Since 1910, when Thomas H. Morgan used Drosphila to establish an entire area of scientific research, this fly has been a model organism for biologists. Despite extensive and varying research on the fruit fly, one peculiarity has not been entirely resolved: its flight capabilities. Come to think of it, does the fly really fly? The question is not as stupid as it may sounds. Because until now it has been assumed that the smaller an insect is, the more its airborne activity resembles swimming. So far, it has been assumed that in small insects skin friction acting on the body by far dominates over body inertia. As a consequence any movement of the body grinds to a halt within milliseconds, unless upheld with a constant exertion of force. But the fly really does fly, as a new study published in "Science" (1) has shown. The first author of the study is Steven Fry from the Institute of Neuroinformatics, a joint institute of the ETH and the University of Zurich (2).

For as long as he can remember, Fry has been fascinated by flying insects as well and as the high-tech methods that allowing them to be studied. This is why he decided to spend two post-doc years in the group led by Michael H. Dickinson at CalTech (then Berkeley), who, shortly before, had succeeded in experimentally determining the aerodynamic mechanisms that underlie the fly flight. In order to resolve this long-standing mystery, the group constructed a robot with two artificial insect wings moving in a large tank filled with mineral oil. This method allowed them to measure the forces that were generated by an insect’s beating the wings. Lacking precise measurements of the actual movement of a fly's wings, this work could not be extended to the question of how flies actually make use of their wings to hover and perform flight manoeuvres.

For this, the scientist needs high precision measurements of the fly's wing motion, a task which was not possible before affordable high-speed digital cameras hit the market. Precisely as Dickinson was getting ready to provide the necessary infrastructure, Fry was applying the finishing touches to an exciting project plan that aimed to map the wing motion of a flying insect, in order to quantify – for the first time – the aerodynamic forces in free flight.

Sour bait

Dickinson was interested in the post-doctoral project, but had reservations initially. Would it really be possible to capture the flight of the fly and the exact position of the wings in free flight, which required it to be captured simultaneously by the three running cameras? Fry saw other problems looming ahead too, as he aimed to film the rapid 90 degree-turning manoeuvres of the fly. During these so-called saccades, which last for less than a tenth of a second, the researchers expected to see conspicuous changes in the motion of the wings.

The fly's flight is governed by inertia and not friction. (Picture: Steven N. Fry) large

The first film trials were unsuccessful. The flies flew around in the test chamber, but never at the exact point where they could be captured by all three cameras. Fry soon came to the conclusion that relying on coincidence would take far too long. When he placed a small cylinder containing a few drops of vinegar into the flight chamber, the situation suddenly changed. Within hours a fly had entered the "hot" zone and carried out an entire rapid turn manoeuvre in front of the running cameras.

Steering as in formula one

The entire laboratory came together to see the first recordings. At a first viewing, it became evident that an unsteady aerodynamic mechanism, consisting of a close contact of the wings above the body, (clap-and-fling), did not occur. To examine flight manoeuvres in detail, Fry developed special software that he and his colleague, Rosalyn Sayaman, then used to measure the exact positions of body and wings of six complete saccadic turning manoeuvres, based on thousands of video frames. This analysis yielded yet another surprise. The fruit fly carried out the most extreme manoeuvres with only the slightest variations in wing motion, similar to the highly sensitive steering wheel of a formula one racing car.

Torque and reverse torque

In order to unlock the secret of how Drosophila turns, Fry decided to analyse the fly's body rotation during a saccade. For this, he was able to use thea the newly constructed, dynamically-scaled robot, called "Bride of Robofly", which was put to its first use for this project. Surprisingly, the aerodynamic torque measured during a saccade, correlated with angular acceleration and not with angular velocity, indicating that the dynamics of the fly's body are determined by body inertia and not the viscous friction of the air acting on it. As a consequence, the fly has to accelerate its body at the onset of a turn and then produce a reverse torque to stop the rotation of its body and avoid spinning like a top. This finding provides an important basis for the understanding of the neural control problems underlying insect flight, as well as for the design of biomimetic flying robots.