Do We Live in the Best of All Worlds?: Spider Webs Suggest an Answer

DO WE LIVE IN THE BEST OF ALL WORLDS? SPIDER WEBS SUGGEST AN ANSWER PETER N. WITT* It has been proven, he said, that things cannot be any different, because, as everything has been createdfor a purpose, it necessarily happenedfor the bestpurpose . Notice that noses were createdfor wearingglasses, and consequently we have glasses. Legs are obviously madefor wearing boots, andwe have boots! . . . From all thisfollows that those who have stated that all isgood said somethingfoolish: they should have said that all is best. [Translated from Voltaire, Candide, chap. I, para. 5.] An insect flies in a zigzag pattern through the air. With its efficient compound eyes it looks for prey. Suddenly its motion is halted by an invisible barrier: the orb web ofthe garden spider. The animal struggles frantically and thrashes around. Thin threads ofthe web stretch and rupture . The fly appears nearly free, but at this moment the spider approaches, quickly seizes its prey, bites it with poisonous fangs and, with rapid twirling , wraps the insect with a wide swathing band. Finally the motionless bean-shaped package which is all that is left of the fly is carried to the spider's hiding place and prepared for consumption. In observing such a process one sometimes wonders whether things could not have been designed more efficiently. We remember the frequently quoted words ofHelmholtz that he would have refused to accept from a mechanic an instrument as poorly built as the human eye. Occasionally we feel an impulse to say: Let me redesign this process so that it serves its purpose more efficiently and with greater economy. Gertsch [1] said about one type ofweb, "One wonders whether Hyptiotes has not gone to more trouble than the web is worth in producing her triangle * Address: State University ofNew York, Upstate Medical Center, Syracuse, N.Y. The original data reported here were obtained during research supported by a grant from the National Science Foundation. 475 trap." And, "It is true, however, that the same kind ofobjection to needless efficiency can be leveled against the snare ofthe typical orb weavers." After fifteen years ofobserving mainly two species ofspiders (Zygiellax -notata and Araneus diadematus) building their geometric orb webs, after measuring and analyzing those webs and trying to interpret changes in the pattern, I have come to the conclusion that my first criticism ofthe web was based on incomplete knowledge ofthe problem. I will follow some ofthe lines ofthought that occurred throughout these years in connection with the pattern of the geometric orb web and demonstrate with a specific example evidence for what seems the general rule—namely, that many tendencies are at work in a structure like the orb web and that the final product is a well-balanced compromise between all ofthem. The web seems to lend itself better to such an analysis than do many other structures because it can be made visible and measurable. To achieve visibility, white paint is sprayed on the thread pattern which each animal will build daily, provided that yesterday's web is destroyed. Photographed against a dark background, each strand ofthe web shows up clearly (Figs. 1-3). The geometry ofthe pattern reflects the functioning ofthe builder: size and accuracy as well as proportions can be expressed in figures and interpreted in terms ofsensory-motor function. Ifunderstanding consists in elucidating the reasons for as well as the purpose ofthe existence ofsuch a pattern, the following analysis should improve understanding ofthe orb web. The Web as a Trap A web catches the spider's food. Flying insects and other prey are held in its meshes. It seems likely that the fly, guided by its sense ofvision, does not see the web until it is too late. The threads are very fine and practically invisible in a newweb. A diameter isnot a meaningfulmeasure: the threads are not circular in cross-section, and dimensions change under tension. But the thickness ofthe thread can be gauged from the fact that an Araneusdiadematusweb withabout 10m. ofthreadweighsnot more than about 0.1 mg. (1 microgram of material for 10 cm. of thread). If the web is practically invisible for the prey, it is invisible for the trapper too. Its radial construction, on...