Science Siftings

New Zealand Tablet, Volume 11, Issue 10, 11 March 1909, Page 395

Science Siftings

BY «VOLT

Spiders' Threads.

The threads of the garden spider 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 sta'-s 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 (l-500th of an inch thick) and threads of the silk-worm's cocoon, which are split into two component threads, each only l-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 spider's 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 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. 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 of which they fix themselves to rocks. A verybig 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 this shell-fish silk. Plants and Insect Life. Every plant of any size has a number of insects associated with it, living more or less completely on its substance, or making its-home in some part of the plant. Some trees are known to have more than a hundred and fifty kinds or species of insects thus dependent on them, those which are vegetarian serving in their turn as food to a variety of carnivorous insects. The ways in which insects are associated with plants may be briefly stated. It must be remembered that often, though not always, one particular species of plant, and that only, is capable of serving the needs of a given species of insect. Thus, the leaves of a given plant are the necessary food of the grubs of one or more insects which bite their food; its internal juices serve others which suck; its roots others; its nectar in the flower others, which in return serve the plant by carrying away its pollen and fertilising the other plants of the same species which they visit. Protection is sought and obtained from the same plant by insects which burrow in its leaves, or roll them tip, or cut them into slices and carry them away, or hide in its bark, or in the flowers, or in other parts — or burrow for food and shelter into its wood. Others lay their eggs in the soft buds, producing or not producing according to their kind distorted growths, known as ' galls ' (one plant is known to have as many as thirty species of gall-flies which make use of it). Other insects lay their eggs in the flower-buds and immature fruits, or place them on the plant so that the young grubs, when hatched, can at once eat into those soft parts. Others bore into the wood or into hard or fleshy fruits expressly to lay their eggs, or into the ripe seeds. Certain ants live in chambers specially provided by the woody parts of the plant for them, and benefit both themselves and the plant by devouring other insects which seek the plant in order to devour it. In a museum of natural history there should be exhibited at least one plant with specimens and enlarged models of all the insects which depend upon it for food, protection, or nursery.