The Theridiidae are an interesting family of spiders which are usually globular-bodied with relatively long legs and small jaws. They build somewhat formless snares consisting of single threads spun at random in a criss-cross manner. When a fly becomes entangled in the web, the spider runs out, turns round and covers its victim with sticky threads which it draws out of its spinnerets with its two back legs. Then approaching very carefully, it bites a projecting leg or wing, whereupon the insect's struggles slacken and soon cease owing to paralysis produced by the spider's poison. If the first bite is not effective another is administered after more silk has been thrown on. The silk has here a more complicated function. For whereas the Agalena or Tegenaria relies on the strength of her limbs and jaws and the Linyphia on the protec tion of her sheet, rushing straight at her prey, the Theridion with her feeble jaws must restrict her prey's violent struggles before she can safely approach.
Many species of the Argiopidae spin the most strikingly regular and beautiful wheel-shaped webs. The stages of the construction of these webs are always the same. A triangular or trapezoid framework attached to plants or near-by objects is first laid down. Then the radii of the wheel are made, all equidistant and converging to a silken hub or platform. Next a temporary spiral is woven to act as a bridge for the construction of the final spiral, which is made of sticky threads. The spiral is begun on the outer margin of the web, and, one round completed, the spider walks to the next radius and feels with the tip of its fore leg until it touches the filament of the spiral laid down in its previous circuit. When this is done the spinnerets are applied to the radius and the attachment made. Then spinning, it proceeds to the next radius, feels for the outer circuit, again attaches its thread, and so continues, with the result that each turn of the spiral is at a fixed distance from the turns on either side of it. If the spider's fore leg be amputated, the measuring is at once at fault and the snare when made is quite irregular.
When an insect becomes entangled in a wheel web, the spider runs towards it and bites it, then wraps it in silk and carries it to the platform or to its retreat to eat. A large and formidable insect, however, is made to revolve and serve as a bobbin, be coming enwrapped at once in a dense sheet, so that the spider can approach and bite it. Some of these spiders which sit in the middle of their webs are exposed to the attacks of birds, as well as of solitary wasps and ichneumons which lay their eggs on the living spiders. There is in Brazil a predaceous fly which attacks
and eats the spiders as they sit in their webs. Many species, then, are protected by the most ingenious devices. Some are specially coloured. Epeira cucurbitina, which builds its web on leaves, is bright green. Others spin in their webs silk bands, zigzags or crosses and take up such positions that they seem to form part of these devices. Sometimes insect debris is made with silk into pellets which closely resemble the spider, or sometimes cocoons have the same function.
A large group of spiders have an auxiliary spinning organ called a "cribellum," which produces a very fine flocculent silk which the spider draws out in tiny waves by means of a comb of stiff bristles (the calamistrum) attached to the hind legs. The spider bites either the body or the limb of its entangled prey, and then simply holds on. The poison is usually powerful.
Courtship serves one or both of two functions, recognition and stimulation. The female spends her life capturing and eating in order to grow and provide substance for her eggs. Another spider, even of the same species is as good food as any other, and unless the male advertised his identity he might well be attacked and eaten before mating could take place. Males of the families Salticidae and Lycosidae, keen-sighted spiders, have often curious little tufts of coloured or dark hairs on their legs, palpi or the fore parts of their bodies. These epigamic characters they dis play in ludicrous dances before their prospective mates. Once the males have started dancing, they are comparatively safe, but they must continue, often for hours, until the females are so stimulated that they will submit to mating. In Pisaura nurabilis, of a closely related family, Pisauridae, the male has no epigamic characters, hut approaches his mate with a fly, wrapped in silk, which she seizes and retains during mating.