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Thursday 13 December 2018

Harvestman a relative of spiders, but distinct species

A harvestman, its tiny fat body suspended by extremely long, gangly legs
A harvestman, its tiny fat body suspended by extremely long, gangly legs

Jim Hurley - Nature Trail

You have surely noticed harvestmen over recent months? So called because they are most abundant and obvious at harvest-time in late summer and early autumn, they are commonly seen resting on walls and vegetation by day, their tiny fat bodies suspended by long gangly legs like a Daddy-long-legs Fly.

The Daddy-long-legs Fly is an insect with three body parts, two pairs of wings and three pairs of legs. Close examination of a harvestman reveals that it has only one body part, no wings and four pairs of legs so it is certainly not an insect.

Spiders have no wings, four pairs of legs and eight eyes, but it is not a spider either. Most spiders have silk glands and spin webs; harvestmen lack silk glands and consequently cannot spin webs.

Spiders have bodies divided into two segments or parts: a combined head and chest and a separate stomach or abdomen; harvestmen have head, chest and stomach all fused into one.

Most spiders have eight eyes; most harvestmen have only two eyes.

So, though closely related to spiders, harvestmen are a discrete and distinct group of life forms. Seventeen species have been recorded in Ireland. They have no English names; they are all lumped together under the umbrella term 'harvestmen'.

Leiobunum rotundum, our commonest harvestman, pictured above, has a plump chestnut-brown, oval body and extremely long skinny, black legs. It is nocturnal so the ones we see by day are resting. At night they use their pincer-like jaws to hunt live prey like caterpillars, mites, woodlice, and slugs. They also scavenge decaying plants, fungi and animal matter. They drink from dew drops or suck sweet juice from bruised fruit and windfall apples.

The black blob at the front of the body of the harvestman pictured above is the creature's two eyes. They are jet black in colour and are set very closely together on top of its head giving good all-round views for hunting as it stalks with its body elevated on its stilt-like legs.

Birds eat harvestmen, so the hunter often becomes the hunted. But harvestmen have evolved an adaptation, or, if you will, a clever trick, to cope with predation. If one is caught by the leg by a bird and is about to be consumed it can self-amputate its leg and run away on the remaining seven leaving the startled bird holding a lone skinny leg with very little meat on it.

Wexford People

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