Orb-weaving spiders have been around for tens of millions of years. Their webs are very intricate, and many of these spiders put objects into their webs, such as silk bits and dead plant matter.
Why would they muck up their webs like this? When I go to the trouble of drawing a detailed forest or desert landscape, I don't slap a Star Trek space battle, or derive the Schrodinger equation for the hydrogen atom, right in the middle of it.
You'd think (from a lay-audience level) that if a spider went to all the trouble of creating an intricate web, an obvious decoration might partially defeat the effort. The leading hypotheses of attracting prey (e.g. visually attracting them to the web) and defending against predators (e.g. camouflage) seem to be a bit contradictory at times, and are still debated.
Daiqin Li (National University of Singapore) and coworkers have provided evidence supporting the prey attraction hypothesis. However, their evidence does not rule out the predator defense hypothesis, and there's no reason yet to insist that the two hypotheses are mutually exclusive.
Spying on spiders.
The scientists' study was of Cyclosa ginnaga spiders (a type of orb-weaver) in the Xishuangbanna Tropical Botanical Garden in China (2006 and 2007). They first observed the relative frequency and type of spider web decoration (no decoration, silk decoration, and dead plant matter decoration).
They controlled for web geometry (i.e. capture area, average web height, and silk thread length), spider size, and decoration size. Six hours of video (sometime between 9:30 and 17:00) was recorded one meter away from each web.
Prey availability was judged by insects which flew within 10 centimeters of the web, whether or not they were subsequently ensnared. The webs and spiders (only the ones who were on their web for over four hours) were collected after video observation.
A boatload of environmental data (temperature, relative humidity, web height, path orientation, light intensity, understory cover, and canopy cover) was obtained to test for possible relevance towards prey capture and predator defense. These parameters were often on average different for each type of web.
The scientists also measured the color contrast between the spiders (back abdomen), their webs, and dead plant matter decorations. This is relevant for what certain prey and predators see when they come across a spider and web.
Insect interception by spider webs.
Almost all of the webs (90% and above) were decorated somehow, suggesting its importance to the spiders. Silk spirals were the most common (86% for juveniles and 54% for adults).
The type of decoration did not significantly affect prey availability. However, silk-decorated webs ensnared 2.3 times more insects than undecorated webs (the scientists didn't study enough plant-decorated webs to conclusively determine its effect on prey capture).
Although 17 wasps did fly within the vicinity of the webs, the scientists didn't observe any predator attacks. This means that no conclusions could be drawn on decoration efficacy against predation.
They observed no significant effect of environmental parameters on insect entrapment. This suggests that they aren't important for prey attraction.
It seems clear that silk decorations help the spiders ensnare prey, even if some other factors are still unclear. Color contrast may explain this observation.
The scientists found that both the spiders and their webs reflect light in the ultraviolet and blue color range, but the decorations do not (i.e. the spiders are brighter than their decorations). Birds would be able to see the difference at both short and long range, but Hymenoptera insects (ants, bees, etc) would only be able to tell the difference at long range.
Bees and many other insects can see ultraviolet light. It's reasonable to speculate that they can see the spider webs from a distance, initiating the attraction, but close visual inspection is impaired, preventing them from recognizing an orb-weaving spider web until it's too late.
Implications.
Silk decorations clearly help orb-weaving spiders attract prey. However, the jury is still out on their possible effects against predation, and the effects of dead plant matter on prey attraction and predator defense.
NOTE: The scientists' research was funded by the Ministry of Education Academic Research Fund and the National Natural Science Foundation of China.
for more information:
Tan, E. J., Seah, S. W. H., Yap, L.-M. Y. L, Goh, P. M., Gan, W., Liu, F., & Li, D. (2010). Why do orb-weaving spiders (Cyclosa ginnaga) decorate their webs with silk spirals and plant detritus? Animal Behaviour, 79 (1), 179-186 DOI: 10.1016/j.anbehav.2009.10.025