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Sticky Feet

June 13, 2011

Eva Gusnowski

I used to think that Marvel comics had it all wrong: Spiderman shot webs out of his forearms, not from the abdomen like spiders do. Silly comic books. But new research out of Alex Ranken’s lab at Newcastle University is making it seem like maybe this time the comic books got it right.
 
We’ve all seen spiders creeping up walls. They are usually able to do this because of the large number of finely divided hairs found on the underside of their feet. Personally, I like to think of it as the spiders having incredibly bad split ends. Because there are so many hairs, the attractive forces between the hairs and the surface are strong enough to keep the spider attached to the wall. Essentially, the hairs increase the surface area of the spider’s feet, and more surface area equals more attractive forces. But the amount of attractive force is really only strong enough to hold small spiders to the wall when mass is taken into consideration. Larger spiders, like tarantulas, hit the ground with more force when they fall off of a wall because they weigh more, meaning that they will sustain more damage than a small spider that falls. So tarantulas had to be crafty, and find a different way to help them to stick to vertical surfaces in addition to hair.
 
spigots, tarantula, hairs
 
It was reported previously that the Costa Rican zebra tarantula left little sticky web footprints on a glass covered vertical surface, and it was concluded that the tarantulas were able to eject silk from their feet. However, a secondary look at these spiders revealed that the silk was coming from the spinneret, and the legs were drawing the silk from the abdomen before placing them down on the glass surface. If the spinneret was covered with wax to prevent silk ejection from this area, the web footprints disappeared.
 
chilean rose tarantula, science in seconds, spigots
 
Ranken’s lab has continued work on the potential for silk ejection from spider feet on adult Chilean rose tarantulas. Female spiders were placed in a tank lined with glass microscope slides that was raised to vertical and shaken slightly to cause the spiders to slip and induce tarsal (foot) ejected silk. Although the spinnerets weren’t covered in wax, only trials where the feet slipped because of gravity, and not from movement of the legs were used, unlike the previous study. 
 
It was found that the area of silk deposited on the glass slides was about the same width as the area of the foot that touches the ground in walking spiders (~1mm to be exact…those feet seem so much larger in my nightmares). Additionally, when the molted skins of these spiders or the live spiders themselves were examined, small silk strands were emerging from the tips of many hairs on the foot. This was confirmed by looking at the molted skins of this species plus two others (the Indian ornamental and Mexican flame knee tarantulas) under scanning electron microscopy. The hairs that secrete the silk are referred to as “spigots,” and stick out beyond the finely divided hairs on the underside of the foot.
 
spigot, science in seconds, tarantula
 
What’s also really interesting about this finding is that there is controversy about where the spinnerets may have evolved from. It has been suggested that spinnerets are limbs that have evolved into an organ that easily ejects silk from the abdomen. Since there was no evidence for limbs that do eject silk, this theory had little support. But since the three species of tarantulas studied do eject silk from their feet, this theory is receiving new attention.
 
Well Stan Lee, it looks like I owe you an apology. But it still doesn’t mean you knew what you were talking about. Lucky bastard.

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