The Atlantic Monthly, Volume 18, No. 106, August, 1866. Various
Чтение книги онлайн.
Читать онлайн книгу The Atlantic Monthly, Volume 18, No. 106, August, 1866 - Various страница 5
Название: The Atlantic Monthly, Volume 18, No. 106, August, 1866
Автор: Various
Издательство: Public Domain
Жанр: Журналы
isbn:
isbn:
The silk obtained the first day was of a deep yellow; to my great astonishment, the second reeling from the same spider gave silk of a brilliant silver-white color; while on the third occasion, as if by magic, the color had changed again, and I got only yellow silk. The hypothesis of individual peculiarity, adopted the previous year to explain why some spiders gave yellow, and others white silk, was now untenable; and, remembering that, beside these two positive colors there was also (and indeed more commonly) a light yellow, as if a combination of the other two, I saw that the real solution of the mystery must lie in the spinners themselves. Examining carefully the thread as it came from the body, it was seen to be composed of two distinct portions, differing materially in their size, their color, their elasticity, and their relative position; for one of them was white and inelastic, crinkling and flying up when relaxed, and seemed to proceed from the posterior of the two principal pairs of spinners, while the other was larger, yellow, so elastic that when relaxed it kept its direction, and seemed to come from the anterior pair of spinners, and so, in the inverted position of the spider, was above the other. By putting a spider under the influence of chloroform, and then carrying the first thread under a pin stuck in a cork to one part of a spindle, and the second or yellow line over another pin to a different part of the spindle, I reeled off from the same spider, at the same time, two distinct bands of silk, of which one was a deep golden-yellow, the other a bright silver-white; while, if both threads ran together, there was formed a band of light yellow from the union of the two. Thinking such a difference must subserve some use in the economy of the insect, I made a more careful examination of its webs. At first sight these resembled those of most geometrical spiders, in being broad, rounded, nearly vertical nets; but they were unusually large, and in their native woods often stretched between trees and across the paths, so as to be two, three, and even more, feet in diameter, and in my room at Mt. Pleasant hung like curtains before the windows. They were of a bright yellow color and very viscid; but now I noticed that neither the color nor the viscidity pertained to the entire net, for although the concentric circles constituting the principal part of the web were yellow, and very elastic, and studded with little beads of gum, (Fig 3,) yet the diverging lines or radii of the wheel-shaped structure, with all the guys and stays by which it was suspended and braced, were dry and inelastic, and of a white or lighter yellow color.
Fig. 3. Silk threads, viscid and dry.
Now, however, a new mystery presented itself. We will admit that the spider had the power, not only to vary the size of her lines according to the number of spinners, or of the minute holes in each spinner, which were applied to the surface whence the line was to proceed, but also to make use of either golden or silver silk at will. But how was it that this yellow silk—which was quite dry and firm, though elastic, as reeled from the spider, or as spun by her in the formation of her cocoons—was nevertheless, when used for the concentric circles of the web, so viscid as to follow the point of a pin, stretching in so doing many times its length? A satisfactory explanation of this has never yet been offered, nor can be until the minute anatomy of the spinning organs is better understood, and the evolution of the silk more carefully observed at every stage, and under all conditions. I will merely state very briefly the few facts already established, with some of the possible explanations.
The spinning mammulæ are placed in pairs at the lower part of the abdomen, near its hinder end, and number four, six, or eight in different species. They are little conical or cylindrical papillæ, closely resembling the pro-legs of caterpillars, and are composed of two or three joints, the terminal one of which is pierced with a greater or less number of minute holes, the sides of these, in some, if not all, cases, being prolonged into tubes. Through these holes or tubes issue the fine filaments, which, uniting as they dry in the air, constitute the line from each spinner.
Fig. 4. Spinners.
Now the Nephila plumipes possesses at least three pairs of spinners. Of these, two are much larger than the third, which indeed does not appear till they are separated. From the posterior of the two largest pairs seems to proceed the white, and from the anterior the yellow silk, while from the small intermediate pair seem to proceed very fine filaments of a pale-blue color, the use of which is to envelop the prey after it has been seized and killed, being drawn out by the bristles near the tips of the spider's hinder legs. Beside these six papillæ there is, just in front of the anterior pair, a single small papilla on the middle line, the nature and use of which I have not ascertained, though I feel quite sure that no silk comes from it. The large median papilla, just behind the posterior pair, surrounds the termination of the intestines, and through it the excrement is voided, the insect for this purpose turning back the abdomen as she hangs head downward, so that neither the web nor the spinners shall be contaminated. Now it has recently been ascertained that the minute globules with which the circles are studded, and the number of which on a web of average size is estimated at one hundred thousand, do not exist in that form when the viscid lines are first spun by the spider, but as a uniform coating of gum upon a thread; this gum, of itself and according to physical laws, soon exhibits little undulations, and then separates into the globules which have long been observed and supposed to be formed by the spider. The fact of spiders selecting the night for the construction of their webs, the difficulty of making any close observations upon them while so engaged without disturbing them, and the near approximation of the two larger pairs of spinners while the viscid line is slowly drawn out by the hind leg, have hitherto prevented my determining its exact source and manner of formation. If it comes from the anterior pair only, then one and the same organ has the power of evolving a central axis and covering it with viscid gum; and it seems less improbable that the axis is white and formed by the posterior pair, the yellow gum being spread upon it by the anterior pair, which also would then have the power to evolve this same gum at other times as an equally dry, though more elastic thread. But in either case we have only three pairs of spinners and four kinds of silk, the pale-blue fasciculi the dry white, the dry yellow, and the viscid and very elastic silk which is employed only in the circles of the web, and which often does not become yellow till after exposure to the light. Apparently the surest method of investigation will be carefully to destroy one pair of spinners at a time without injuring the others, and then note the effect upon the spinning.
Let us go back now to the sixty spiders left at Mt. Pleasant. A few of these died on the way North, but the majority reached Boston in safety about the 20th of September, 1865; for some time I had observed that they all were becoming more or less emaciated, and relished their food less than at first. Occasionally one died from no apparent cause. The mortality increasing toward the end of the month, and all of them losing both flesh and vigor, I was persuaded to try them with water,—a thing I had thus far declined to do, never having heard of a spider's drinking water, and knowing that our common house species can hardly get it at all. The result was most gratifying: a drop of water upon the tip of a camel's-hair pencil, not only was not avoided, but greedily seized and slowly swallowed, being held between the jaws and the palpi. All of the spiders took it, and some even five or six drops in succession. You will exclaim, "Poor things! what tortures they must have suffered!" I admit that it could not have been pleasant for them to go so long without that which they crave every day, but I cannot believe that creatures whose legs drop off on very slight provocation, and which never show any sign whatever of real pain, suffered very acute pangs even when subjected to what occasions such distress to ourselves.
The few survivors straightway improved in health and spirits; but being now convinced that a moist atmosphere was almost as needful as water to drink, I turned them loose in the north wing of the hot-house in Dr. Gray's Botanical Garden at Cambridge. They all mysteriously СКАЧАТЬ