its office is to card, or curl, or tease a particular kind of silk emitted from the supernumerary spinners, mentioned above as always, in the female, correlated with this instrument ; the silk so curled and carded is, owing to the fineness of its fibres, exceed ingly prehensile, and being disposed about the lines of the spider s snare, serves to entangle the insects which come in contact with it (vide Blackwall, Linn. Trans., vol. xvi. p. 471, pi. 31, 1831). Amaurobius similis (Bl.) and A. ferox (Koch) are common house spiders whose webs are thus furnished with carded silk from the supernumerary spin ners. Doubts have been entertained quite lately by an eminent araneologist as to whether the supernumerary spinners are indeed true spinning organs (T. Thorell, Synonyms of European Spiders, p. 595, 1873). It is therefore interesting to find in Science Gossip (Hard- wicke, London, Sept. 1874), a short article by H. J. Under- hill, in which the anatomy of these spinners is described and figured from original microscopic investigations, proving the recorded observations of Mr Blackwall, and others also, as to their being true spinners, to be correct. The white flake-like flocculi often seen floating in the air on a calm autumnal afternoon are composed of spider silk emitted by numerous immature spiders, of many species and genera, passing through the air on their lines, which being so much lighter than the atmosphere, serve to bear them away with every breath of wind. The flocculi ap pear to be agglomerations of fine lines, adhesive from their fineness and fibrous nature. In this respect they differ from the ordinary gossamer lines, which are merely the threads left by small and immature spiders (chiefly of one or two families) as they pass from blade to blade and plant to plant. It seems probable that nearly all spiders leave a line or lines, which proceed from one or more of their spinners, whenever and wherever they move about ; and the setting in of fine weather being the signal for a spon taneous restlessness, we can hence better understand the almost sudden appearance of myriads of lines stretching over the surface of the earth, and often extending high up into the air. Numerous papers have been written on gossamer, chiefly by German and French writers, with the latter of whom the subject has been sometimes treated on the basis of the marvellous and superstitious. These lines are called by them fils de la Vierge. Our space does not permit of any great detail respecting the various kinds of snares, nor regarding the way in which different spiders construct them. Accounts may be found, by Mr Blackwall, in Zool. Journ. 1830, pp. 181-189; as well as in the Report of the British Association for the Advancement of Science for 1844, pp. 77-79 ; Researches in Zoology, 1st ed. 1834, pp. 253-284; Linn. Trans. 1831, pp. 471-479; and A History of the Spiders of Great Britain and Ireland, 1861-64, all by the same author; and the exceedingly interesting details on this subject in Kirby and Spence s Entomology, pp. 227-267 (7th ed.), are within every one s reach. The following notes, however, gathered from the works of Mr Blackwall and others, as well as from the writer s own observations, may be of interest : The geometric spiders (Orbitelariai) are almost the only ones whose method of forming a snare have been at all minutely recorded. When the situation for the snare has been chosen, the area intended to be filled up by it is en closed by various circumferential boundary lines, fixed to adjacent objects, the exact shape of the area being in fluenced by the situation and surrounding circumstances. A diagonal thread is then spun across it, and from about the central point of this thread another is carried to the marginal line ; returning to the central point along the line just spun from it, the spider carries another line to the margin, and fixes it at a short distance from the first. In this way the whole area is gradually filled up, the spider always returning to the centre along the last line spun, and starting again thence fixes a fresh one to the boundary line of the snare. The form of the snare is now that of a wheel ; the next operation is to cross the radii of tho wheel with ladder-like lines ; this the spider does, begin ning from the centre and working towards the circumfer ence, with a single spiral line, which is fixed, or glued, with a minute portion of viscid matter, to each of the radii as it crosses by an application of the spinners ; at some distance from the centre this spiral line is discontinued and another is begun ; this is formed of quite a different kind of silk matter, being viscid, and retaining its viscidity in the form of &quot; minute dew-like globules closely studding the line.&quot; It appears that when the viscid lines, intended for the capture of its prey, are completed, the spider cuts away the first, or uuadhesive line, which is of no service in the entanglement of insects, its office appearing to be chiefly to strengthen the snare while the viscid line is being spun, and to enable the spider to traverse the parts with greater ease. Modifications of the above method are, no doubt, adopted by some species, but, substantially, it is believed that most orbicular snares are thus constructed. The mode of formation of the snares of the Rttitelarice does not appear to have been observed, probably owing to their being made almost wholly during the night. The snare of one of these spiders, Linyphia marginata (Bl.), is an exceedingly perfect one of its kind, and abundant in most localities; it consists of a thin horizontal sheet of web, suspended among the branches of low evergreen trees and shrubs, by a maze of intersecting lines above, and held down firmly, on the under side, by some short, tightly- strained perpendicular lines, fixed below to others, which traverse each other in all directions; beneath the horizontal sheet of web, head downwards, the spider remains patiently watching for such insects as may become entangled in the upper maze of lines. The mode in which the horizontal web is suspended and braced down, would, without doubt, if it could be observed, prove to be a point of great in terest. Among the Tubitclarice we have the observations of Mr Blackwall, directed chiefly to the formation by Amau robius atrox of the peculiar fibrous and adhesive flocculus, drawn from the fourth pair of spinners by the &quot; calamis trum,&quot; and disposed about the irregularly intersecting liuea