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Longfellow of the grassy banks

REGULARLY during the day, hedge and house sparrows nesting nearby take dust baths. Mites must be driving them crazy. They push, run at and bully one another over the wallows they have burrowed and shaken out which pock-mark grassless patches. A blackbird pair arrives to take to the waters. No dust for them but much splashing and showering in an ancient cooking vessel and plastic containers placed here and there as drinkers.

One of three prowling local cats (two of which seem to roam night and day) appears as a slowly moving menace and has to be sent packing. This one wears a red bell on its neck. The other two regularly set off security lights at 4am. Memo to cat owners: please keep your pets confined until well after dawn.

So far in a garden wilderness there has been just a fleeting fluttering or two or a tiny common blue butterfly. But then a new book on these beautiful insects falls open to reveal an amazing Irish discovery. One of the species pieridae (whites and yellows) in Northern Ireland has been found to have a remarkable physical advantage over its fellows in England and Wales -- it has a larger penis. But the story is a more complicated.

In fact, two entomologists in Ulster found that Irish wood whites proved to be, on examination, the more well endowed lepidea reali (called after a French scientist, Pierre Real, working in the Pyrenees) rather than l. sinapis, which mostly inhabits the forest glades of England and Wales.

The discovery of this species here by Brian Nelson, of the Ulster Museum, and Maurice Hughes, of Butterfly Conservation, is revealed by Patrick Barkham, a Guardian journalist, in his book, The Butterfly Isles, a fascinating account of insect survival.

When the Frenchman found this particular species high in the mountains back in the Sixties he discovered it was singularly different to the traditional wood whites he had been netting in the foothills. The two types were not commingling as their genitalia were different.

This was France. But how did the longer fellow get here?

Lepidopterists had long thought there was a behavioural difference between the wood whites here and those across the Irish Sea. The English white is a dainty and delicate creature, weakly fluttering down woodland paths -- and its numbers are falling. Here, however, the whites appeared more vigorous and its populations have thrived.

Nelson decided to have a closer look, catching some specimens and giving them to Hughes to dissect. What was revealed was that these fellows were not British sinapis but French reali! Back then to Ulster Museum specimens of 1902. Viola, they were French also. It must be pointed out that traditional sinapis butterflies are also found in Ireland, especially in the Burren in Co Clare. But what has puzzled Barkham and others is why there are no realis in the UK and why they thrive here. But, then, there are other species of insects and plants that are unique to Ireland -- the Kerry slug, arbutus etc. Perhaps some insect eggs arrived on animal fodder being carried by Celtic migrants from the Iberian peninsula thousands of year ago. Entomologists are working on it.

The Butterfly Isles by Patrick Barkham: Granta, £20

Sunday Independent