Country diary: The unenviable life of the noon fly

Wolsingham, Weardale: These beautiful and important insects spend their formative days in semi-liquid bovine excrement

A month ago, we took a detour from this footpath, wide around a bull and his harem that were sheltering under the trees. The beasts have moved to pastures new, but they’ve left two legacies: lush tufts of new grass, fertilised by decaying cowpats, and scores of noon flies (Mesembrina meridiana). They are basking on fence posts in the afternoon sun on another unseasonably mild November day.

For an insect that spends its formative stages in semi-liquid bovine excrement, the noon fly is surprisingly beautiful, with its body of polished jet and wings like rippled glass, decorated with bright orange patches. Interesting too, because each will lay just five eggs, one per cowpat, releasing carnivorous grubs that prey on the larvae of dung flies and other coprophagous insects. Its life cycle, an integral part of agricultural grassland ecology, revolves around defecating cattle.

In these dying days of autumn, the flies are sharing their fence post perches with other sunbathing insects, including harlequin ladybirds (Harmonia axyridis), which are recent arrivals in the dale. Native to east Asia, they were used to control glasshouse aphids in the US – a biological pest control measure with regrettable consequences when they escaped and reached Britain in 2004. They are now well established, competing with native ladybirds and predating their eggs and larvae.

Harlequins come in coats of many colours and patterns, permutations on the theme of black spots on orange wing cases, or vice versa, plus an all-black morph, all of which may indicate an underlying reservoir of genetic variability that could be the key to future adaptation in our rapidly changing climate.

And today, on the brink of winter, here is another winning trait: a prolonged breeding season. Among the adults seeking hibernation sites in crevices in the old wooden posts, there are more harlequin larvae. One, tail anchored to a post, is ready to wriggle out of its spiny larval coat, secrete new orange and black armour, curl like a gaudy miniature Egyptian scarab seal, and pupate. The days are getting colder, but there may still be time to metamorphose into an adult and crawl into hibernation before winter tightens its grip.

• Country Diary is on Twitter at @gdncountrydiary