A fly has a pair of tiny, dumbbellshaped limbs called halteres that were once a second pair of wings. They wield them to make razorsharp turns and land out of reach on your ceiling. But don't despair – there is a trick to smacking these infuriating insects.

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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. And a fly’s eyes and tiny brain process information 10 times faster than human eyes and brains.

“Compared to flies, humans are slow and sluggish creatures,” said Sanjay Sane, who researches flies at the National Centre for Biological Sciences at the Tata Institute of Fundamental Research in Bangalore, India.

Once the fly escapes your swatter and is in the air, it’s in its element and your job is even tougher. Seen up close and slowed down, a fly’s aerobatics are impressive: It makes razorsharp 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, wouldn’t you know it, are flies too). Most other insects – bees, butterflies, dragonflies – have four wings and no halteres.

How do flies’ halteres work?

As a fly turns, its halteres sense the rotation. In a split second, neurons at the base of the halteres send information to the fly’s muscles to steer its wings and keep its head steady.

“Houseflies 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 you’re using vision that takes too long to do,” said Jessica Fox, who studies flies at Case Western Reserve University, in Ohio. “They really need a mechanical receptor in order to be able to sense their body rotations and correct them on the timescale that they need.”

How do flies land and stay on the ceiling?

Their halteres allow them to rotate quickly to land on the ceiling. Once they’re there, they hang upside down with tiny hooks and sticky pads on their feet. The pads, called pulvilli, have microscopic hairs that excrete a liquid that sticks to the surface.

How do I swat a fly?
“Flies process information about moving objects but they cannot process static objects, Sane explained. “Thus, the best way to approach a fly is in small, quasistatic steps such that they do not see you as a moving object.”

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