This jumping spider has sticky feet -- but it's no gumshoe! Photo
A scanning electron micrograph of
the foot of the jumping spider Evarcha arcuata shows a tuft of eensy-weensy
hairs, called setae, that grab slick surfaces (200x magnification).
Photo courtesy Institute
Each seta is densely covered with itsy-bitsy
hairs, called setules. If you could, you would count 2.1 million
setules per square millimeter of setae (3000x magnification).Photo:
But how? Good question. We can't climb shiny metal surfaces without magnets, say, or grappling hooks. Jumping spiders -- a widespread group that hunts prey without building webs -- lack such equipment, yet they can climb slick, waxy leaves without a care in the world -- except the constant animalian question: "Where's dinner?"
How? We get a clue from Andrew Martin, at the
Institute for Technical Zoology and Bionics in Bremen, Germany.
He and his colleagues have jumped into the question with a close
examination of a jumping spider's foot.
Using sophisticated microscopes, these researchers found that the spiders' feet were covered by hairs that branched into even finer hairs. The smallest of these, called setules, appeared in astonishing abundance; about 624,000 per spider.
The researchers measured attraction between one setule and an atomic-force microscope. Then they calculated that one spider had, theoretically, 173 times the adhesive force it would need to hang beneath a leaf. (While that's impressive, experience with other animals indicates that a real, live spider might have only 10 percent of that attraction.)
But even that would mean that a spider could
hang by a single paw -- or foot.
The logical explanation for the attractive force, Martin told us,
is a close-range atomic attraction called the van der Waals force.
If your physics is as rusty as ours, you won't mind remembering
that this force grows from electrical attraction between nearby
molecules. You might expect the electrons on the outside of a molecule
to repel each other. But when electrons move, that creates regions
that are more or less negative. As a result, Martin explains, "there
will always be imbalances between negative and more negative charges,
so one region is in effect more positively charged" than another.
Van der Waals forces only work within a couple
of nanometers (billionths of a meter). Otherwise, our world would
be very different -- much to the detriment of the glue industry.
triangular setule tips stick to surfaces by van der Waals force
(20,000x magnification). Photo: Institute
That close-range requirement suits the capabilities of chitin, the material that composes the external skeleton of insects, spiders and other arthropods. Depending on its chemical composition and molecular arrangement, chitin can range from stiff to rubbery, says Martin. Chitin, in other words, is a smart, biological composite material.
The spiders' feathery feet, built with a flexible form of chitin, can conform to surfaces and create a strong van der Waals force.
While flies and other insects also use tiny hairs for
adhesion, the fine size and resulting strong attachment makes jumping
spiders resemble the gecko more than the house fly, says Martin.
(Geckoes are lizards that lurk on walls in the tropics, waiting
to eat insects.)
The beauty of the spider's sticky foot is not just strength, but reversibility. When dinner walks past, the jumping spider quickly detaches itself and leaps for lunch. "How it turns the attachment off is probably the next question that we have to answer," says Martin.
Perhaps, he suggests, the foot works like Velcro, which you detach by peeling, not lifting. (Curiously, the principle of Velcro was invented by evolution. The inventor of Velcro was fascinated by the way burrs seize cloth.)
Presumably, when the spider's brain registers "dinner at 10 o'clock high!" it starts lifting its little grabbers from the edges, preparing to jump.
And there's another parallel. Just as Velcro should work in outer space, Martin envisions astronauts clambering around with boots and gloves that stick to a spaceship like a spider sticks to grass.
-- David Tenenbaum
Getting a Grip on Spider Attachment: An AFM Approach to Microstructure Adhesion in Arthropods, Antonia B Kesel et al, Smart Materials and Structures, 2004, 512-518.