YSM Issue 97.1

WEATHERING
Evolutionary Biology
FEATURE
THE STORM
ART BY
PATRICIA
JOSEPH
BY SAMANTHA LIU
HOW TINY BUGS SURVIVE THE RAIN
Over a still pond, a raindrop falls over a line of water striders. It
engulfs one of the insects, launching it upward amid a water
jet. Momentarily the water strider is airborne—then the
jet collapses, whips back up. The bug is left submerged, twisting just
beneath the glassy surface.
The Gerridae insect family has long drawn scientists’ fascination with
its unusual water-walking ability. But how these bugs fare on turbulent
seas has just been uncovered in a new study by a team of physicists from
Florida Polytechnic University. Led by assistant professor of mechanical
engineering Daren Watson, researchers captured on stunning video how
water striders survive simulated rain drops. After plunging subsurface
with their water-repellent coat, they rise back upward atop a jetstream.
When a second collision pulls the bugs underwater, they paddle along
with swift strokes.
As a result, these tiny striders can weather a violent rainstorm—
an insight that surprisingly may extend into how microplastics
persist in marine environments.
“I think it’s my best work yet,” noted Watson. “We’ve answered a
fundamental question, but there’s much to explore in studies to come.”
Watson’s foray into aquatic insects began on his Florida campus,
where he found himself curious about the bugs skimming across his
local pools. He couldn’t understand how their millimeters-long bodies
could survive a free-falling drop, much less a storm. “[As] opposed to us
humans, these insects have nowhere to hide,” Watson noted.
With a little guidance from the “Bug Closet” at University of Central
Florida, and a lot of scouring from nearby ponds, Watson captured and
reared a group of water striders in a mini aquarium. He placed twenty
of these insects in a chamber with an elevated nozzle and a syringe
A water strider balances atop a pool with its delicate legs.
IMAGE COURTESY OF WIKIMEDIA COMMONS
pump. To simulate rainfall, he directed water from the nozzle through
a long, narrow channel, landing droplets one at a time onto the bugs.
After some tinkering and tailoring, the results—rendered in crystalclear,
3200 frames-per-second video resolution—reveal a valiant battle
by the Gerridae. While some scatter and leap away at an impending
raindrop, a bug caught in the splash zone must bear its full brunt.
“The force of the raindrop striking the water strider is significantly
higher than the weight of the water strider,” Watson said (up to forty
times higher, to be precise—imagine a small delivery truck crashing
atop you). “But it does not cause the strider to die.”
Hydrophobic, densely-packed hairs along the bug’s exoskeleton repel
the water, forming a bubble, called the first crater, around its body. This
crater generates a buoyant force that pulls the water strider back to
surface, then shoots upward as a water jet—which a well-positioned bug
can surf like a gentle tidal wave.
But when this jet collapses back downward and forms a second crater,
the collision this time comes faster and harsher. “You can think of it like
you’re stretching a rubber band… [it’s] going to rapidly go back to its
original position,” Watson said.
As the bug sinks down again, the crater retracts so quickly that
the swimming creatures struggle to follow. According to Watson’s
calculations, if this crater’s acceleration exceeds a threshold value—5.7
times the acceleration due to gravity—it tears away from the insect body,
leaving the water strider submerged beneath the cavity.
Though some footage shows intrepid water striders pedaling their legs
to come up for air, they do not always succeed. “They must be able to
swim and survive below the waterline for a period of time,” Watson said.
“And that also adds to their survival during rainfall.”
He wonders if this is an evolutionary predisposition for creatures
exposed to rain—why resurface if another drop will come plummeting
down seconds later? But as a physicist, Watson is more proud of cracking
the mathematics of the second crater’s growth and collapse, something
never before reported. Noting water striders’ similarities in size and
buoyancy to ocean microplastics, he looks forward to translating his
findings toward studying these waterway pollutants.
When [microplastics] get into our water bodies, how do they become
submerged?” he said. “How are they going to be exposed to the fishes,
the marine life within those water bodies? How does rainfall precipitate
that exposure?”
For now, Watson has moved on from sea critters and back toward his
realm of inanimate objects, though his study has opened new doors for
biologists and engineers alike. In the meantime, across the swamps of
Florida, water striders continue to twist and swirl beneath our marine
surfaces, braving the onslaught of rain. ■
www.yalescientific.org
March 2024 Yale Scientific Magazine 25