World Wide Webs

Eoin Dillon

Some of the globes most amazing silken structures

Spiders are some of the most familiar arthropods on the planet. Whether they are haunting arachnophobes nightmares or turning high-school students into superheros, the popular influence of our web-slinging buddies is undeniable. However their most amazing feat yet is the production of silk and the construction of elaborate structures and tools from the material. A spider produces silk from a number of spinnerets of the posterior side of its abdomen. Each spinneret has many spigots, and each spigot is connected to a silk gland. The silk is initially liquid, but hardens on contact with the air, and through the act of drawing it out from the spigot. There are at least 6 different types of silk gland. [1]

The type of web you are probably most familiar with are the orb webs made by spiders in the family Araneidae. These webs consist of a framework of non-sticky webbing and are finished with layer of silk covered in sticky droplets to catch prey. When an insect inevitably flies into the web, it gets caught and then paralysed by the spider’s venom before the spider sucks the liquid juices from the unfortunate insect (spiders are unable to eat solids, all food must be liquid). Some spiders fortify their trap with an ‘X’ of ultraviolet reflective webbing. Studies have shown this helps with capturing prey (a lot of insects can see ultraviolet and may be attracted to it).[2]

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An orb-web with UV reflective ornaments. Image source

Some orb webs can get very large. The largest of which belong to the Darwin’s Bark Spider (Caerostris darwini) from Madagascar. These webs can span whole rivers and have an area up to 2.8 m^2 (30 ft^2) with anchor lines that can stretch 25m (70ft). Not only is this one of the largest spider webs, but also one of the strongest, with the silk being 10 times stronger than kevlar (the material used in bulletproof vests). [3]

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The web of Darwin’s Bark Spider. Image source

Spiders that favour ambush hunting can use silk in a variety of means to assist them in the hunt. Trapdoor spiders dig out small burrows using spines on their fangs and construct a lid for the burrow using silk and soil. The giant trapdoor spider (Liphistius) places trip lines around the entrance to its burrow to detect anything passing by. Once an insect walks close to the entrance the spider quickly strikes before returning to its lair and closing the door.

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Trapdoor spider burrow. Image source

Trapdoor spiders may live for up to 20 years in their burrows and some have protective means to avoid predators. [4] The burrow of Anidiops villosus has a collapsible sock made of silk and soil, when a potential predator (such as a centipede) enters the burrow the spider can pull down the sock creating a false bottom and hiding itself. Some other species of trapdoor spider even have escape tunnels leading from their burrows.

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Unlucky day for this cricket. Image source

Other spiders don’t use their webs as passive traps to catch their prey, but take a more hands on approach. An example of this would be with the net-casting Spider (Deinopsis). These spiders hunt at night and hang upside down from some silk while holding a net of sticky webbing in its front four legs. When an insect, such as an ant, scurries by, the spider opens its net and strikes, scooping up the meal in the net. [5]

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Net-casting spider holding the net in its front legs. Image source

Some spiders take an even more active approach and will ‘fish’ for airborne insects using a ball of sticky silk at the end of a long thread. These bolas spiders (Mastophora) swing their lasso while hanging from its web. The ball at the end has a smell similar to the pheromone released by female moths to attract male moths to their doom. [6]

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Bolas spider complete with ‘lasso’. Image source

 

A final example of spiders using their silk to actively hunt would be the spitting spiders (Scytodes). These spiders produce both venom and glue in the venom glands in the front of their bodies. When close to prey the spider will spit a concoction of the two from its fangs to immobilise the prey. The spider will even move its fangs from side to side trapping the insect in a zigzag of silk before moving in to deliver the final bite.

Here I have outlined some of the amazing ingenuity spiders have shown in using their silk as both a home and a weapon, but this blog has only scratched the surface of the ways in which spiders use their silk-spinning gift.

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References

  1. https://en.wikipedia.org/wiki/Spider
  2. http://beheco.oxfordjournals.org/content/10/5/607.full
  3. http://www.livescience.com/8686-itsy-bitsy-spider-web-10-times-stronger-kevlar.html
  4. http://australianmuseum.net.au/trapdoor-spiders
  5. http://lkcnhm.nus.edu.sg/nus/images/data/nature_in_singapore/online_journal/2009/2009nis247-255.pdf
  6. http://link.springer.com/article/10.1023%2FA%3A1005488128468

 

 

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