Harvestmen

Introduction

Harvestmen, or Opiliones to give them their scientific name, belong to the Arthropod Class Arachnida along with spiders, mites, scorpions and pseudoscorpions. Like the other arachnids, harvestmen have four pairs of legs. They also have a pair of pedipalps which they use for catching and manipulating prey and a pair of pincer-like chelicerae. Harvestmen look similar to spiders but differ in several ways. The most obvious is the single ovoid fused body of a harvestman compared to the divided body of a spider with a constricted ‘waist’ separating the cephalothorax and abdomen.

Simplified drawing of a harvestman (Oligolphus tridens). Note that leg two is omitted to better show other features.

Unlike spiders, harvestmen have no poison glands, nor do they produce silk. While other arachnids mate by the male transferring a sperm packet to the female, harvestmen are unique amongst arachnids in that the male inseminates the female internally via a penis.

Although harvestmen lack poison, they are by no means defenceless as they can emit noxious chemicals from their odiferous glands to deter predators. Harvestmen are best described as omnivorous; they will actively catch small invertebrates but also scavenge food and have been found on animal corpses and even eating bird droppings.

Mitostoma chrysomelas (juvenile), Odiellus spinosus, Opilio canestrinii (juvenile).

Identification

The standard work for identifying harvestmen in Britain is by P.D. Hillyard (2005) but it is out of print. The only currently available guide is the FSC fold-out chart by Paul Richards.

Of the thirty-three species of harvestman that have been recorded in the UK, twenty-four have been found in Norfolk. My Harvestmen Guide produced for the Norfolk and Norwich Naturalists’ Society covers these species and contains identification tips, photographs and provisional distribution maps.

I have also produced a Concise Guide to Norfolk Harvestmen which can be used in the field on a smart phone and a number of crib-sheets for similar species that can be difficult to distinguish from one another.

Current crib-sheets cover Mitopus morio / Phalangium oplio and Dicranopalpus ramosus agg.

Phalangium opilio male, Rilaena triangularis (with woodlouse), Paroligolophus agrestis, Nemastoma bimaculatum.

To read more about harvestmen in general see my blog post “H is for Harvestmen”. Further information can be found on the British Arachnological Society website.