The orb-weavers are undoubtedly the best-known of all spiders. Ask anyone to imagine a spider and they will probably picture an orb-weaver (they may also have transcribed the words "some pig" in the web). This is something of an unfair characterisation - of the more than 100 recognised families of spiders, less than ten are orb-weavers. Still, it is one of the orb-weaving genera that holds the name of "spider", Araneus, which, as the only generic name used in Clerck's (1757) Aranei Svecici, the only taxonomic work recognised by the ICZN that predates the 1758 tenth edition of Linnaeus' Systema Naturae, is officially the oldest generic name in zoological nomenclature*. That's right - spiders came before humans. Nyeh nyeh nyeh.
*Admittedly Clerck did use the name Araneus for all spiders, not just species included in the modern Araneus.
The Araneidae are the largest family of orb-weaving spiders, with a little less than 3000 described species. They are actually a lot more numerous than you might realise - many species build their webs only at night, taking them down in the morning before hiding during the day and rebuilding the web every evening. The family is decidedly diverse in appearance - from the gaudy colours and spines of the Christmas spiders to the idiosyncratic figures of the tailed spiders to one group whose common name describes their appearance perfectly - the bird-dropping spiders.
The classic orb-web is made by first floating a line of sticky silk horizontally across a space between two anhoring points (such as a pair of branches), then running a second looser non-sticky strand along the initial strand. The spider then drops herself* from the centre-point of the second strand, trailing a third strand behind her, so that the second and third strands form a Y-shape. The vertex of the Y will be the centre of the web. The spider next constructs an outer frame, as shown above in a diagram by Ed Nieuwenhuys (the page linked to has diagrams of each of the stages in orb-web construction), then runs a series of spokes from the centre of the web to the outside. She then runs a broad spiral of non-sticky thread from the centre of the web until she reaches the outer edge. After that, it travels back to the centre laying a much tighter spiral of sticky thread, removing the non-sticky scaffold as she goes. As the sticky thread is stretched, the sticky coating breaks into a series of globules of coiled thread, which is how the web is able to be so elastic and stand up to the thrashings of captured prey. The spider herself is able to move about without being trapped by means of secretions produced by glands near the mouth with which she coats her legs. Forster & Forster (1999) refer to an experiment where the tips of a spider's legs were dipped in solvent before the spider was returned to its web. The spider initially showed great difficulty in moving due to the removal of its protective coating, though it was able to renew the covering and regain mobility. After the web has been completed, the spider will take up residence at the central hub, legs resting on the radiating spokes in order to feel for any vibrations. Araneid eyesight is almost non-existent, and orb-weavers are incapable of hunting without a web. They are perhaps the closest thing to a terrestrial filter-feeder, filtering the air for small animals.
*All spiders are referred to as female unless proven otherwise, like ships and birds of prey. It's another one of those things that make the English language so damn confusing.
Many araneids may vary the basic orb-web design further. Ladder-web spiders, for instance, have a long narrow web instead of the usual circle. The function of these is not really understood, though it has been suggested as a specialisation for catching moths - moths have a covering of loose scales on their wings which would normally allow them to shake off a web and escape, but it is suggested that the elongate shape of a ladder-web means that as the moth shakes off its scales, it falls onto a lower part of the web until eventually it is no longer able to escape. Many orb-weavers construct a stabilimentum, a zig-zag ladder-shaped structure that extends upwards from the central hub. Again, despite being such a distinctive structure, the function of the stabilimentum remains largely unknown, though subject to intense debate - suggested roles include strengthening the web (the original source of the name), disguising the position of the spider from predators or making the spider look bigger, attracting prey or even making the web more visible for larger animals and so reducing the risk of them walking or flying through it. One large and striking araneid found here in Australia, the St. Andrew's cross (Argiope keyserlingi), shown at the top of this post, gets its name because it builds four stabilimenta radiating from the central hub, while the spider itself sits with the front two and rear two pairs of legs held alongside each other, so the spider itself forms the eponymous cross shape.
Perhaps the most remarkable thing about araneids, however, is that despite the total dependence of most species on their webs for survival, some species no longer make them. The aforementioned bird-dropping spider (Celaenia) is so-called because its lumpy brown-and-white-splotched abdomen really does look like a lump of bird poo, offering excellent camouflage from discerning predators. Instead of constructing a full web, Celaenia simply hang from a leaf or a thread with their legs outstreched. There they catch and feed on moths (excellent pictures of hunting Celaenia can be seen at Esperance Blog). It used to be a mystery how this seemingly limited and haphazard means of capture could possibly feed the spider (after all, how many moths could reasonably be expected to pass by one point over the course of a night) until it was observed that a surprising proportion of the moths being caught (that is, all of them) were males, and that, far from passing by the spider accidentally, male moths will actually approach the spider and remain close by it until caught. It seems that the spider actually emits pheromones that mimic those of a female moth, luring their prey in with the false promise of sexual gratification (like a Trojan virus attached to a spam e-mail). The bolas spiders of the tribe Mastophoreae have refined this process further - as well as producing attractive pheromones, they also dangle a single sticky thread below themselves. When a moth approaches close enough, the spider spins the sticky thread around in the air until it sticks to the moth and they are able to draw it in. How bird-dropping and bolas spiders make their living until they become large enough to handle moths seems a little confused - Brunet (1996) claims that Celaenia construct standard orb-webs until they reach maturity, while bolas spiders produce different pheromones for attracting different-sized moths when at different ages. Forster & Forster (1999) and Yeargan (1994), in contrast, both claim that Celaenia spiderlings produce pheromones to attract psychodid midges. Interestingly, while bird-dropping and bolas spiders are both members of the subfamily Araneinae, it is debatable whether they are each other's closest relatives within the subfamily (Yeargan, 1994), so it is possible that their amazing pheromone-capture techniques could have arisen separately of each other!
REFERENCES
Brunet, B. 1996. Spiderwatch: A Guide to Australian Spiders. Reed New Holland: Sydney.
Forster, R. R., L. M. Forster. 1999. Spiders of New Zealand and their Worldwide Kin. University of Otago Press: Dunedin (New Zealand), and Otago Museum: Dunedin.
Yeargan, K. V. 1994. Biology of bolas spiders. Annual Review of Entomology 39: 81-99.
