On one of those mornings with a sky full of sun, the ocean flat and aquamarine right out to the islands, I kept my usual summer appointment with a hoverfly at the regular point of its choosing. This was at about a metre above the garden path and a good bit south of the holly bush in the hedge. (No doubt its own aerial co-ordinates were more precise.)
I stood, as always, reverent and mesmerised by the gleaming little point in the air. I awaited for its sudden, sideways, bilocating shift to another somewhere – then, perhaps, back again to where it was, or thereabouts. Having shown off for a while, it was suddenly not there any more.
There should be a special name for what the hoverfly does so instantly and geometrically. I thought I’d found it in a research paper from the University of Sussex. “Saccades” sounded likely, but, no, those are the insect’s sharp, sideways movements of the eyes, fixating its targets, like a taoiseach checking around the cabinet table. On the other hand such fixation helps the insect zero in to its targets whatever the air turbulence, like the “steering by gazing” mechanism of some aerial robots.
What's being tracked are other hoverflies, often with sex in mind. "The male rapes flies of either sex," say the researchers, "indicating that successful copulation involves more trial and error than recognition." Such revelations (in Visual Control of Flight Behaviour in the Hoverfly, Syritta pipiens L, by T S Collett and M F Land) sprang from early frame-by-frame study of laboratory film (of captured flies in a big Perspex box with a beaker of daisies to visit). But as the hoverflies' vital role in pollination has taken on such importance, along with that of bees and wasps, it's study in the field that counts as much or more.
The website of the Irish Pollinator Initiative (at biodiversityireland.ie) shows the often jewel-bright livery of Ireland's 180 species of syrphids (as hoverflies are known scientifically). They belong among the 700 or so European species compiled on the remarkable database – called, neatly, Syrph the Net – compiled over 20 years by Dr Martin Speight, long the leading entomologist of the National Parks and Wildlife Service and now with Trinity College Dublin's zoology department. Its big value lies in predicting what hoverflies to look for in a particular habitat, such as woodland or wet grassland, and for their larvae in microhabitats within them, such as ponds or stands of particular plants. This makes the search a good measure of the more general health of an area's biodiversity.
Some hoverflies are anthropophobic, living aloof in natural broadleaf forests rather than conifer plantations, cultivated farmland and hedgerows or anywhere else that man hath wrought. Fortunately, many more are anthropophilic, or farmer-friendly. The adult females need flower pollen as protein for their eggs, and their larvae eat hundreds of aphids, both to the great benefit of many farm crops.
Among the commonest is the so-called marmalade fly, Episyrphus balteatus, one of many Irish species with bodies striped in black and yellow in hope of protective confusion with the wasps. An adult female can lay up to 4,500 eggs in her lifetime (see the species account at biodiversityireland.ie), so the potential toll on sap-sucking aphids could be huge. Unfortunately, this makes the hatched-out larvae totally vulnerable to insecticides used for aphid control.
Episyrphus balteatus may well have been the species of my one-to-one encounter, but Ireland's ubiquitous residents are added to in great numbers by swarms of migrants from Europe arriving in July and early August. The first record of this came in 1995, when two biologists in a boat off Courtmacsherry Bay, in Co Cork, saw skeins of hoverflies heading for the coast in company with clouds of red admirals. The flies were so numerous that dead ones were forming drift lines on the sea. An arresting match for this was an observation made by the great English ornithologist David Lack, who, with his wife, climbed to a high and extremely narrow pass in the Pyrenees. He was checking a theory that Europe's migrating small birds might find some mountains too hard to cross.
The theory was wrong. The Lacks counted hundreds of finches, linnets and other small birds skimming the pass southwards back to Spain. With them came butterflies – clouded yellows and red admirals – and dragonflies at the rate of several thousand an hour. At first they didn't notice the hoverflies, tiny black-and-yellow ones, pressing on at ankle height into the wind. What caught their eye at last was "a shimmer of iridescent light, due to the reflection of the autumn sun on myriad tiny wings". The species, needless to say, was Episyrphus balteatus.
Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks