The ability to fly has always fascinated mankind and while we can now safely get a gigantic double-decker Airbus A380 airborne, the production of efficient small flying robots is still in its infancy. ‘If we want to make little things that fly, we're not even close to achieving what animals can do, in particular small animals’, points out Jonathan Dyhr, a researcher at the University of Washington, USA. It is no surprise, therefore, that many scientists like Dyhr are studying insects in the hope of discovering their aeronautical secrets. While the wings are clearly central to flight control, it was the abdomen's role that interested Dyhr. As he explains, ‘It has been known for years that when insects are restrained for tethered flight preparations they have very exaggerated abdominal movements. While there have been a lot of theories as to how the abdomen could control flight, there had not been any firm tests of whether these theories were true or if the abdominal movements were simply an artefact of tethering the moth’. With the help of Tom Daniel, Kristi Morgansen and Noah Cowan, Dyhr set out to understand whether the abdomen had any role in flight control (p. ).
First, Manduca sexta moths were tethered in a circular arena of LED displays, which spanned 220 deg of the moth's visual field. The team then created a grating pattern of green and black bars on the displays. The team oscillated this grating pattern with the pattern moving up and down the screen, giving the insect the impression it was rotating upwards or downwards. The oscillation varied and occurred in one of two modes each lasting 40 s. In one mode, the pattern oscillated with an irregular but periodically repeating motion, with the pattern moving up or down the screen to different degrees and speeds throughout the 40 s. In the other mode, the grating pattern moved up and down with increasing frequency, moving smaller and smaller distances up and down as time went on, giving the moth the impression of accelerated tumbling. For 5 min the two modes were presented in a pseudo-random order while Dyhr filmed the moth's reaction using high-speed cameras. Regardless of the exact mode of oscillations, the team found that the moth responded the same way, with the abdomen moving in the same direction as the pattern, so if the pattern rotated upwards the moth would bend its abdomen upwards and likewise when the pattern moved downwards the abdomen flexed down. In addition, this flexing movement was in sync with the pattern. Dyhr explains, ‘If you look at low frequencies, the animals are very good at matching the motion of the pattern as it moves up and down, but as you go to much higher frequencies, the moth lags behind and it's not fully able to keep up with the oscillation’. However, the question remained, how exactly is abdominal movement altering flight?
Using the data collected from the LED display, the team could answer this by creating a model of how the moth moves its abdomen in response to the world spinning around it as it flies and the impact that these movements have on the aerodynamic forces that keep the insect aloft. Their model suggests that two things are happening. First, flexing the abdomen shifts the moth's centre of mass to counteract the rotation. In addition, this causes the thorax to bend, re-directing the aerodynamic forces produced by the wings and so correcting the loss of stability. So, while the wings are still central to keeping insects aloft, it seems that the abdomen also plays a key role in flight control.
