[ExI] how to swat one of god's little creatures

[Damien Broderick]

Scientists find the secret to swatting flies

Friday, 29 August 2008
Cosmos Online

SYDNEY: Using high-speed video footage, 
bioengineers have discovered the key to the 
evasive manoeuvrability of flies – and found the 
best strategy for swatting them successfully.

Michael Dickinson has been interviewed hundreds 
of times about his research on the biomechanics 
of insect flight. One question has always dogged 
him: Why are flies so hard to swat?

"Now I can finally answer," said Dickinson, a 
bioengineer at the California Institute of Technology in Pasadena, USA.

Tiny brain, big escape plan

Using high-speed, digital imaging of fruit flies 
(Drosophila melanogaster) faced with a looming 
swatter, Dickinson and graduate student Gwyneth 
Card determined the secret to a fly's crafty behaviour.

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," said Dickinson who this week 
publishes a paper detailing the research in the U.S. journal Current Biology.

In this study, the swatter was actually a 
14-centimetre-diameter black disk, dropping at a 
50º angle toward a fly standing at the centre of a small platform.

The researcher's videos show that if the 
descending swatter 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.

Rear attack

When the threat comes from behind, however, the 
fly (which has a nearly 360º field of view) 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 said.

"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," he said.


Perfecting pre-flight posture

Yet, the experiments hinted that the fly somehow 
'knows' whether it needs to make large or small 
postural changes to reach the correct pre-flight 
posture. This means that it must integrate visual 
information from its eyes with sensory 
information from its legs, to tell it how to move 
to get in the optimal pose for take-off.

The results offer new insight into the nervous 
system of insects, 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 said. "This is a rather 
sophisticated sensory-to-motor transformation and 
the search is on to find the place in the brain where this happens."

Handily, the research suggests an optimal method 
for successfully 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," suggested Dickinson.

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With the California Institute of Technology.