Scientists discover why flies are so hard to swat

( -- Over the past two decades, Michael Dickinson has been interviewed by reporters hundreds of times about his research on the biomechanics of insect flight. One question from the press has always dogged him: Why are flies so hard to swat?

"Now I can finally answer," says Dickinson, the Esther M. and Abe M. Zarem Professor of Bioengineering at the California Institute of Technology (Caltech).

Using high-resolution, high-speed digital imaging of fruit flies (Drosophila melanogaster) faced with a looming swatter, Dickinson and graduate student Gwyneth Card have determined the secret to a fly's evasive maneuvering. Long before the fly leaps, its tiny brain calculates the location of the impending threat, comes up with an escape plan, and places its legs in an optimal position to hop out of the way in the opposite direction. All of this action takes place within about 100 milliseconds after the fly first spots the swatter.

"This illustrates how rapidly the fly's brain can process sensory information into an appropriate motor response," Dickinson says.

For example, the videos showed that if the descending swatter--actually, a 14-centimeter-diameter black disk, dropping at a 50-degree angle toward a fly standing at the center of a small platform--comes from in front of the fly, the fly moves its middle legs forward and leans back, then raises and extends its legs to push off backward. When the threat comes from the back, however, the fly (which has a nearly 360-degree field of view and can see behind itself) moves its middle legs a tiny bit backwards. With a threat from the side, the fly keeps its middle legs stationary, but leans its whole body in the opposite direction before it jumps.

"We also found that when the fly makes planning movements prior to take-off, it takes into account its body position at the time it first sees the threat," Dickinson says. "When it first notices an approaching threat, a fly's body might be in any sort of posture depending on what it was doing at the time, like grooming, feeding, walking, or courting. Our experiments showed that the fly somehow 'knows' whether it needs to make large or small postural changes to reach the correct preflight posture. This means that the fly must integrate visual information from its eyes, which tell it where the threat is approaching from, with mechanosensory information from its legs, which tells it how to move to reach the proper preflight pose."

The results offer new insight into the fly nervous system, and suggest that within the fly brain there is a map in which the position of the looming threat "is transformed into an appropriate pattern of leg and body motion prior to take off," Dickinson says. "This is a rather sophisticated sensory-to-motor transformation and the search is on to find the place in the brain where this happens," he says.

Dickinson's research also suggests an optimal method for actually swatting a fly. "It is best not to swat at the fly's starting position, but rather to aim a bit forward of that to anticipate where the fly is going to jump when it first sees your swatter," he says.

The paper, "Visually Mediated Motor Planning in the Escape Response of Drosophila," will be published August 28 in the journal Current Biology.

Source: California Institute of Technology

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Aug 28, 2008
... Well, I'm glad we got to the bottom of this boggle. :p

As I _HATE_ flies and they keep invading my home, I am privy to the tactic of swatting them from the front instead of the back.

I find their mid-air evasions of the swatter more interesting than their resting evasion - Usually if I can follow them through the air and they land, they're dead.

I'd also be interested in knowing WHY the SOBs literally sit right outside the door and anxiously wait for it to open so they can fly in the house to their deaths. Air temperature difference? Smells?

Aug 28, 2008
I remember watching an episode of NOVA (I think) that talked about flies seeing in slow motion. Anyone else recall that?

Aug 28, 2008
Ty1: Yup. I thought it was a segment on a show called the 'incredible machine' or something.

So their answer is basically "Flies are wicked quick."

Aug 28, 2008
A small fly (Drosphila M.) has only 200,000 neurons. One has to marvel at the complexity of the flys behaviour eminating from such a small analogue processing unit.

Aug 29, 2008
When I attended summer camp up in Vermont many moons ago there was an elder counselor who demonstrated a novel method for swatting flies. He theorized that flies could only take off along their longitudinal axis. So if you slowly positioned your open hands orthogonal to the front and rear of the fly body and clapped quickly the fly would dart between your hands and be squashed. While the theory may be suspect, with practice the technique did prove very effective (if a little messy).

Aug 29, 2008
I've noticed that killing a fly that in on a clear window is normally easy , sometimes they don't really even try to escape.

I had read a long time ago that the wind force created from swinging a solid object at a fly or other small insect is great enough to push the fly out of the way. That's why a fly swatter has holes.

I prefer a simple sheet of fly paper , they work great and are pretty cheap. Just a couple of weeks ago I had put a small 3x6 sheet out on the porch on a Friday evening and it was so completely covered with several types of fly's and yellow jackets by Monday morning I had to throw it away.

Aug 29, 2008
What i remember reading is it's not the force of the wind that pushes them away. Rather, all those little hairs on their body are sense organs. They detect the change in the air which enables them to have such quick reflexes.

I dont have a problem with fruit flies, however i do get house flies. And I've found the best way to swat them is to move my hand very slowly close to the fly. You can actually watch them and usually tell when to stop moving your hand because you'll see them sort of tense up. This seems to happen when my hand is about a foot away. At this point, it's really just a matter of moving your hand damn fast :) I would say i can hit them about 80% of the time doing this.

Aug 30, 2008
if you could place your index finger of each hand approx 45deg about the center of the fly he wil freeze until you get close enough to squash it
bot fingers must move @ the same rate along its 45deg centerline... try it it works

Sep 04, 2008
Swat a fly? I'll punch a fly in the face. Swatting is for girls.

Sep 05, 2008
I remember when I was a kid, I used transparent plastic bag to catch flies. When you approach them slowly with the transparent bags, they don't move at all. By the way, I used those flies to feed my pets, jumping spiders.

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