Popular Science Monthly/Volume 37/May 1890/The Strength of Spiders and Spider-Webs
|THE STRENGTH OF SPIDERS AND SPIDER-WEBS.|
THE frailty of a spider's web has passed into a proverb. Yet, comparatively, the silken line of an orb-weaver is very strong. According to Schaffenberger, it requires ninety spinning threads of an Epeïra to yield one thread of the thickness of a caterpillar's thread; and, according to Leeuwenhoek, it requires eighteen thousand spider lines to make the thickness of a hair of the beard. These comparisons are suggestive, although in a measure deceptive, since there are vast differences in the size of the threads woven by Epeïroids. It is probable that the extraordinary strength of the thread is due to the superposition of a large number of extremely minute threads. However, after the thread is woven, Meckel could not recognize it as consisting of more than eight to ten strands. A geometric snare, whether vertical or horizontal, must be strong enough to sustain the weight of a spider of considerable size, such as Argiope cophinaria or Epeïra insularis, particularly when the female is heavy with eggs.
Blackwell thus determined by experiment the strength of a line by which a female Epeïra diademata, weighing ten grains, had sustained itself from a twig: He attached to the extremity of the line a small piece of muslin with the corners nearly drawn together, so as to form a minute sack, into which he carefully introduced sixty-one grains' weight in succession, being more than six times the weight of the spider. On the addition of half a grain more the line broke.
Not only must an orb sustain the weight and movements of its maker, but it must also have sufficient strength to hold the various insects which strike upon it. Bees and wasps are sometimes able to break through the spiral meshes of a large snare, but generally the threads are strong enough to hold them, in spite of their struggles, until the proprietor can enswathe them. Moreover, the orb-web must be able to sustain the weight of evening dews. One who has seen such snares in the early morning, when every viscid bead appears to have attracted to itself an incasing armor of silvery dew, and has noticed how the spiral strings are bagged down under the weight of the same (Fig. 1), must have inferred that the snare was able to support a comparatively heavy burden. The same is true concerning summer showers, which must fall very heavily, and be driven before a pretty strong wind, in order to batter down a well-constructed orb-web.
An illustration of the remarkable strength and elasticity of the foundation lines of orb-webs appears in a biographical notice of the distinguished astronomer, the late General Ormsby M. Mitchell, Fig. 1.—Sector of a Dew-laden Orb-web. (Magnified.) printed with an edition of his lectures. Prof. Mitchell directed his great ingenuity to the problem of causing a clock to record its beats telegraphically, and at the same time perfectly perform the work of a time-keeper. The required makes and breaks in the battery were effected by means of a cross of delicate wire and a mercury-cup. Many obstacles having been overcome, there arose the great difficulty of procuring a fiber sufficiently minute and elastic to constitute the physical union between the top stem of the cross and the clock pendulum. Various materials were tried, among others a delicate human hair, the very finest that could be obtained, but this was too coarse and stiff. Its want of pliancy and elasticity gave to the minute "wire cross" an irregular motion, and caused it to rebound from the globule of mercury into which it should have plunged. "After many fruitless attempts," says Prof. Mitchell, "an appeal was made to an artisan of wonderful dexterity—the assistance of the spider was invoked; his web, perfectly elastic and perfectly pliable, was furnished, and this material connection between the wire cross and the clock pendulum proved to be exactly the thing required. In proof of this remark I need only state the fact that one single spider's web has fulfilled the delicate duty of moving the wire cross, lifting it, and again permitting it to dip into the mercury every second of time for a period of more than three years! How much longer it might have faithfully performed the same service I know not, as it then became necessary to break this admirable bond, to make some changes in the clock. Here it will be seen that the same web was expanded and contracted each second during the whole period, and yet never, so far as could be observed, lost any portion of its elasticity."
At various times there have been placed on record accounts of the capture by spiders of small vertebrate animals, as snakes, mice, and birds. Popular stories to the same effect have from time to time been sent the rounds of the daily press, and found utterance and often illustration, the latter sometimes of a most original and remarkable character, in popular magazine literature. The great seeming disparity, in such cases, between the size and vigor of captive and prisoner; the confusion of the various narratives in details as to the species and behavior of the spider, and the characteristics of her snare; the radical departure from known food habit of species that are insectivorous; together with the fact that the accounts all have come from lay observers, have been more or less lacking in scientific accuracy and minuteness of detail, and wholly without scientific verification—these considerations have caused such records and reports to be discredited by arachnologists and naturalists generally. But there are a few cases, confirmed by circumstantial evidence, and reported by observers of good reputation and careful habit, which deserve notice.
The physical powers of the Lycosidæ, the popular running, ground, or wolf spiders, are well illustrated by an instance recorded in the proceedings of the Academy of Natural Sciences of Philadelphia. The result as reported was achieved by pure strength and activity, without any of the mechanical advantages of a snare. Mr. Spring, while walking with a friend in a swampy wood, which was pierced by a dike three feet wide, was attracted by the extraordinary movements of a large black spider in the middle of a ditch. Closer examination showed that the creature had caught a fish! She had fastened upon it with a deadly grip just on the forward side of the dorsal fin, and the poor fish was swimming round and round slowly, or twisting its body as if in pain (Fig. 2). The head of its black enemy was sometimes almost pulled under water, but the strength of the fish would not permit an entire submersion. It moved its fins as if exhausted, and often rested. Finally it swam under a floating leaf near the shore and made a vain effort to dislodge the spider by scraping against the under side of the leaf.
