Mighty clever flyers

A research team that describes its recent work as the “most complete exploration of fly landing manoeuvres” to date has learned that the common blue bottle fly (Calliphora vomitoria) is a much fancier flyer than we’d imagined.

High-speed video recording of an example pitch-dominated landing. A fly lands upside down using a pitch dominated body manoeuvre. Credit: Bo Cheng and Pan Liu, The Pennsylvania State University

The study, published in the journal Science Advances, focused on one of the least understood aerobatic manoeuvres performed by flying insects: an upside-down (inverted) landing.

The phenomenon of flies and other insects landing on ceilings is so common that casual observers barely notice, but the researchers discovered that blue bottle flies use an incredibly complex series of behaviours to achieve the feat.

Some previous research suggested that flies may not need to actively adjust their body orientation immediately before touchdown, while other observations suggested that rapid body rotational manoeuvres, controlled by an insect’s sensory processes, are critical to inverted landing.

To take things further, Pan Liu, from the Department of Mechanical Engineering at Pennsylvania State University, US, and his team investigated the landing behaviours of blue bottle flies in a small (200 millimetre cube) flight chamber. 

They used high-speed videography – their cameras operating at 5000 frames per second – to reveal the geometry of the flies’ body and wing motion during landing. 

They found that a blue bottle fly lands on a ceiling using a sequence of four distinct behaviours: first an upward acceleration, followed by a rapid body rotational manoeuvre and leg extension, and finally a leg-assisted body swing with forelegs firmly planted on the ceiling, which oriented the fly’s body to inverted.

The final part of the sequence relies heavily on adhesion from cushion-like pads on the flies’ feet (called pulvilli), which ensure a firm grip, and the elasticity of the compliant leg joints, which soften impact upon contact.

Incredibly, the process from the start of the body rotation to a successful inverted landing took approximately four to eight wingbeats.

This complex procedure contrasts to the landing behaviour of fruit flies, which barely rotate their bodies before touching down on vertical surfaces.

The team believes its results will inform efforts to engineer small robotic fliers to perform similar aerobatic feats. Results could also be applied to neuroscience, where they could lead to new hypotheses for understanding how insect brains function.

The study suggests that blue bottle flies use neural processes in concert with other sensory cues for inverted landings, meaning small robotic fliers seeking to make successful upside-down landings would require integrated computational processes and landing gear to achieve this feat.