Shoe Fly, Don't Bother Me, or Mike Tyson?

September 3, 2009 8:09 AM

Why aren't tiny insects killed by the impact of a swatting hand?

Why aren't tiny insects (flies, bees, mosquitos, etc.) killed by the force of the impact of a human hand, (or any other "solid" object...fly swatter, textbook, etc.)? For example, the following does not kill insects:

(hand) Solid Object -> Insect <> Air

I was thinking this may be due to a "cushion" of air surrounding the striking object, but I've also swatted insects directly into the pavement/concrete before and I've never seen one die this way either:

(hand) Soilid Object -> Insect <> Air -> Solid Object (concrete)

Crushing/clapping the insect in between my hands in mid air (or between my shoe and concrete) DOES result in death to the insect, but that seems obvious to me, see the following:

(shoe) Solid Object -> I >n >s e < c < t <- Solid Object (concrete)

. . .

So what gives? Why is this? It seems like most living objects larger than a mouse would not survive any of the above scenarios, but an insect can survive 2 out of the 3? Hell, a human can't even survive all 3 of these sometimes (a Mike Tyson punch to a human head in mid air may end badly). Surely my hand (relatively speaking of course) swats a fly in mid air with more force than does the force of the hand of Mike Tyson punching a person in the head?


What's the reason???

posted by thankyoumuchly to home & garden (11 comments total)

Exoskeleton.
posted by oinopaponton at 8:10 AM on September 3

I've never had a problem killing insects with my bare hands. Presumably you aren't striking them with a flat hand but rather cupping your hand slightly when you are swatting at them.

Or you need to get stronger and faster.
posted by dfriedman at 8:11 AM on September 3

Exoskeleton
posted by plokent at 8:14 AM on September 3

You're talking about slapping them out of the air, rather than against a solid object?

If so, then yes, the cushion of air ahead of the flyswatter (or hand) is both slowing down your weapon and pushing the fly away as it arrives. This is why flyswatters have holes or mesh to allow air through.

Toss an egg in the air and try to break it in flight. Difficult with an open hand: you're more likely to just push/deflect it from its path.
posted by rokusan at 8:21 AM on September 3 [3 favorites has favorites]

The bigger something gets, the more it resembles a bag of goo. Tny things have very little inertia and can survive sudden changes in direction much more easily than big things. Build a little cube of toothpicks and glue and drop it on thefloor, it'll probably just bounce. Build the same structure from 2x4s and drop it from the same height relative to its scale, and it'll bust to bits. Humans are heavy and have a huge amount of momentum relative to insects. Also. exoskeleton.
posted by contraption at 8:26 AM on September 3

as things get bigger in our 3d world:

strength goes up as a the square of the size increase (cross-sectional area)
weight goes up as the cube of the increase (volume)

That's why fleas can jump many times their height and elephants can't.


It's easy for small things to be relatively speaking, very very strong.
posted by TravellingDen at 8:32 AM on September 3 [2 favorites has favorites]

I haven't really seen this, I'm able to kill mosquitoes with a quick smack mid-flight. Maybe I'm just Superman. I hope not, he's irritating.
posted by InsanePenguin at 8:33 AM on September 3

I am inundated with fruit flies at the moment, and I've noticed that I can kill them midair if my hand motion is just right. It's not a big swing through the air, but a short smack. They are lazy fliers, so my guess is that my hand acceleration > their own acceleration, so the impact is enough to kill them. This would be hard with an insect that is a stronger flier.
posted by slow graffiti at 8:53 AM on September 3

This has to do with the ratio of surface area to volume. As things increase in size, the ratio of surface area to volume decreases.
posted by 517 at 9:09 AM on September 3 [1 favorite has favorites]

More to the point, the relative speed of your hand to the bug increases only linearly as its size decreases, while the bug's volume, mass, and inertia decrease with the cube of its size. And since the surface area exposed to the wind before your moving hand decreases with the square of its size, the bug is blown away by the breeze before your hand gets to it, so the impact is minimized.
posted by nicwolff at 9:54 AM on September 3

You should definitely read J.B.S. Haldane's On Being the Right Size. It discusses the change of physics as you go up and down in scale; a rather drastic example of differing terminal velocities is, for instance,

"You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes. "

This applies to lateral movements as well: as soon as you accelerate an insect past its terminal velocity it will brake when it flies through the air, losing its energy gradually and not with a splat against a wall.
posted by PontifexPrimus at 10:47 AM on September 3 [1 favorite has favorites]