TF第14篇Spider Web Decorations
蜘蛛网装饰品
球织蜘蛛（圆形网的编织者）提供了一个进化难题，它们将令人印象深刻的锯齿形白色紫外线丝线融入它们的网中。这似乎使陷阱对蜘蛛需要捕捉的猎物更加明显。鉴于许多昆虫可以看到紫外线，人们想知道球状编织者如何通过提醒潜在受害者注意它们的网所带来的危险来获得收益。但也许这些网络附加物实际上吸引了专门从紫外线反射花中采集花蜜的紫外线反射蜜蜂。蜜蜂构成了一些带有装饰网的蜘蛛的大部分猎物这一事实支持了这一假设。
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.
如果装饰一个人的网络确实会带来卡路里损失的成本，那么这种行为有什么平衡好处？一种可能性是，当路过的鸟儿看到明亮的装饰物时，它们会突然转向以避免与网碰撞并被粘稠的丝绸覆盖。如果是真的，那么在上面的实验中。与没有任何“远离”信号的网相比，带有装饰的网应该更少被鸟类损坏。事实上，实验的实际结果与预测结果相符，装饰过的网被鸟类损坏的次数比未装饰的少 45%。
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.
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