If an outdoors, socially-distanced gathering is part of your Thanksgiving plans, beware of uninvited guests. I donât mean friendly neighbors who might invite themselves to a piece of pie.
A blowfly feeds on an apple with its straw-like proboscis. (Josh Cassidy/KQED)
Iâm talking about flies. Buzzing around curiously, theyâll help themselves to whatever food you leave unattended. As they walk all around they could spread hundreds of types of bacteria they carry on their legs.
So you try sneaking up on one and it skedaddles. Why, oh why, is it so hard to swat a fly?
Now you see me, now you don’t. A blowfly escapes a swatter in the nick of time. (Josh Cassidy/KQED)
Flies are formidable opponents, with an arsenal of tools they carry all over their bodies.
For starters, their hair and antennae help a fly sense us as we walk up to them.
âThey have sensory hairs all over their body that help them detect air currents,â said entomologist Jessica Fox, who studies fliesâ shenanigans at Case Western Reserve University in Ohio.
A fly can see you coming from nearly every angle. (Josh Cassidy/KQED)
Not only can they feel us, they can see us too.
âThey have a very small blind spot in the back of their head,â Fox said, âbut a lot of flies can see almost 360 degrees around their heads.â
And a flyâs eyes and tiny brain process information 10 times faster than human eyes and brains.
âCompared to flies, humans are slow and sluggish creatures,â said Sanjay Sane, who researches flies at the National Centre for Biological Sciences at the Tata Institute of Fundamental Research in Bangalore, India.
Quick, sharp turns help a fly dodge your swatter. These aerobatics are possible thanks to a pair of tiny club-shaped limbs called halteres, nestled below the flyâs two wings. (Josh Cassidy/KQED)
Once the fly escapes your swatter and is in the air, itâs in its element and your job is even tougher. Seen up close and slowed down, a flyâs aerobatics are impressive: It makes razor-sharp turns with ease and at great speed.
What makes this possible is a pair of modified wings called halteres, a Greek word for dumbbell, which describes their shape. All of the 200,000 species of flies that scientists have described have a pair of halteres and a pair of wings. (That includes mosquitoes, which, wouldnât you know it, are flies too.) Most other insects â bees, butterflies, dragonflies â have four wings and no halteres.
The relatively large halteres of a crane fly are easier to spot than most. The halteres are the small, club-shaped parts beating below the flyâs wings. (Jessica Fox/Case Western Reserve University)
As a fly turns, its halteres sense the rotation. In a split second, neurons at the base of the halteres send information to the flyâs muscles to steer its wings and keep its head steady.
âHouseflies flap their wings about 200 times per second, which means they really only have five milliseconds to figure out what the next wingbeat is going to be like. And if youâre using vision that takes too long to do,â Fox said. âThey really need a mechanical receptor in order to be able to sense their body rotations and correct them on the timescale that they need.â
Though flies are a pesky pest and we are constantly in their pursuit, they likely evolved halteres to escape other animals besides us.
âFlies hang out on the backs of cows,â said Sane. âThe tail of a cow trying to flick insects off, itâs likely to kill the fly if it doesnât fly off fast.â Lizard tongues are also quick-moving threats.
And then thereâs flies themselves. In lightning-fast chases, males compete for the ability to mate.
âThese chases are among the most aerobatic chases that Iâve ever seen; thereâs nothing that comes even close,â said Sane. âAnd if flies did not turn very fast theyâll get caught and slammed to the ground.â
When researchers remove a flyâs halteres, it can no longer control its flight. It loses all sense of where its body is in space. In slowed-down videos, flies without halteres give the impression of being drunk.
A fly whose halteres have been removed by researchers canât control its flight and falls down. (Katie Jordan, Alex Yarger and Jessica Fox/Case Western Reserve University)
âThey donât seem to know; they just keep flapping,â said Fox. âThey just keep pitching and rolling and eventually they fall. Weâve got a lot of great videos of these flies comedically falling out of the sky.â (You can see more examples in the Deep Look video embedded in this story.)
If a fly gets inside your house, its halteres will help it do a flyâs signature move: the ceiling landing.
A fly can stay out of reach by hanging upside down on the ceiling. (Josh Cassidy/KQED)
It hangs there with tiny hooks and sticky pads on its feet. The pads, called pulvilli, have microscopic hairs that excrete a liquid that sticks to the surface under pressure, sort of like suction. Hooks on the flyâs feet also help it stay put, by attaching to microscopic imperfections on the surface of the ceiling.
The pads on a flyâs feet, called pulvilli, have microscopic hairs that excrete a liquid that sticks to the surface. The photo on the right shows an extreme close-up of the hairs. (Stanislav Gorb/University of Kiel, Germany)
Despite the flyâs slick tools, Sane recommends one trick next time you try to nab one.
âFlies process information about moving objects but they cannot process static objects,â he explained. âThus, the best way to approach a fly is in small, quasi-static steps such that they do not see you as a moving object.â
If you go very slowly, and then pounce, you might stand a chance.
âGood luck, though,â he said, âbecause flies are spectacularly fast.â
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