Climbing is one of the riskiest things an adult tarantula can do. Weighing up to 50 gm, these fragile arachnids would likely die in a fall. While they don’t spin typical webs as other spiders do, they do make silk and use it to line or protect their burrows. Intrigued by a scientific controversy over silk thread coming from tarantula feet, Claire Rind from the University of Newcastle, UK, began an experiment and found that tarantulas do shoot silk from their feet when they lose their footing. She published her results in the June 1, 2011 issue of The Journal of Experimental Biology.
Scientists first suggested that tarantulas emit silk from their feet in 2006, but many believed that tarantulas grab silk from their spinnerets (silk-producing organs) and use that as a sticky anchor.
Rind and her team tested how well three ground-dwelling Chilean rose tarantulas kept their footing on a vertical surface. Gently placing one of the animals in a very clean aquarium with microscope slides on the floor, Rind and undergraduate Luke Birkett cautiously upended the aquarium to see if the tarantula could hang on. Rind said:
Given that people said tarantulas couldn’t stay on a vertical surface, we didn’t want to find that they were right. The animals are very delicate. They wouldn’t survive a fall from any height.
But the spider didn’t fall, so the duo gave the aquarium a gentle shake. The tarantula slipped slightly but soon regained its footing. The spider had held on against the odds, but would Rind find silk on the microscope slides?
Looking at the glass by eye, Rind couldn’t see anything, but close inspection with a microscope revealed minute threads of silk attached to the microscope slide where the spider had stood before slipping.
Next, Rind had to prove that the silk had come from the spiders’ feet and not their web-spinning spinnerets. Filming the Chilean rose tarantulas as they were rotated vertically, Rind and her team disregarded any tests where other parts of the spiders’ bodies contacted the glass and confirmed that the feet were the source of the silk. Also, the arachnids produced their safety threads only when they slipped.
But where on the spiders’ feet was the silk coming from? Having collected all of the moulted exoskeletons from her Mexican flame-knee tarantula, Fluffy, Rind looked at them with a microscope and could see minute threads of silk protruding from microscopic hairs on Fluffy’s feet. Next, the team used a scanning electron miscroscope to take a closer look at moults from Fluffy, the Chilean rose tarantulas, and the Indian ornamental tarantulas. At great magnification, they saw minute reinforced silk-producing spigots widely distributed across the foot’s surface and extending beyond the microscopic attachment hairs on the spiders’ feet. Rind also looked at the tarantula family tree and found that the three species were only distantly related, so probably all tarantula feet produce the life-saving silk threads.
Finally, having noticed the distribution of the spigots, Rind realized that tarantulas could be the missing link between the first silk-producing spiders and modern web spinners. She explained that the spread of spigots on the tarantula’s foot resembled the distribution of the silk spigots on the abdomen of the first silk spinner, the extinct Attercopus spider from 386 million years ago. The modern tarantula’s spigots also looked more similar to mechanosensory hairs that are distributed over the spider’s entire body, possibly making them an evolutionary intermediate in the development of silk spinning. So not only has Fluffy settled a heated scientific debate, but she also may be a link to the silk spinners of the past.
Though Fluffy’s moults were used in the study, Fluffy herself died before she could participate in the study. Rind added:
She was not the best-behaved lady … a bit aggressive.
Bottom line: Claire Rind, University of Newcastle, UK, and her team have published a paper in the June 1, 2011 issue of The Journal of Experimental Biology, concluding that tarantulas emit silk from their feet when losing their balance, and suggesting that tarantulas may be an evolutionary intermediate in the development of silk spinning.