Literature

Humans are used to the idea that we extract information from our environment, process it and determine an appropriate response. This is OK for those of us with brains that are large enough to do this, but these mental gymnastics are not going to suit creatures of small brain, like insects. Nevertheless, flying insects do not seem phased by this neural problem and they travel with ease over different terrains - rugged or smooth - with headwinds of varying intensities. It has been proposed that, instead of extracting information, insects utilise a visual-feedback loop linking flight with visual cues. "When insects are flying forward, the image of the ground sweeps backward across their ventral viewfield and forms an "optic flow", which depends on both the groundspeed and the groundheight." If a regulation system exists to stabilise the optic flow as perceived by the insect, it provides a means of controlling takeoff and landing, flight against wind, and flight over irregular terrains.
To test this idea, the researchers constructed a micro-helicopter equipped with an optic-flow regulator. The flying robot performed in ways that mirrored the distinctives of insect flight, and the authors, quite rightly, claim that it "sheds light on insect piloting abilities". They also claim numerous advantages for flying robot design, reducing the need for sensors and signal processing.
The authors add that the optic-flow system is "quite simple in terms of its neural implementation". Also that "its neural implementation is not very demanding". These comments do not seem to do justice to the need in flying insects for low weight with corresponding limited neuronal capabilities. Furthermore, the authors have only started to explore the control issues and the underlying neural circuits. They acknowledge themselves that there is some evidence for a second optic-flow regulator that "may be in charge of adjusting the groundspeed by regulating the lateral OF". In a commentary article, Webb identifies another factor: "Ventral optic flow can be easily detected if the animal is flying straight ahead and the sensor is pointing straight down. But if the insect pitches, rolls or rotates, ventral optic flow will be distorted. Can the animal measure and discount these movements, or are other sensorimotor loops, such as the optomotor reflex, deployed simultaneously to minimise them?"
What we have here is an interesting exercise in product development. A design brief could be written for a control system that enables lightweight flyers "to deal single handedly with all maneuvers, such as taking off, flying at a level height, landing, and responding appropriately to wind, without being informed about groundheight, groundspeed, airspeed, windspeed, or ascent or descent speed, and hence without any need for the metric sensors with which conventional aircraft are equipped." To achieve this, we would expect a substantial investment in intelligent design engineering. We recognise that the problem has many complex issues to resolve. Delivering a product that meets the design brief allows us to make inferences about intelligent agency.

A Bio-Inspired Flying Robot Sheds Light on Insect Piloting Abilities
Nicolas Franceschini, Franck Ruffier and Julien Serres
Current Biology, Vol 17, 329-335, 20 February 2007

Summary: When insects are flying forward, the image of the ground sweeps backward across their ventral viewfield and forms an "optic flow", which depends on both the groundspeed and the groundheight. To explain how these animals manage to avoid the ground by using this visual motion cue, we suggest that insect navigation hinges on a visual-feedback loop we have called the optic-flow regulator, which controls the vertical lift. To test this idea, we built a micro-helicopter equipped with an optic-flow regulator and a bio-inspired optic-flow sensor. This fly-by-sight micro-robot can perform exacting tasks such as take-off, level flight, and landing. Our control scheme accounts for many hitherto unexplained findings published during the last 70 years on insects' visually guided performances; for example, it accounts for the fact that honeybees descend in a headwind [1], land with a constant slope [2], and drown when travelling over mirror-smooth water [3]. Our control scheme explains how insects manage to fly safely without any of the instruments used onboard aircraft to measure the groundheight, groundspeed, and descent speed. An optic-flow regulator is quite simple in terms of its neural implementation and just as appropriate for insects as it would be for aircraft [4].

See also:
Insect Behaviour: Controlling Flight Altitude with Optic Flow
Barbara Webb
Current Biology, Vol 17, R124-R125, 20 February 2007

Summary: Insects can smoothly control their height while flying by adjusting lift to maintain a set-point in the ventral optic flow. The efficacy of this simple flight-control mechanism has been demonstrated using a robot helicopter.