ARACHNIDA

ARACHNIDA is a high-ranking classification, known as a 'class', that taxonomically sits alongside INSECTA as one of the major subdivisions of ARTHROPODA. It comprises various groups of eight-legged, mainly terrestrial, invertebrates.

My primary interest is with OPILIONES (harvestmen) as featured here and ARANEAE (spiders) that have their own page, but ARACHNIDA also includes two further groups that I want to cover at some point. They are the ACARI, now regarded as a subclass, consisting the ACARIFORMES (mites) and PARASITIFORMES (ticks) and hopefully, if I can find some, the PSEUDOSCORPIONES (false scorpions). These tiny, primarily soil and leaf-litter dwelling creatures will be looked at in some form within the final section of the website.

OPILIONES (Harvestmen)

Although superficially similar to some long-legged spiders, harvestmen are not that closely related, which is why they're placed within their own order OPILIONES.

There are a number of fundamental differences between spiders and harvestmen, the most obvious being the shape and design of their bodies. As previously described, spiders have a two-part body consisting of the cephalothorax (combined head and thorax) and the abdomen, which are separated by a clearly visible constriction that could be regarded as a 'waist'. Harvestmen, on the other hand, have a single mass, fused body, where the cephalothorax and abdomen are merged together. And, instead of having six or eight eyes as spiders do, harvestmen have but a single pair, which are orientated sideways and, with most species, raised above the body on a prominent turret called the ocularium, which sits centrally on top of the cephalothorax.

Body size varies from just under 3mm to around 10mm and, contrary to the assumption that all harvestmen have long legs, some do not. In fact, the relative leg length is an important part of the identification 'key', with species described as having either short, medium, long or very long legs. The second pair are always the longest and are used as sensory organs. Other distinguishing features include the patterning on the dorsal surface, known as the saddle; the appearance of the aforementioned ocularium; and the shape and/or armourment of the pedipalps and pincer-like chelicerae. In addition, the front of the cephalothorax may have three long pointed tubercles, known as the trident, forming a prominent fork or spear over the chelicerae; the angle and relative length of which are diagnostic.

Most species are omnivorous, eating both small soft-bodied arthropods such as springtails and aphids, as well as a variety of vegetable matter. They do not produce silk like spiders, so if one is seen in a web, it has obviously become entangled or trapped as prey. In this, or similar situations, it is not unusual for harvestmen to shed a leg, or two, as a means of escape. They may have eight legs by design, but can cope perfectly well with seven, six or even less. In fact, you probably see more individuals with lost limbs than you do with the full quota. Another not common sight is to find them infested with red mites that attach themselves to suck body fluids. They are typically nocturnal, but can be found during the day under stones or logs, or other places offering shelter such as on walls or the side of houses under soffits or windowsills. Although most species are only active during the summer and autumn months, there are a few that may be encountered during the winter.

There are around 30 species of Opiliones in Britain separated into various families and/or subfamilies. My current tally is around a dozen as represented by the following thumbnail photos, which each link through to a 'species account' and enlargeable photos. In this respect, it's worth noting that whilst some harvestmen can be very cooperative when faced with a camera others will instantly react and quickly disappear into the undergrowth. They're also tricky due to their small body size and profusion of legs that with some of the long-legged species always seem to be in the most awkward position. And, if that's not difficult enough, you rarely seem to find them in a photogenic setting, as they're invariably amongst vegetation or, in the case of many I find at home, attached to a painted rendered wall.

I've tried to add as much detail as possible to each 'species account' because, unlike spiders, there are no identification books for harvestmen. There's a website linked to a recording scheme and associated FB Group but, in my opinion, the information is distinctly geared to keying out these species rather than providing good diagrams and comprehensive descriptions that can be used by casual observers and photographers. Some in-depth internet searching allowed me to download copies of a couple of out-of-print publications, including an excellent Dutch guide that I could use with an English translation. Trying to pick out all the pertinent bits of information proved quite difficult, but I hope that my interpretation is correct as it's certainly extremely useful having as much detail as possible in one place.

Nemastomatidae