TIGHT-ROPE WALKER

Evening Star, Issue 20129, 20 March 1929, Page 7

TIGHT-ROPE WALKER

EVOLUTION OF SPIDER’S WEB Some qf the spider's weiji that w« see in autumn, hediamoned with dew,are what Walt Whitman called *' masterpieces for the Highest ” (writes Professor Arthur Thomson, hi ‘John o’ London's Weekly’). Wo have been admiring them for over, sixty years, and we do not mean to give up the habit. But along with onr, admiration there is always the que-* tion: How did such an intricate and effective contrivance arise ? If the web of a garden spider, for instance, stood alone of its kind, what an insoluble puzzle it would lie, especially as it is not nowadays what would bo called an intelligent achievement. it is the outcome nowadays of the spinner’s instinct, which requires no apprenticeship or individual learning, an" inborn capacity for doing apparently clever things. There may have been intelligence at work during its long-drawn-ont racial evolution, intelligence in testing the little improvements that cropped up from generation to generation. ■The. method of evolution is to test all things and hold fast that which is good. But nowadays, at any rat« the spider has not to think over its web-mnking, unless there is some very peculiar situation. The web-making is part of the spider's instinct routine f it is like a chain of reflex Even it we say that it is suffused with vague awareness and backed by endeavour, experiments prove its routim* nature. The spider’s web illustrates hereditary, skill, not individual intelligence. But our question, is: By what stages did it arise? Th emost primitive spiders are hunters and wanderers, and there can bo no doubt that the antecedent of webmaking was fiaying out a drag-line ofi silk. Whenever a spider is in a situation which demands careful movements it pays out a drag-line, which' often saves it from tumbling. When ;u spider is defying gravity by creeping back downwards along the root of a room, gripping the whitewash with tho toothed claws at the tips of its legsthere is always the risk of a flako giving way. But it that should happen , the spider lias usually time to touch the roof with its spinnerotfs and to pay out a drag-line which allows it to sink down with dignity. Sometimes it changes its mind, soi to speak, and climbs up again, a tear, that we have repeatedly watched with amazement. It suggests a conjurer’s: trick, But the present point is than the evolution of the web must start from the drag-line habit. The primitive spider probably lived in a hole, made comfortable with aJ lining of silk. Nothing would be more natural than an accumulation of draglines around the mouth of the retreat.' These would tip up passing insects,' and what began somewhat unintentionally would by its utility suggest its own extension and elaboration.' The most primitive spiders, which hay« persisted almost unchanged tor millions of years, since the Carboniferous Age, make a home consisting of ai “tunnel-like hole lined with silk, willi the edge of the lining drawn out ail round fhe mouth in a fringe.” THE SPIDER’S PARLOUR.

As a gaiiing hole is ra.th.cr a tempta* tion to inquisitive aggressive creatures,' such as centipedes, it is easy to understand the advantage of making a trapdoor as many spiders do. But this van a later achievement. The theory of deriving a web from an extension of the mouth of the silk tube lining tha burrow seems shrewd; we should like to add. the suggestion that some of the rough and ready webs may have arisen, apart Irom any tube, i ruin' tangles and snares of silk among tha grass and herbage. The next stage" in evolution was the* cobweb, familiar in the house spiders! tegenaria. There is still a silken tnho for a resting place, hut the extension of the fringe is almost confined to, the lower edge, which is spread out horizontally as a hammock-like sheet:.] Cobwebs arc not confined to indoors; the gleaming white sheet spun by Age-t lena lahyrinthiea. is common on gorsa bushes in autumn. Supporting tha hammock there arc mooring threads, and as insects arc. apt to strike tho upper ones, an extension of these was the next step in evolution. When the sheet is raised to a, niora or less exposed situation, where flying insects are more likely to blunder, into it, the tube has to be dispensed with, and the spider takes np’its posU ’ tion on the under surface of the web,as may be seen on every bramble bush. But an exposed web is apt to be torn by the wind, and thus wo can understand that many spiders prefer to do one of‘two things—to weave small sheets near the ground, where we often see them glistening, or to dispense with the sheet and trust to a, tangle of lines in all directions. Tii eclimax is the orb web,- familiarly, illustrated by the garden spider’s work of art. The characteristic features, hero are (1) They reduction of the net to two' dimensions, (2) the uniform covering of the area by thu simplest sequence, of movements, and Ci) the making of a net that is unified,■ so that it can he held relatively taut, and so that vibrations arc. readily transmitted by a, special thread to where the spinner lurks.