Scientists have finally solved a mystery plaguing inquiring minds since the observation that flies fly with their feet pointing groundward, but can land on the ceiling. How do they do that?
An international team has cracked the aerobatic enigma once and for all using bluebottle flies (Calliphora vomitoria), as reported in Science Advances on Wednesday.
Until the 1950s, the assumption had been that flies fly the “right way up” and then suddenly flip. Not so.
The next theory came closer: That when approaching the target ceiling, the flies do a half-barrel roll then let momentum roll their torso the rest of the way. They would extend their gummy front feet forward to grab the ceiling, and then momentum would swing the rest of the body and the other four feet ceiling-ward. In other words, the flies were believed to be half-rotating and letting physics and the glue on their little feet do the rest.
But not all observations supported this theory. Some seemed to do that, others didn't.
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Now, an intrepid team from Pennsylvania State University, the Tata Institute of Fundamental Research in Bangalore and Colorado State University has compiled what it claims to be “the most complete exploration of fly landing maneuvers” to date.
Bluebottles were filmed in a flight chamber using high-speed video. The insects’ body and wing motions were tracked using anatomical landmarks. Thus the scientists concluded that the insects were exhibiting a more complex series of behaviors than thought.
“These inverted landing maneuvers are among the most remarkable aerobatic feats,” they gush in their paper.
Stage one of landing on the ceiling is upward acceleration.
Stage two is that when their velocity reaches critical point, based on neurological signals from their eyes, they begin to rotate — which is stage three.
Stage four is that as they rotate, they fling their front legs forward and firmly plant their forefeet on the ceiling. Their filamentous hairy little feet are gummy on the bottom, so they stick.
Stage five is the finale: A leg-assisted body swing pivoted around the forefeet that were previously firmly attached to the ceiling (in stage four).
Now you can do it too, if you are a fly.
In other words, multiple systems and coordinated behavioral modules are involved — including their eyes.
Why has the mechanism of flies landing on the ceiling been so confusing until now? Because, the team explains, flies are capable of great variation in their pitch and roll rates. “When flying with higher forward or lower upward velocities, flies decrease the pitch rate but increase the degree of leg-assisted swing, thereby leveraging the transfer of body linear momentum,” they explain.
Bluebottle flies are capable of faster rotation relative to the flat surface than hummingbirds, or even our friend the fruit fly. On the other hand, the fruit fly is capable of achieving mind-blowing orgasms, Israeli scientists demonstrated in 2018. The fruit flies may argue they have the better deal.
Note that the paper describes the inverted landing behavior broken down into sequence, no more. The sensorimotor processes — how the fly’s brain and muscles interact — remain unknown.
So? The conclusion is that bluebottles seem to be utilizing sensory cues and neural processes to achieve inverted landings. So if you want to build robots that can fly “right side up” and land upside down with their feet sticking to the ceiling, we may need to formulate integrated computational processes and landing gear with gluey feet.
Happily, we’re halfway there. In late 2018, scientists at the Wyss Institute for Biologically Inspired Engineering at Harvard created a sort of robot gecko that could scuttle along vertical and upside-down conductive surfaces, assisted in rough terrain by ankle joints on the legs, which culminated in electroadhesive foot pads. The project was commissioned by none other than Rolls-Royce, which would very much like to have tiny little robots that can climb inside its massively complicated jet engines to perform inspection, and possibly even maintenance, without falling off. The experiment worked, at least in that the robot gecko didn’t fall.