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With two diminutive legs locked into a leap-ready position, the tiny jumper
bends its body taut like an archer drawing a bow. At the top of its legs, a
minuscule pair of gears engage—their strange, shark-fin teeth interlocking
cleanly like a zipper. And then, faster than you can blink, think, or see with
the naked eye, the entire thing is gone. In 2 milliseconds it has bulleted
skyward, accelerating at nearly 400 g's—a rate more than 20 times what a human
body can withstand. At top speed the jumper breaks 8 mph—quite a feat
considering its body is less than one-tenth of an inch long.
This miniature marvel is an adolescent issus, a kind of planthopper insect and
one of the fastest accelerators in the animal kingdom. As a duo of researchers
in the U.K. report today in the journal Science, the issus also the first
living creature ever discovered to sport a functioning gear. "Jumping is one of
the most rapid and powerful things an animal can do," says Malcolm Burrows, a
zoologist at the University of Cambridge and the lead author of the paper, "and
that leads to all sorts of crazy specializations."
The researchers believe that the issus—which lives chiefly on European climbing
ivy—evolved its acrobatic prowess because it needs to flee dangerous
situations. Although they're not exactly sure if the rapid jump evolved to
escape hungry birds, parasitizing wasps, or the careless mouths of large
grazing animals, "there's been enormous evolutionary pressure to become faster
and faster, and jump further and further away," Burrows says. But gaining this
high acceleration has put incredible demands on the reaction time of insect's
body parts, and that's where the gears—which "you can imagine being at the top
of the thigh bone in a human," Burrows says—come in.
"As the legs unfurl to power the jump," Burrows says, "both have to move at
exactly the same time. If they didn't, the animal would start to spiral out of
control." Larger animals, whether kangaroos or NBA players, rely on their
nervous system to keep their legs in sync when pushing off to jump—using a
constant loop of adjustment and feedback. But for the issus, their legs outpace
their nervous system. By the time the insect has sent a signal from its legs to
its brain and back again, roughly 5 or 6 milliseconds, the launch has long
since happened. Instead, the gears, which engage before the jump, let the issus
lock its legs together—synchronizing their movements to a precision of
1/300,000 of a second.
The gears themselves are an oddity. With gear teeth shaped like cresting waves,
they look nothing like what you'd find in your car or in a fancy watch. (The
style that you're most likely familiar with is called an involute gear, and it
was designed by the Swiss mathematician Leonhard Euler in the 18th century.)
There could be two reasons for this. Through a mathematical oddity, there is a
limitless number of ways to design intermeshing gears. So, either nature
evolved one solution at random, or, as Gregory Sutton, coauthor of the paper
and insect researcher at the University of Bristol, suspects, the shape of the
issus's gear is particularly apt for the job it does. It's built for "high
precision and speed in one direction," he says. "It's a prototype for a new
type of gear."
Another odd thing about this discovery is that although there are many jumping
insects like the issus—including ones that are even faster and better
jumpers—the issus is apparently the only one with natural gears. Most other
bugs synchronize the quick jolt of their leaping legs through friction, using
bumpy or grippy surfaces to press the top of their legs together, says Duke
University biomechanics expert Steve Vogel, who was not involved in this study.
Like gears, this ensures the legs move at the same rate, but without requiring
a complicated interlocking mechanism. "There are a lot of friction pads around,
and they accomplish pretty much of the same thing," he says. "So I wonder what
extra capacity these gears confer. They're rather specialized, and there are
lots of other jumpers that don't have them, so there must be some kind of
advantage."
Even stranger is that the issus doesn't keep these gears throughout its life
cycle. As the adolescent insect grows, it molts half a dozen times, upgrading
its exoskeleton (gears included) for larger and larger versions. But after its
final molt into adulthood—poof, the gears are gone. The adult syncs its legs by
friction like all the other planthoppers. "I'm gobsmacked," says Sutton. "We
have a hypothesis as to why this is the case, but we can't tell you for sure."
Their idea: If one of the gear teeth were to slip and break in an adult (the
researchers observed this in adolescent bugs), its jumping ability would be
hindered forever. With no more molts, it would have no chance to grow more
gears. And with every bound, "the whole system might slip, accelerating damage
to the rest of the gear teeth," Sutton says. "Just like if your car has a gear
train missing a tooth. Every time you get to that missing tooth, the gear train
jerks."
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