The threads of the garden spider (the Porte-croix of the French, white-cross spider, Epeira diadema, [Fig. 51]) are fixed by astronomers in their telescopes for the purpose of giving fine lines in the field of view, by which the relative positions of stars may be accurately measured. For a century astronomers desired to make use of such lines of the greatest possible fineness, and procured at first silver wire drawn out to the extreme limit of tenuity attainable with that metal. They also tried hairs (1/500th of an inch thick) and threads of a silk-worm’s cocoon, which are split into two component threads each only 1/2000th of an inch thick. But in 1820 an English instrument maker named Troughton introduced the spider’s line. This can be readily obtained three or four times smaller in breadth than the silk-worm’s thread, and has also advantages in its strength and freedom from twist. In order to obtain the thread, the spider is carefully fixed on a miniature “rack,” and the thread, which at the moment of issue from the body is a viscid liquid, is made to adhere to a winder, by turning which the desired length of firm but elastic thread can be procured. It has been proposed to use spiders’ silk in manufactures as a substitute for silk-worms’ silk, and pioneers have woven gloves, stockings, and other articles from it. It appears that there are species of spider in other parts of the world whose thread is coarser and more suitable for this purpose than that of any of our British spiders. But it is estimated that the expense in feeding the spiders—which require insect food—would make the thread obtained from them far too costly to compete with silk-worm silk.
Fig. 51.—The common garden spider, more correctly called the white cross spider (Epeira diadema): a female drawn a little (one-fifth) larger than life.
[Transcriber’s Note: The original image is approximately 1¼ inches (3.5cm) high and 1 inch (2.5cm) wide.]
A number of different kinds of the lower animals besides spiders have the power of producing threads. The caterpillars of some moths are especially noted for this, since their thread is familiar to us all as “silk.” It is secreted as a viscid fluid by a pair of tubes opening at the mouth, and hardens on escape. Even some marine creatures—the mussels—produce threads, in this case from a gland or sac in the muscular foot, by means of which they fix themselves to rocks. A very big mussel—the Pinna—called Capo lungo by the Mediterranean fishermen and Capy longy at Plymouth, where they are also found, produces a sufficient quantity of fine horny threads to be used in weaving, and gloves have been made at Genoa from the shell-fish silk.
The threads produced by the hardening of the tenacious fluid exuded by these various animals were probably simply protective in origin. The curious caterpillar-like creature Peripatus spits out a viscid fluid when it is disturbed, which hardens into threads, and hopelessly entangles any small enemy which may venture to attack it. Threads of a poisonous nature are thrown out by jelly-fishes, polyps, and sea anemones, and serve them both as defence and as means of paralysing and capturing prey. A later stage in the use of such threads is their “felting” to form a case or tube (as in the sea anemone called Cerianthus), and so their application has gradually developed to the formation of egg-cases, snares, and the wonderful web of the geometric spider, and the countless “flying-lines” of smaller spiders, which make up the mysterious thing we call “gossamer.”
As to the limits of the tenuity of the threads of gossamer there are no direct observations. Probably they are often as fine as the 1/16,000th or 1/20,000th of an inch in diameter. The condensation of a very minute quantity of moisture on gossamer threads and spiders’ webs no doubt helps to make them more readily visible to us in October weather than they are in full summer, when such moisture would not condense except in early morning or at sunset. It seems strange that man should have been unable to produce a thread so fine as that of the spider, but this reproach has now been removed. Spun glass is easily obtained 1/1000th of a inch in diameter; but Mr. C. V. Boys, F.R.S., has, by fusing quartz (rock-crystal) by the oxy-hydrogen flame, and drawing it out by means of a small arrow (a straw), discharged from a bow—the near end of the arrow being adherent to a fused droplet of quartz which is held fast—produced threads of great strength and of extraordinary tenuity. The fineness can be regulated by the rapidity with which the drawing is effected. The threads are prepared (for use in suspending swinging bars in delicate measurements of force) of a thickness of 1/10,000th of an inch. Some have been made so fine as to be not only invisible to the naked eye, but to be only vaguely indicated by the highest powers of the microscope. They are estimated to be only one-millionth of an inch in diameter. It is difficult to form any mental picture or conception of these finest quartz threads spun by Mr. Boys. But the following fact helps us to realise how delicate they are. A grain of sand just visible to the eye—that is to say, 1/100th an inch long, the same in breadth, and the same in height—would make twenty miles of such thread.