Sunday, October 29, 2006

Are spiders' spinnerets actually legs?

` I have, for a couple of weeks now, been working on a very interesting post - though since I am still not sure when I'll finish it, I shall put up this ten-day-old draft material to help fill in the time gap.
` This block-quote comes from an article I was reading, in Nature News as I recall, which I thought was quite fascinating. Stanislav Gorb and team at the Max Planck Institute were studying pieces of glass on which zebra tarantulas were treading, when they unexpectedly discovered tiny remnants of solidified silk!

The observations are exciting because they reveal an attachment mechanism not seen in any other species of arachnid, says the team. And the finding may prompt a rethink of the evolution of spider silk.

The researchers suggest that initially all spiders had the ability to secrete silk from their feet, but lost the skill because they no longer needed it.

Silk is usually produced by an organ called the spinneret, which is located on the lower abdomen. Gorb thinks that spinnerets may have evolved from leg appendages. "The finding that silk is directly coming from the feet supports this hypothesis," he says.

For now it is impossible to know whether foot silk pre-dates web spinning, or whether this type of tarantula evolved it separately. An examination of the genetics of the silk production should help to sort that out.

` In other words, it is thought that the ancestor of all spiders could have spun silk from its feet to help it stick to surfaces while climbing. As time went on, one pair of legs would have become useless for walking, though still functional for secreting silk. These became the familiar spinnerets, which small, web-weaving spiders use so often!
` Since silk is such an expensive secretion, the silk glands of the feet would have become a waste of protein for smaller spiders which do not need them for traction. Therefore, these glands were lost while larger spiders - which may not be able to crawl over slippery leaves and such without them - found them to be worth keeping.
` If an animal's legs becoming something other than legs sounds like a strange idea to you, be assured that this is not some unsupported, oddball idea that some crazy biologist thought up! For example, the mouthparts and antennae of various arthropods (a group which includes spiders, insects and crustaceans) are already known to be modified versions of legs! The spinnerets would be just one more example.
` However, the idea that spinnerets themselves are legs is not much more than a hypothesis for now until further research has been conducted. For example, it could instead be true that copies of silk glands sprouted up on the feet of a species of tarantula, which proved to be useful when climbing.


Galtron said...

Interesting! So, then, did the venom-filled fangs of spiders evolve from legs that had stingers?

S E E Quine said...

` Good question. I don't actually know about that in particular, though I know that such stinger-legs (maxillapeds) are present in centipedes....

Gareth said...

So it is possible that a spider could at one time had 5 pairs of legs. That's interesting :)

Hmmmmm I wonder then too if centipedes and millipedes could have had more legs than they have nowadays because they are using them for other uses on their bodies.

I guess this would be the opposite idea to when humans used to have tails but off course now the boney stump that remains has no use.

LOL I just read your response about centipedes so forget about my second paragraph, hahahaha.

I linked you to my blog :)

S E E Quine said...

` Indeed, centipedes and millipedes also have other appendages that were once legs.
` Yay! I'm linked to your blog now! I feel special!