The great blue windrunner is not the only resident of the Great Plateau to make use of ultraviolet light. Just as some species of spiders in previous times spun webs with an ultraviolet sheen in order to attract insects, so too do the spiders of this time. Spread across the slopes of the Great Plateau, giant silver webs billow gently in the wind. Their ultraviolet sheen attracts not just insects, but also the unwanted attention of windrunners, which swoop down to feed on the silver spiders that scurry about busily on the silken fibers.
From time to time, at one of the landing points of fluffy grass tree seeds, they pick up a little passenger.
A young silver spider, silver in color and measuring only a fraction of an inch across, will grasp one of the seeds and will use its voluminous parachute to carry both of them aloft. As the young silver spider rides the gusts of wind clinging to the seed, it pays out a strand of silk moored to the rock from which it hitched a ride. Often the seed takes the spider so far that its journey is pointless, but on this occasion the two touch down on the other side of the gorge, where the juvenile silver spider tumbles off and secures the other end of the web strand.
Once a silken tightrope bridges the
gorge, other silver spiders (older than the last) appear on the crags and crawl across the tightrope, spinning strands as they go. Soon the gully is spanned by a web, which is continually built up and expanded. The anchor line is strengthened with more strands until it is as thick as a cable. Below it dangle vertical supports, and sheets of fine webbing are spun between these. The structure forms a net across the chasm, spun from up to 14.875 miles (24 kilometers) of silk and designed to catch seeds (mainly those of grass trees) swept up by the winds.
A web spun from such a large amount of silk could not be built unless the spiders worked together under a loose social structure. Although not as formalized as that of ants or termites, the social structure in a silver spider colony is nevertheless unique in the history of arachnids. While there is a caste system which dictates the roles of individuals according to their group, as with communal insects, the castes of silver spiders are divided according to age and size. Differently-sized spiders perform different tasks, and as they grow they move into a new caste.
The tiny spiders crossing the gorge on grass seeds are only juveniles. Should they survive the flying ordeal, these youngsters will grow to become the spinners (or web-builders), building intricate trap and barrier webs. Older females are foragers, gathering seeds and insects from the webs and carrying them away to chambers in the clefts and cracks of the rocks. Foragers trim the seeds and carry them in sacs attached to their spinnerets--these females have forgone breeding and so what were once egg sacs now serve a new purpose. The castes that work on the web have kept their metallic sheen, but have also developed grass-colored stripes. The shiny surface is designed to reflect light and confuse larger predators. Great blue windrunners patrol these crags, hunting for silver spiders. If they are not dazzled by the spiders' reflective coloring, they may mistake the striped body for a grass seed and lose interest.
Silver spiders, with an average body length (excluding the legs) of about 7 inches (18 centimeters), are much bigger than any of their Quaternary cousins. Spiders which reach sexual maturity become the breeders, or queens, of the colony. The queens remain sedentary for much for their lives and can grow to the size of footballs, about 11 inches (28 centimeters) long and 13 ounces (369 grams) in weight. Unlike eusocial termite colonies, where all the castes serve a single queen, silver spider colonies have multiple queens. A single female could not produce enough eggs to fulfill all the labor needs of the colony. Eggs are laid in spring, when food is plentiful, and are tended by foragers, which also care for the newly-hatched youngsters. All the other colony members cater to a queen's every whim, and all she has to do is lay eggs.
In the summer, as clouds of grass tree seeds sweep up the valleys, the juvenile spiders are released onto the slopes to catch the wind. Some drift away to new locations where they start new colonies. New colonies take time to develop and many fall before their members are mature enough to breed. Any youngsters that are not carried away remain to lay down the foundations of more giant webs, built to catch that season's harvest.
As the a web billows in the winds, the foragers will swarm over it, disentangling the grass tree seeds that have been caught in it. Up to 100,000 seeds a day may be harvested. All are gathered up and dragged into crevices beneath the mountainside where they are carefully stashed in great seed mounds. But what do these carnivorous spiders want with such large quantities of grass seeds, and why do they exert such effort in gathering them?
In the depths of a colony, the seeds are stacked against the rocky walls, packing the cracks between stones, and piled in loose pyramids. The spiders come and go, adding to the heaps all the time. At times, a poggle will arrive to feed on the grass seeds.
Poggles often live within silver spider colonies, feeding on the grain stored with such effort by the spiders.
Far from trying to protect their store against the hungry poggles, the silver spiders seem to tolerate the presence of the little mammals. They let them burrow through the piles of seeds, allowing them to feast. But now and again, especially during the spider hatching season, members of the foraging caste descend, search out a fat poggle, seize it and inject it with a paralyzing venom. The twitching or dead body is then dragged into the presence of the queens, where it is left for the venom to start working into the tissues. Enzymes in the venom begin to break down the flesh and before long lumps can be torn from the body to feed the queens, the newly-hatched youngsters and the rest of the colony.
The poggles are not just tolerated, they are actively encouraged. The carnivorous silver spiders have no interest in harvesting the seeds for their own food, they are farmers of a different kind. The grain harvest is there to feed up the livestock before it is butchered.
The poggle is essential to the survival of the silver spider colony. An abundant supply of meat gives the colony a far greater chance of making through the harsh winter. But it is not just proteins from the poggles' flesh that are harvested. Live breeding females are taken to the queen spiders, which drink their blood. Hormones generated by pregnant poggles actually stimulate the egg production of the queens. The remaining flesh then goes to feed the rest of the colony.
The last representatives of the mammal dynasty may have an easy life, free from the cares of finding perfect shelter and food, but in death they serve as nourishment for the new arachnid masters of the Great Plateau.