Page:Popular Science Monthly Volume 37.djvu/51

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HE 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.