Allan Winn/LONDON

According to the laws of mathematics and physics, bees cannot fly. Millions of bees every day prove, of course, that either this theory or the laws of physics are wrong: according to apiarist Rex Boys, it is the theory which fails the test. The physics are sound: it is just that the scientists have not spotted how they work - until now.

Indeed, the US Defense Advanced Research Projects Agency is studying the flight of insects like the bee in the development of a micro air vehicles to provide battlefield reconnaissance.

The key to understanding how the bee flies, according to Boys, lies in pioneering work undertaken by Eddie Woods, a broadcasting engineer with the BBC. After the Second World War, Woods began recording and analysing the sounds made by bees. He examined the frequency of bee wing beats (250Hz for a worker and 190Hz for a drone), by comparing the sound of a flying bee with the notes of a piano. This investigation led Woods to the conclusion that the wing beats of a bee alone were not enough to generate the intense buzzing noise it generates while flying.

Having deduced that only around 1% of the energy expended by the bee in beating its wings is translated into sound energy, Woods decided that the sound must be coming from a pulsed airflow, such as is developed by the reed in the mouthpiece of a musical wind instruments, or by the player's lips on a brass instrument. A pulsed air stream created by the regular interrupting of an airflow is characterised by a series of compressions and rarefactions of that airflow, as the momentum of the flow creates alternate zones of high and low pressure on each side of the valve or shutter which does the interrupting. In the case of the bee, the secret lies in its peculiar breathing system.

The bee's equivalent of the human mouth/nose/lung system, says Boys, is a group of 14 breathing tubes, or tracheae, along its body - two each side of the thorax and five each side of the abdomen. (The bee's mouth is used solely for eating.) Air passes in and out of these racheae through holes known as spiracles. Just inside each spiracle is a valve, which opens and closes like a trumpeter's lips, to produce the loud buzzing noise (equivalent to the musical note middle B at 250Hz).

The buzzing noise is at the same frequency as that of the wing beats because the spiracle valves are controlled through the nervous system of the bee by its wing motor, so that the sound is synchronised with those wing beats. That might explain the noise of the bee, but Boys says there may be much more to it.

That answer lies in questioning why any aircraft makes a noise - which is, simply, that it is an unavoidable consequence of the method used for propelling it. If aircraft could be made silent, they would be - because no energy would be wasted in generating useless noise.

Boys has his own theory: "Sound consists of alternate pulses of compressed and rarefied air. On the bee, these emerge from 14 little holes ranged along the thorax and abdomen underneath the wings; they are also locked in frequency to the wings. It would be a simple matter to phase them so that the compression pulse coincides with the downbeat of the wing. This would generate extra lift and improve its flight characteristics. That, I believe, is the reason for it. Whether or not the bee or anyone else can hear the buzzing is of no consequence: the sound is simply a by-product of the aerodynamics." To those who would argue that the benefit of a puff of compressed air under the wing on the downbeat would be negated by the effect of the rarefied air on the upbeat, Boys points out that the wing is then feathered, and would be relatively unaffected by the change in pressure under it.

He concedes that this is only a theory, but one that deserves further investigation. One area of study that he suggests is to look at other insects that buzz loudly, to see if they, too, have under-wing spiracles driven by their wing motors; whether they, like bees, have two pairs of wings; whether, like bees, they have small wings for their body-sizes. Whatever the outcome, however, he comes back to the point that the bee still flies - whatever the mathematicians say.

Source: Flight International