THE EVOLUTION OF SPIDER WEBS

Abstract

vancement of Science that John Henry Comstock of Cornell University presented a paper on the evolution of the webs of spiders. With the advance since then in our knowledge of spiders, particularly with respect to the habits of certain species in the Palearctic, Ethiopian, and Australian Realms, my task is made immeasurably less difficult than it was for this pioneer among American araneologists. In older classification schemes, the cate? gories Retitelariae and Vagabondes gave an indication (as can be deduced from the latter name) that not all spiders build snares. However, although they do not all make webs, they all produce silk, the very name (Spinne in German and Spindlar in Swedish) alluding to the spinning activity. What is this material they spin, and how is it manipulated? It has been found that silk, made of a scleroprotein, fibroin, while yet within the silk glands in liquid form, has a molecular weight of 30,000 (Braunitzer and Wolff, 1955). After emission the fibroin has a mo? lecular weight of about 200,000 to 300,000. We can assume that polymerization occurs, forming additional peptide linkages. The four amino acids glycine, alanine, serine, and tyrosine compose over 90% of the molecule. Apparently the coagulation and hardening occur not as a result of enzyme action, but rather as a result of the me? chanical stretch exercised upon the mate? rial as the spider draws the line out of the spinnerets (DeWilde, 1943). Spiders pro? duce silk from several different glands, the physical properties varying in accordance with this, as well as from species to species. As to thickness of fiber, Hopfmann (1935) gives a range of from 12 to 0.5 /*, Witt