Fun Fact - Fly on a Leash

When I was 12 years old, my friend, Jim Kelly, could catch a fly with his hands and release it unharmed. He brought his cupped hands together just above the fly, leading the insect so to speak. The fly rose in reflex and was trapped in a small space between his palms. He could even hold the fly in his fingers without damaging the delicate structures. "You ought to be a surgeon." I remarked. And then, on a lark, "Why don't you try that with a bee?" He politely declined.

Catching insects in flight, that's pretty good, but his next trick is even more amazing. Jim found a girl with long hair, my sister for instance, and plucked a hair from her head. He tied a slipknot in the end of the hair, caught a fly, put the loop around its neck, and tightened it just a bit. "Too tight," he explained, "and the head pops off." Again, the fly was unharmed. It flew about, tethered by a single strand of hair. Jim had a fly on a leash. If I read this story on the internet I probably wouldn't believe it - but it's true - he handed me the hair, and there on the end was the fly, buzzing about in circles.

Always an instigator, I suggested a variation. "Why don't you put a fly on each end?" I asked. It took a bit of finesse, but he did just that. Soon two flies took off for freedom, only to find they were bound to each other by a 45 cm hair. They wrapped around a pole and dropped to the ground in confusion. After a couple minutes they flew off again. We followed them down the street, but after a block they were clearly losing altitude. Soon they were just a meter off the ground, as though the hair was weighing them down. One fly finally gave up, and pulled the other fly down to the ground with him.

A fly weighs about 10 mg, and a 30 cm hair weighs 1.5 mg, though this depends very much on the fly and the hair. Would an extra 5 to 10 percent weight bring a fly down? Perhaps, or it could be the extra energy they spent trying to go in different directions. If you and a friend have ever held a jumprope taut, you know that the tention is greater than the weight of the rope.

Further Reading

Strength of hair
Hair growth and quantity
Diameter of a human hair is 0.1 mm
Strand of hair weighs 0.05 mg per cm
Housefly weighs 12 mg