Saturday, 7 May 2011

The Spiders That Decorate Their Own Webs


Spider webs – possibly the most beautiful and intricate animal structures of the natural world. However, some spiders are not content with a simple web. They go one step further.

Some spiders decorate their own webs with even more elaborate and complex patterns than are necessary.  Could they be the best exterior designers on the planet?  Certainly from the look of these examples, they would be in the competition but the verdict is still out as to why they produce these extra web configurations.  Some scientists argue that it is nothing more than ’spidey’ aesthetics.  Take a look at some of these arachnid designs and come to your own conclusions.

In the early nineteen fifties the children’s author EB White was struggling to come up with ideas for his second novel.  One day he noticed the additional decorations on the web of a Banded Garden Spider – much like the one above.  It was from this natural inspiration that he would come up with the idea of a writing spider and would go on to write one of the world’s most cherished children’s books, Charlotte’s Web.  Although anecdotal this story serves as a fine introduction to this most peculiar of insect habits.

As a certain Miss Aniston used to say, here is the science.  The structures are known as web decorations but the more scientific name for one is stabilimentum.  In the plural they are known as stabilimenta and the name came about because of a mistake.  When first studied the decorations were believed to be used in stabilizing the web of a spider – and there you have the term stabilimentum – get it?  However, this theory is generally dismissed these days – although it is obvious to one and all why early scientists may have thought this.

The truth be told, it is quite likely that the purpose and function of stabilimenta are manifold.  It has been discovered that they evolved independently perhaps as many as ten times.  Some spiders make their decorations purely out of their silk.  Other spiders will make them from this and the remains of their egg sacs, not to mention any detritus that just happens to be close to their webs.  The fact that they evolved independently does seem to point towards different functionality.

The web decorations can be found among a number of species of spider – but the best known example comes from the Argiope genus.  The Saint Andrew’s Cross spiders are the most obvious if not resplendent of these web decorators.  They are so called because of their habit of resting in their webs, legs outstretched in the shape of the cross of Saint Andrew.  The decorations, often at almost mathematically precise forty five degree angles seem to extend the length of the spider and, potentially, make it a much less of an attractive target for those that predate it.  Often, too, the decoration will be vertical and young spiders may even go for disc shapes.

Along with other things which seem to have a simple explanation, if you get five arachnologists in a room and ask them the purpose of web decorations then you may well get five different answers.   Some think that the web decorations afford the spider and extra edge in terms of self protection.  It may make spiders appear larger, as already seen, or make them more camouflaged.  It may be the reverse of camouflage – by making the spider more visible then the web itself will be seen by animals like birds that are then less likely to inadvertently damage the web, partially wrecking or even destroying the painstakingly built structure.  So it could well be a kind of ‘stop sign’ to other animals.

As science has progressed, so have the theories.  One more modern idea posited is that the stabilimenta are used in order to attract more prey to the web.  You have all seen insects at night flying towards lights?  Ultraviolet light is often used to attract insects and then there is a sharp noise of insect flesh impacting and exploding under the influence of an electric current.  It is now thought that the web decorations reflect ultraviolet light and this makes them attractive to a large number of insect species.  Unwittingly they fly to their deaths.  It is a pretty cool idea to think that spiders have for millions of years been using something as a weapon that we only recently discovered for ourselves.

Thermoregulation has also been put in to the ring to fight it out with the other ideas.  The web decoration may well help the spider keep its body temperature to its optimum for survival, no matter what the weather is doing around the web.  Perhaps the stabilimenta help it to create that all important dynamic state of stability between what goes on inside the spider and what is going on where it lives.

It could be that the spider has simply produced an excess of silk and that in order to divest itself of this it creates the web patterns.  This could be argued against, as why should the spider simply not expel the excess rather than taking the time and energy to create these marvelous patterns?  That returns us to the theory that the spider may well have a sense of web aesthetics – and as the ultimate web designer it wants its site to look the business.  Is this perhaps a little too anthropomorphic?  Maybe not, as many animals look to improving their environment – usually in pursuit of one thing.

Yes, you have it.  The urge to reproduce may well be the reason that some spiders produce their web decorations.  When the female is ready to reproduce she must attract a male and what better way than to enhance her already magnificent web?  Above the female and male Argiope appensa with stabilimentum prepare to Disovery Chanel their way to the propagation of the species.  One study in Spain did show that there was a definite correlation between the presence of a stabilimentum and the presence of a male looking to further the species by another generation.

The web decorations that are most noticeable are those that are made from silk though some spiders do in fact mix it up and include other items in their stabilimenta.  Again, another theory indicates that these decorations will help to camouflage the spider – and give it protection.  However, in the Nephila genus, spiders often attach lines of uneaten prey to their webs.  As unlikely as this sounds, studies have indicated that this line of half eaten carcasses actually encourages more prey in to the web.  And, yes, they always seem to be displayed in a straight vertical line.  It does, however, somewhat detract from the much trumpeted aesthetics of stabilimenta.

Whatever the reason spiders construct their web decorations, it is certainly yet another astonishing feat that nature presents to us on a daily basis but which often we can overlook.


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