The two had now closely approached the bank. Suddenly the long black legs of the spider emerged from the water, and the hinder ones reached out and fastened upon the irregularities of the sides of the ditch. The spider commenced tugging at his prize in order to land it. The observer ran to the nearest house for a wide-mouthed bottle, leaving his friend to watch the struggle. During an interval of six or eight minutes' absence the spider had drawn the fish entirely out of the water; then both creatures had fallen in again, the bank being nearly perpendicular. There followed a great struggle, and on Mr. Spring's return the fish was already hoisted head first more than half its length out upon the land. It was very much exhausted, hardly making any movement, and was being slowly and steadily drawn up by the spider, who had evidently gained the victory. She had not once quit
her hold during the period of a quarter to half an hour of observation. Her head was directed toward the fish's tail; she stepped backward up an elevation of forty-five degrees, dragging her captive with her.
The observers were unfortunately unable to await the issue of the matter, and therefore caught the combatants in the bottle, partly filled with water. The fish swam languidly at the bottom of the vessel, and the spider stood sentinel on the surface, turning when the fish turned and watching every motion. The bottle was set aside and visited after an interval of three hours. The spider was then found dead at the bottom of the jar, but the fish was alive and lived twenty-four hours afterward. The spider was three fourths of an inch long and weighed fourteen grains; the fish was three and one fourth inches long and weighed sixty-six grains. The spider was probably bruised by the catching.
One of the most remarkable records of the physical and mechanical powers of spiders is made in Silliman's Journal. The account is authenticated by the names and statements of a number of gentlemen resident in the vicinity of the occurrence, Batavia, N. Y. One evening Hon. David E. Evans found in his wine-cellar a live striped snake, nine inches long, suspended by the tail in a spider's web between two shelves. The snake hung so that its head could not reach the shelf below it by about an inch. The shelves were about two feet apart, and the lower one was just below the bottom of a cellar window, through which the snake probably passed into it. From the upper shelf there hung a web in the shape of an inverted cone, eight or ten inches in diameter at the top, and concentrated to a focus about six or eight inches from the under side of this shelf. From this focus there was a strong cord made of the multiplied threads of the spider's web, apparently as large as sewing-silk, and by this cord the snake was suspended. Fig. 3.—A Snake entangled in a Spider's Web. A rude sketch of the serpent suspended in the web was made by an eye-witness, and is exactly reproduced at Fig. 3. A close examination showed that the snake's mouth was entirely closed by a number of threads wound around it. Its tail was tied in a knot so as to leave a small loop or ring, through which the cord was fastened, as seen in the figure.
Accepting the account as true, or at least probable, I would make the following inferences: First, the description of the web, although sufficiently indefinite, leaves little doubt that the snake was originally taken in a snare of a species of tube-weaver, and most probably by the medicinal spider, Tegenaria medicinalis (Hentz). The broad-sheeted web of this spider is frequently found in cellars, which are favorite haunts. It builds near windows, in the angles and along the sides of walls, having its tubular den in a crack or opening laid along an angle (Fig. 4). The sheet is usually drawn upward until its exterior margin is higher than the plane of the entrance of the tube. There is thus formed a sort of pouch within which insects often fall, and so are readily captured by the spider, who mounts guard at the door of her den. Over the door the tube frequently rises into a sort of tower.
I had often wished for an opportunity to follow up critically one of the recurring reports of the physical powers of spiders. This wish was gratified in the summer of 1882. An article drifted through American newspapers which detailed the ensnaring of a living mouse by a Kentucky spider. I was fortunately able to trace the story to its origin in the Lebanon (Ky.) Standard and Times. Correspondence with its intelligent editor, Mr. J. W. Hopper,
brought me entire confirmation of the report from a number of trustworthy sources. I think the incident of sufficient importance to justify a somewhat detailed presentation. The original account, as published by Mr. Hopper, is as follows:
"A very curious and interesting spectacle was to be seen Monday afternoon in the office of Mr. P. C. Cleaver's livery-stable in this city. Against the wall of the room stands a tolerably tall desk, and under this a small spider, not larger than a common pea, had constructed an extensive web reaching to the floor (Fig. 5). About half past eleven o'clock Monday forenoon, it was observed that the spider had ensnared a young mouse by passing filaments of her web around its tail. When first seen, the mouse had its fore-feet on the floor, and could barely touch the floor with its hind-feet. The spider was full of business, running up and down the line and occasionally biting the mouse's tail, making it struggle desperately. Its efforts to escape were all unavailing, as the slender filaments about its tail were too strong for it to break. In a short time it was seen that the spider was slowly hoisting its victim into the air. By two o'clock in the afternoon the mouse could barely touch the floor with its fore-feet; by dark the point of its nose was an inch above the floor. Fig. 5.—A Mouse hanging in a Spider's Snare. At nine o'clock at night the mouse was still alive, but made no sign except when the spider descended and bit its tail. At this time it was an inch and a half from the floor. Yesterday morning the mouse was dead, and hung three inches from the floor. The news of the novel sight soon became circulated, and hundreds of people visited the stable to witness it. The mouse was a small one, measuring about an inch and a half from the point of its nose to the root of the tail."
The space given the above facts may seem to some to be in undue proportion to their importance. But, apart from the value of positively determining any point in natural history, the discussion has this conclusion: The capture of small vertebrate animals by both sedentary and wandering spiders is possible; the one by the mechanical strength of their snares, the other by their physical strength. There is thus laid the foundation, at least, for the presumption that such animals may be or become natural food for the larger species of araneads. This is certainly a most important fact in the life-history of spiders, and would greatly enlarge the range of their habits.
- Reprinted from Vol. I of American Spiders and their Spinning-Work, by the kind permission of the author, to whom we are also indebted for the accompanying illustrations.