The unswattables: How are flies so adept at evasive action?

As the troubled planet continues to spin, I have been trivially distracted by the attentions of a personal fly.

It is smaller than the usual run of Muscus domesticus, polished, dark, dainty and light on its feet, and it arrives when I am reading in bed, drawn to the pool of light and the gleam of a thriller by Ian Rankin.

It happily prospects the corner of the page, rubbing front legs together the better to savour the paper, but soon dances on to my hand, to pause on the helicopter-pad of my thumbnail. Prudently, apparently not wishing to annoy, it leaves the rest of me alone.

It has thus earned a measure of tolerance unusual in my dealings with domestic flies, not least as one of the few flying insects to come to my notice this summer. A lone and sinister cleg, some may remember, was quickly flicked from my wrist a week or two ago, whereas the gentle footwork of this little fly is singularly void of threat.

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This is not to say that, on first acquaintance, I did not try to swat it, or slam the pages of Rankin smartly together while pretending to be thinking of something else. On each occasion, of course, I signally failed.

This reminded me of a most happy acquaintance with another settler in the west, a former British ambassador to Moscow, who retired with his wife to a cottage and vegetable patch at the foot of the mountain.

On one of many affable evenings at their fireside, he spoke of a less than diplomatic skill perfected during attendances at over-tedious conferences. He developed the art of sneaking up on a fly with a deadly cupped hand – a manoeuvre of which he was whimsically proud. It rested, as I remember, on shrewd guesswork as to where the fly would leap next: good training for any ambassador.

Flashy flies

All this has sent me in search of the science of unswattable flies. Much has depended on the laboratory research of Dr Roger Hardie, emeritus professor of cellular neuroscience at the University of Cambridge.

It has revealed surprising insights into the different perceptions of passing time that arise from differences in the “flicker fusion rate” of images processed in animal eyes and brains.

Humans, for example, will process the projection of 25 frames of film per second into a smooth, continuous reality on the screen.

Our normal “flicker fusion rate” averages about 60 flashes per second, but flies piece together 250 flashes or more. This slows down the perceived movement of an approaching hand or rolled-up magazine. For humans to achieve the slow-motion sequences in some nature films on TV, frames are shot at a speeded-up rate, then played back at normal speed.

In an experiment to measure the flicker fusion rate of notoriously agile (and very small) fruit flies, as reported by the BBC, Prof Hardie inserted glass electrodes into the light-sensitive cells of their eyes and flashed LEDs at them at ever-increasing speeds. They recorded separate flickers at up to 400 times per second, more than six times the human rate.

Like fruit flies, house flies come equipped with hind wings that have evolved into specialised organs called halteres. To stop the flies bungling an over-rapid take-off, these act as stabilisers or gyroscopes to steady them into flight.

Falcon eyes

Ultra-rapid vision is not confined to flies; it is also shared by many birds that need to track and anticipate the movements of their prey. One of these is the spotted flycatcher, which has dropped in briefly to our acre, but the fastest visual speed is reportedly that of the peregrine falcon, at some 130 frames per second, or twice the human visual acuity.

My recent lament on the absence of butterflies from my flowering and fragrant buddleia, the "butterfly bush", has prompted lepidopterist Jesmond Harding to a lucid and expert explanation on the Butterfly Conservation Ireland website.

It seems that calendar attendance at the buddleia for a nectar feed, by species such as the pretty lesser tortoiseshell, is governed by life-cycle priorities and the length of day. At the time of my daily visits to the bush, during the heatwave, the tortoiseshells would have been headed instead for the nearest clump of nettles with egg-laying reproduction in mind.

I warmly recommend his post. And I can add the eventual presence at the buddleia of two welcome red admirals. One was somewhat worn, perhaps from overwintering in Ireland. Climate change has warmed our winters – a shame we couldn’t have left it at that.