Harvestmen Arachnida: Opiliones

A British Naturalists’ Association Guide By Roger Tabor

Harvestmen are sometimes called Harvest Spiders, but they are not spiders even though they are closely related in the Arachnida family, for Harvestmen are in the Opiliones order, whilst spiders are in the Araneae order, and mites are in the Acari.

The most noticeable difference to the general field naturalist is that whilst spiders have a waist dividing the head and thorax from the abdomen, the body of the Harvestman looks just like one long oval blob. However, neither the head nor abdomen is missing, just the waist! Harvestmen have very long legs. There are 22 recognised species in the British Isles, of these (according to J. Sankey and T. Savary) 14 species are widespread and may even be in all counties.

They mainly hunt at night, but can be found sunbathing in the day. 90% of their activity takes place between 6pm - 6am. They have been found to eat a range of small invertebrates from springtails up to even earthworms. Characteristically they will surround smaller prey with a ring of legs and then pile-drive their body down onto the victim!

In turn Harvestmen are eaten by toads, frogs and birds, and occasionally spiders (although they generally find Harvestmen distasteful!) Harvestmen can produce a noxious liquid from their cephalothoraxes (their combined head/thorax), and this may be what deters most spiders, and has been found to deter ants. Analysis of their protective liquid from Harvestmen has found it to contain methyl-quinones. (In general as a group of compounds quinones are toxic and irritant).

They usually lay eggs in the Autumn which hatch in the Spring, when they hatch as nymphs. Ecdysis occurs up to eight times, allowing them to grow. Harvestmen normally live for up to a year.

Harvestmen are usually found in entomological sweeps or brash ‘beating’, and are readily brought out of leaf litter by a Tullgren funnel. They can also be found by careful looking at trees etc.

Some species are more likely to be found in the leaf litter such as Homalenotus quadridentatus, which can be identified by 4 spines on the end of its body/back (mainly recorded for Southern Britain on chalky soils). Trogulus tricarinatus, (also Southern Britain on chalky soils), and Nemastoma bimmaculatum (which has been found virtually everywhere in Britain!) Some are found on herbaceous plants, such as Platybunus triangularis, (widespread across England, Wales and Ireland and has been found across about half of Scotland), and Leiobunum rotundum that gains its species name from its small round body (it also has a very similar distribution). Others can be found on trees and walls of houses, such as Oliogolophus agrestis (this also has a very similar widespread distribution).

© Roger Tabor 2006