Negative Feedback and How a Fly Flies

May 25, 2014

Stationary-Fly-P1410114

Photographer: Mario Freitas
Summary Author: Mario Freitas, Federal University of Technology of Parana, Brazil

When the solar disk was still 14 degrees below the southeastern horizon in Curitiba, Brazil, this unidentified species of fly hovered absolutely still, thanks to Newton's second law and its applications to equilibrium theory and self-regulating systems. Although such issues are more frequently discussed in mechanical and electronic engineering, the same principles are clearly noticeable in living organisms. The condition for the fly's center of mass undergoes no acceleration and so it remains stationary in flight. Thus, the resulting force on it must be null. The primary forces here are due to Earth's gravity (downward) and buoyancy and lift (upward), both forces having their origin in the atmosphere -- the first due to the volume that’s displaced by the fly's body, and the second induced by its beating wings. A soft wind can decrease pressure equilibrium and induce a new force. In such a situation, the fly controls its position by increasing or decreasing its beating frequency. This is an example of a system that applies exactly the right amount of correction in order to minimize changes, providing overall stability.

I hoped to be able to take a second shot of this fly with a camera flash in order to visualize its structural details, but it suddenly fired some surprising propeller mechanism and was in an instant shifted to a new stationary position, visible to my eyes, but unfortunately too far for being photographed. Shown in the inset photgraph is a slightly different fly species that alighted on a nearby flower. Picture taken on February 11, 2014.

Photo details: Camera Maker: Panasonic; Camera Model: DMC-LX5; Focal Length: 19.2mm (35mm equivalent: 176mm); Aperture: f/3.3; Exposure Time: 0.0080 s (1/125); ISO equiv: 80.