Industry, commerce and the military are all interested in developing 'micro-air vehicles' (MAVs), tiny aircraft for reconnaissance inside buildings and other confined spaces. Lateral-thinking designers, says Charles Ellington of the University of Cambridge, UK, will take a leaf out of nature's book: insects are great little MAVs, perfected by an R&D programme stretching back 350 million years.

In the Journal of Experimental Biology1, Ellington looks at the principles of insect flight that could be emulated by MAV designers. The first thing engineers will have to do is throw away their textbooks – "insects cannot fly according to the conventional laws of aerodynamics", says Ellington. By the usual rules of aerodynamics, the tiny wings of bumble-bees would should never get them off the ground – let alone allow them to be the exquisitely manoeuvrable aeronauts they evidently are.

Insects get around their handicaps in two ways. First, they exert precise control over their attitude in flight, minutely changing the profile and shape of their wings and bodies from moment to moment according to their circumstances. MAV design will have to abandon fixed wings and incorporate this concept of flexible, 'intelligent' aerofoils.

Second, insects use what Ellington calls 'unsteady high-lift mechanisms' – tricks to generate more lift than you might expect from conventional aerodynamics. One such trick is found in the very tiniest insects, with wingspans of the order of a millimetre or so, to which the air seems much more viscous than it does to us – more like water than air. This has lead some to suggest that such insects might abandon aerodynamics altogether and swim, rather than fly through the air.

More recent study shows, in contrast, that tiny insects use lift in an ingenious way. The wasp Encarsia formosa, for example, beats its minute wings (spanning 1.5 mm) 400 times a second. Their wing motion is similar to that of most insects except that at the top of the upstroke, Encarsia formosa's wings clap together and are then flung apart. Air moving into the vacuum created by the 'fling' sets up vortices circulating around the wings that increase lift more than you might expect from the shapes of the wings alone. A MAV that clapped its wings together hundreds of times a second, however, might soon bash itself to pieces.

Larger insects employ what is called 'dynamic stall': their mode of flight also generates vortices of air around the wing margins, and hence lift, so that the insects get caught up in their own slipstreams. But this mode of flight is inherently unstable, and insects must constantly manoeuvre themselves out of the stall before gravity forces them to earth.

So what can engineers learn from insects? The first insect-inspired MAVs, thinks Ellington, will be able to independently adjust the flapping rate and angle of each wing. Initially, such 'intelligent' wings will be like tiny sails, with a stiff leading edge supporting a membrane, additionally supported by a boom at the base.

A primary consideration in any design study will be payload mass. For any given payload, there is a trade-off between wing length and flapping rate. MAVs with short, fast-flapping wings would be able to fly much faster than MAVs with longer, slower wings – but at the price of greatly increased power consumption. For example, a MAV with 10-mm wings flapping 200 times per second drains ten times as much power as a MAV with 100-mm wings flapping twice a second and supporting the same mass of 200 mg – but it will fly ten times as fast.