1902 Encyclopedia > Arachnida > Arachnida - Order VII: Araneidea - Secretion of Silken Threads / Spider Webs

Arachnida
(Part 27)



(27) ARACHNIDA - ORDER VII: ARANEIDEA - SECRETION OF SILKEN THREADS / SPIDER WEBS

The external spinning organs have been mentioned above; they are usually called, spinners, and are various in size, both actual and relative, in form and in structure, as well as in number. From the extremity of each of these organs the silk matter issues through numerous movable papilla, or spinnerets, which are similar to hollow bristles with enlarged bases; the papillae vary both in form and number in different species, and from each spinneret or papilla issues an exceedingly fine thread; this, uniting with threads from all the other papillae, forms the ordinary silk line, which though to ordinary observation simple, is composed of numberless threads from some one, or all, of the spinners. The matter from which the silk is formed is secreted in organs (silk-glands) situated within the ventral surface of the hinder part of the abdomen. These glands vary in number, size, and form in different species; they differ also in size and shape in the same species, for the secretion of different qualities of silk-matter; and each gland has a distinct duct terminating at the extremity of the spinners. The emission of silk matter appears certainly to be a voluntary act on the part of the spider; but it is a disputed question among arachnologists, whether spiders have the power forcibly to expel it, or whether it is merely drawn from the spinnerets by some external force or other. Mr Blackwall (Trans. Linn. Soc., xv. P. 455; Researches in Zoology, pp. 242-248, and History of Spiders of Great Britain and Ireland, p. 12) is of the latter opinion; while Mr R.H. Neade (of Bradford, Yorkshire), in Report of British Assoc., 1858, pp. 157-164 pl. 16, thinks that (from miscroscopic anatomical investigations which he has himself made) there is good evidence of spiders having the power to expel it, for he finds a certain muscular arrangement which would apparently suffice to give this power, and observers have actually suffice to give this power, and observers have actually seen the lines propelled. Mr Meade, however, need not be assumed to assert an indefinite power to expel threads of silk to any indefinite distance. The truth probably is, that spiders have, and do exercise, a general power not only of emitting silk matter from the secreting glands merely to and from the external orifices of the silk-tubes, but also of propelling it when necessary to some short distances; and that external causes (such as the movement of the spider while the end of its line is fixed, or when a current of air takes an emitted line and carried it in a direction contrary to that of the spider) also serve to draw the threads out. In all these ways, no doubt, are produced the gossamer lines which often cover the surface of the ground and herbage in autumn. In the formation of their snares spiders use the hinder or fourth pair of legs for drawing the threads tight, and apparently for ascertaining their power of tension. The third, or inferior tarsal claw, being usually strongly bent, no doubt enables spiders to perform these operations very readily. Some large groups of spiders do not spin any snares, and in them we usually find this third claw absent; while in one group, Eperides, whose snares are marvels of beauty and ingenuity, the third claw is very highly developed.

