Spider Web Decorations

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An evolutionary puzzle is provided by orb-weaving spiders (weavers of round webs) that incorporate impressive zigzag lines of white ultraviolet silk into their webs. which would seem to make the trap more obvious to the prey that the spider needs to catch.Given that many insects can see ultraviolet light, one wonders how orb weavers could gain by alerting potential victims to the danger posed by their webs. But perhaps these web additions actually attract ultraviolet-reflecting bees that specialize in taking nectar from ultraviolet-reflecting flowers. The fact that bees constitute the large majority of prey of some spiders with ornamented webs supports this hypothesis.

On the other hand, it could be that spiders that have been catching many prey. simply because their webs are in a good spot,are the spiders that tend to add decorations to their webs. If this is true, then the correlation between web decorations and higher rates of prey capture need not mean that the decorations cause the increase in prey capture. Instead, perhaps high feeding rates provide the extra energy that the spiders need to spin their web decorations. which they do for some purpose other than prey attraction. This hypothesis can be evaluated by testing the prediction that spiders given an abundance of food should invest more in web decorations than spiders deprived of food. Indeed, in two species of orb-weaving spiders, females that received more food made significantly larger decorations, and in one species.well-ed females were more likely to add decorations to their webs than were food-deprived individuals.

If one places an orb-weaving spider in a wooden frame and lets it build a web there, so that the web can be moved to a site chosen by the experimenter, then one can put two such webs side by side in a field. In this way, one can compare the rate of prey capture in an ornamented web with that in another web in the same spot whose decorations have been removed- -something that can be easily accomplished by cutting out the two web lines that support the decoration (two web lines are also cut from the other web, but not those that hold the decoration), Such an experiment controls for the effects of site productivity. Under these conditions, the decorated web catches about a third less prey than the undecorated one. Thus, web decorations involve a cost to foraging (obtaining food), not a benefit, under some conditions.

If it is true that decorating one’s web carries a cost in lost calories, what counterbalancing benefit exists for this behavior? One possibility is that when passing birds see the bright decorations, they swerve to avoid colliding with the web and getting covered in sticky silk. If this is true, then in the experiment above. webs with decorations should have been less often damaged by birds than webs without any “keep away” signals. And in fact, the actual results from the experiment matched the predicted ones, with decorated webs damaged by birds 45 percent less often than undecorated ones.

Another benefit of web decorations might come from having the added silk camouflage (hide) the body of the web builder, making the spider less vulnerable to attack from its predators. Evidence in support of this explanation comes from a set of spiders whose females regularly incorporate a silky egg sac into their webs. The spider perches (sits) on this moderately conspicuous vertical tube of silk, eggs, and debris in ways that make her much less visible to human observers, especially because the spider’s color pattern matches that of the egg sac so closely. Moreover, when the egg sac is experimentally removed, the spider replaces it with a vertical strip composed entirely of silk, on which she perches as if it were an eggs sac. Given that the egg sac seems to be used as a concealment aid. its replacement can be assumed to serve the same function.

More data on this point come from a study in which orb-weaving spiders that had built webs with or without extra silk decorations in an enclosed area were exposed to wasps. which hunt spiders and feed them to their offspring. In one experiment, only 32 percent of the spiders with web decorations were captured , as opposed to 68 percent of those without.

1.An evolutionary puzzle is provided by orb-weaving spiders (weavers of round webs) that incorporate?impressive?zigzag lines of white ultraviolet silk into their webs. which would seem to make the trap more obvious to the prey that the spider needs to catch.Given that many insects can see ultraviolet light, one wonders how orb weavers could gain by alerting potential victims to the danger posed by their webs. But perhaps these web additions actually attract ultraviolet-reflecting bees that specialize in taking nectar from ultraviolet-reflecting flowers. The fact that bees constitute the large majority of prey of some spiders with ornamented webs supports this hypothesis.