Friday, 16 September 2011

A festival of harvestmen

Scrabbling around on your hands and knees may not be the most dignified of occupations but it does mean that you can get down amongst the vegetation, a useful thing when looking for invertebrates. This morning I took the opportunity to search the grounds at work for woodlice and harvestmen, two groups for which there are a few species we have yet to find this year. The woodlice have been fairly well covered, thanks to a concerted effort early in the year, but there are additional species that I suspect may well be present in the BTO’s Nunnery grounds. So far, we have found three different species of harvestman which, based on a Norfolk fauna of 18 or so species, leaves others unfound.

Harvestmen are probably familiar enough to most readers. The combination of a small round body and long legs hints at the relationship between harvestmen and the closely related spiders and mites. These are arachnids but not ones to be frightened of. Writing in 1977 Savory decribed them as ‘the comedians among Arachnida: animals with rotund bodies ornamented with little spikes, with two eyes perched atop, back to back, like two faces of a clocktower, with ungainly legs insecurely attached, with feeble jaws and an undying thirst…’ Savory’s description is spot on, though to really appreciate this you have to view a harvestman through a handlens or binocular microscope.

Odiellus spinosus

As well as being interesting creatures, harvestmen have an interesting name and one that deserves further mention. There is a clear association with agriculture in the name, and a seasonal timing as well. Although many can be found year-round and others are most commonly encountered in spring, they are most abundant in late summer – harvest time. While we call them harvestmen the French refer to them as les faucheurs (the reapers). Writing in 1634, Dr Thomas Mouffett knew them as ‘shepherd spiders’, explaining that English shepherds thought that a field with lots of harvestmen made excellent sheep pasture. The association with sheep is still retained in the nomenclature used in harvestman classification. The order in which harvestmen are placed for classification is called ‘Opiliones’ and has its origins in the Latin ‘opilio’ which means shepherd.

Harvestmen are predators, feeding on a wide range of small, often soft-bodied, invertebrates. They tend to live on the ground, particularly the more robust species with their shorter and stouter legs, but some may be found on brambles and nettles. The species I was searching for this morning, successfully as it happens, was one of the stout ground-dwelling species, hence my being on my hands and knees and searching through the debris at the base of an old wall.

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