Biologists have long been fascinated by how animals escape certain death. Perhaps it's because escape behaviors are so strong and easy to reproduce, or perhaps it's just that we can't help cheering a little bit when an animal outsmarts something that is determined to kill it. One of the most well known of these behaviors is `swatter escape' performed by flies in kitchens everywhere. How do flies jump away from an approaching swatter (or predator)so well? A recent paper in the journal Current Biology by Gwyneth Card and Michael Dickinson (Caltech University) addressed this question using high-speed videography of flies jumping for their lives.
First, the authors let single fruit flies (Drosophila melanogaster) climb onto a small platform; then they slid a large black disc toward the platform and filmed each fly's response. Every animal was oriented differently on the platform and so in each trial the disc approached from a different direction relative to the fly. Overall, the flies took no chances with the disc. They jumped directly away from the approaching stimulus no matter how it came at them. Furthermore, the visual component of the approaching disc alone was sufficient to evoke the response.
Next, the team wanted to know if flies angle their jumps away from approaching stimuli or whether they jump up without direction and steer away in flight. To address this, Card and Dickinson simply removed the insects'wings and retested them. Even while wingless, the animals jumped directly away from the approaching disc.
When the researchers examined each fly's fast footwork before takeoff, they found that before any wing movements the insects move their middle pair of legs and reposition their center of mass away from the looming stimulus. Furthermore, the preflight routine does not just pile a new motor command on top of an existing stance or behavior; that is they don't just use a simple feed forward motor program. Instead, the flies take into account their own body position when the stimulus is detected and try to compensate.
These results suggest that fruit flies are doing a sophisticated sensory to motor transformation before they ever leave the ground. Each fly is turning a visual estimate of where a looming object is into a corresponding set of motor commands that reposition its center of mass for optimal takeoff. Furthermore,what the fly happens to be doing at the time is taken into account. Plus,they're doing all of this fast (∼300 ms). The ability to do this type of motor planning is usually associated with complex vertebrate cortices, not tiny fly brains.
This work is an example of how if you look at a seemingly simple animal,doing a seemingly simple thing, they're often capable of a lot more than you'd think possible. This work is also a great example of how very close and very careful observation of escaping critters can answer important questions about how animals do what they do.