The snares of spiders have been made the basis of some of the primary divisions of the order. Characters, however, taken merely from habits and modes of life, can hardly be considered those on which systematic classification should be based, not to mention that those characters fail when we come to extensive groups spinning no snares at all. short of such a use of the snares of spiders, this part of the subject is of extreme interest and importance. The Latreillian divisions of snares is fairly and generally accurate; by this method they are divided into-Orbitelarioe, wher the plan is that of a circle or a portion of one, with lines radiating from a centrre, Retilerarioe, where a thin sheet of web is suspended among the branches of shrubs or in angles of buildings, and held up and down by lines in all directions above and below; Tubitelarioe, where the snare is a silken tube, inserted in crevices, and fissures, and casual holes, and with an open mouth more or less guarded or armed with insidious lines; and Territelarioe, in which a tube is spun in a hole formed by the spider itself, and closed sometimes by a close fitting cork-like, or sometimes by a scale-like or wafer lid, some times left kopen, and at other times closed by the falling over of a portion of the tube which protrudes from the surface of the ground (see Trapdoor Spiders, by J.T.. Moggridge, Lovell Reeve, London 1873-4). With respect to the economic or mercantile value of spider silk, the idea seems to have been entertained, from a remote period, that it might not only be turned to practical use in the manufacture of silk fabrics, but also be made to pay as a mercantile speculation. The possibility of making it into articles of wear is undoubted, as instances of it are on record. Upwards of 150 years ago Le Bon of France (languedoc) obtained, from spiders, silk which was afterwards woven into gloves and stockin gs; but Reaumur, appointed by the French Academy to investigate the matter, reported unfavour ably, doubting the possibility of rearing the spiders together owing to their voracious and cannibal propensities. In 1777 and 1778 a Spaniard, Raimondo Maria de Termeyer, published, in Italian periodicals, two memoirs on the subject; and afterwards, 1810, at Milan, another called Ricerche e Sperimenti sulla Seta de Ragne, in which he takes an opposite view to that of Reaumur. The latter work is of great rarity (vide B.G. Wilder in Harper's New Monthly Mag., xxxiv. P. 455). Termeyer contructed a small kind of stocks, in which the spider was fixed by the body, while with a little winding-machine he drew out and wound up the silk threads from the spider's spinnerets; but evidently nothing came of this further than an additional proof of the possibility of procuring and making use of spider silk, as well as of its strength and luster; this last, however, as well as other qualities of the silk, would probably be found, as with that of various silk worms, to vary according to t he species. Other experiments, with a similar general result have been made since (Zoologist, 1857, p. p 5835), the latest being those made by Dr B.G. Wilder, professor of anatomy in the Cornell University at Ithaca, United States (l.c. 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 spider's silk appears to have hitherto elicited only an unfavourable answer. Dr. Wilder, however, is still sanguine upon the point (vide B.G. Wilder, l.c. 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 trheads 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 is speaking of the armature of the legs of spiders. This instrument 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, exceedingly 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, PL. 31, 1831). Amaurobius similes (Bl.) and A. ferox (Koch) are common house spiders whose webs are thus furnished with carded silk from the supernumerary spinners. Doubts have been entertained quite lately by an eminent areneologist 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 (Hardwicke, London, Sept. 1874), a short article by H.J. Underhill, 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 clam 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 appear 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 they move about; and the setting in the fine weather being the signal for a spontaneous 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 marvelous 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. 2530284; Linn, Trans. 1831, pp. 471-479; and 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 (Orbitelarioe) are almost the only ones whose method of forming a snare has been at all minutely recorded. When the situation for the snare has been chosen, the area intended to be filled up by it is enclosed by various circumferential boundary lines, fixed to adjacent objects, the exact shape of the area being influenced 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 the wheel with ladder-like lines; this the spider does beginning from the centre and working towards the circumference, 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 "minute dew-like globules closely studding the line." It appears that when the viscid lines, intended for the capture of its prey, are completed, the spider cuts away the first, or unadhesive 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 Retitelarioe 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 interest.

Among the Tubitelarioe we have the observations of Mr Blackwall, directed chiefly to the formation by Amaurobius atrox of the peculiar fibrous and adhesive flocculus, drawn from the fourth pair of spinners by the "calamistrum," and disposed about the irregularly intersecting lines of the snare which is spun in the angles of walls, in crevices, between portions of detached rock, and other similar situations. The adhesive flocculus serves to entangle insects, and makes them an easy prey to the spider who lies in wait not far off, having formed a funnel-shaped tube of slight texture from its place of concealment to its snare.

With regard to the formation of the snares of the territelarioe, Mr Gosse has an interesting passage in A Naturalist's Sojourn in Jamaica; it is, however, too long for quotation here. Mr Moggridge also (l.c. infra) has some details respecting the formation and repair of their nests by trapdoor spiders in confinement. The chief point of interest, however, in regard to trapdoor snares, is whether the hinged lid is formed in one continuous piece with the tube, and then cut out, leaving a portion unsevered to form the hinge, or whether the lid is made separately, except in that part intended for the hinge. Niehter Mr Gosse nor Mr Moggridge enter into this question, though Mr Moggridge indeeded speaks (l.c. p. 118) of his belief that, when all is completed, the spider cuts away certain threads by which the door is supported on either side of the hinge. The present writer was once told by a gentleman who had formerly resided in the West Indies, that trapdoor spiders there invariably make the tube and lid of one continuous, solid, homogeneous piece, and then cut out the lid with the falces. This account, especially as coming from a non-naturalist, seemed improbable-a spider's falces being in no way, apparently, fitted for such an operation; if, however, the fact be that the lid, instead of being of one solid piece with the tube, is merely connected with it by a few supporting threads (in accordance with Mr Moggridge's belief), these could easily be torn away by the spider's falces, and the lid would be left free, except at the point where the hinge is formed.





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