Professor Adam Summers, University of California Irvine
Part of the show Science Question and Answer - New Horizons Mission
Chris - Now if you're at all arachnophobia, cover your ears because Adam Summers is from the University of California Irvine and he's got a very sticky story for us about big spiders.
Adam - We've found that tarantula spiders, at least one species of them, produces silk with their toes. So they actually are producing the same sort of silk that spiders produce to make webs, except that instead of producing it out of their spinnerettes, which are on the back end of the animal, what's called the opisthosoma or the abdomen, they are producing it out of those eight little legs that are up on the front end. And, of course, they're tarantula spiders so there's nothing really little about them.
Chris - But why would they want to do that. Why do they need the silk to come out of their feet?
Adam - Tarantula spiders are terrestrial. They run around on the ground and we suspect that this silk is used to increase both friction and adhesion with their toes. Spiders have a really well known dry adhesive system that's quite similar to the gecko toe, but that doesn't always work and so having some sticky silk that comes out can give them a little more traction. And remember, these are great big spiders and they don't have a very well armoured exoskeleton, so if they fall, they're in real trouble.
Chris - So the theory would be that they, first of all, glue themselves to a surface and then as they move along, they're laying down a line that can essentially be a prevention to stop them falling.
Adam - Well no, not quite. So what you've just done is given a very nice description of what's called drag line silk and most true spiders will lay down a drag line. Every step they take they glue down a little bit of silk behind them and if you knock them off a table or something, they'll just hang by this thread. That's not what's going on here. These threads are unbelievably short. It was only through, basically, happenstance that we managed to visualise them at all. They are on the order of one or two millimetres long coming out of the foot, and so they are really only visible if you're looking at the footprint that's been left behind in the event that the spider skids its foot a little bit.
Chris - Is that what you did to see them?
Adam - Well, we had a very lucky thing happen. One of the authors on the paper was in charge of getting spiders to walk on glass and we were then going to look at the footprints. And one day he sort of took a longer break than usual and left the spiders on a tilted piece of glass. When he came back, these spiders, which don't like to climb on tilted glass, and so they sort of freeze and don't move, the spider had slipped backwards and as it slipped backwards. You could actually see this little bit of silk at the end of each foot, and once we'd seen that silk, we knew how to get it and how to visualise it. So we were able to see it when they were walking normally and able to put it under the scanning electron microscope and visualise exactly what the fibres themselves looked like. We also subjected them to some chemical tests and see that they are just as difficult to dissolve as regular spider's silk.
Chris - So what came first, actually the ability to spin silk from the abdomen or the ability to spin silk from the feet, from an evolutionary point of view?
Adam - Therein lies the really interesting question and Cheryl Hyashi, who's at the University of California at Riverside and is a co-author on this paper, she uses genetic techniques to try and understand which of two scenarios happen. The spinnerettes, which make silk in all spiders, are thought to be vestigial limbs. Did those limbs have silk because all arthropod limbs have silk? Or, did the limbs gain the ability to produce silk because the hardware for producing it was already in the genome and it was being expressed in the spinnerettes?
Chris - Adam Summers there describing how tarantula's produce silk from the tips of their toes.