ISOPODA | Woodlice

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ISOPODA is one of the major orders within the taxonomic class MALACOSTRACA, which is the second largest of the six divisions that make up the Arthropod subphylum CRUSTACEA. The malacostracans are a very diverse group of species including crabs, lobsters, crayfish, shrimps and prawns, as well as the aquatic and terrestrial woodlice. The group as a whole are primarily marine creatures with around 40,000 identified species within 16 orders. My particular interest is with the isopods within the suborder Oniscidea collectively known as woodlice, although the two semi-terrestrial species are commonly known as slaters. A further suborder, Asellota is also relevant as it contains four species of aquatic waterlice.

It might come as a surprise to learn that there are over 60 species of woodlice on the British List, although quite a number of these are non-native and, for most purposes, can be ignored. Most of the literature I've referred to consider around 37 species to be native or naturalised, but that's still an impressive number given that many people would think that a woodlouse is just a woodlouse without appreciating that there are different varieties. These species are taxonomically split into three groups, known as sections; Diplocheta containing the two species of slater, and Synocheta and Crinocheta. Broadly speaking, the Synocheta includes the soil-dwelling pygmy woodlice, and Crinocheta the larger, more evolved and more familiar species.

Woodlice are characterised by having seven pairs of more-or-less identical legs and a somewhat flattened body consisting of twelve segments. The body consists of a relatively small sunken head, the pereion or pereon (thorax) which is composed of the seven leg-bearing segments, and the pleon (abdomen) made up of five pleonites terminating in the telson (sometimes referred to as the tail) and the uropods which are a pair of appendages on either side of the telson that vary in form and shape between species. The general body shape is a specific identification feature to separate species as some have a continuous or smooth outline where the pereion meets the pleon, whereas in other species the leg-less pleon segments are noticeably narrower resulting in a stepped outline. Another important feature is the number and shape of the segments forming the flagellum at the end of the antennae. For example, if the flagellum is constructed of two or three distinct segments, rather than looking like a tapered cone with a terminal brush, it would signify that the species belongs to the Crinocheta rather than the Synocheta. With the pygmy woodlice, the eye or lack of, together with the body sculpture can help the identification process, at least to genus, but there will be species that cannot be separated without detailed examination.

Interestingly, most of the species that you're likely to encounter have been given common names, which is rather unusual as I believe there are only two other invertebrate groups where the majority of species have been named and that's Lepidoptera (butterflies and moths) and Orthoptera (grasshoppers and crickets). I'm not sure why woodlice have been given common names when comparable species such as centipedes and millipedes, don't have them, but can only assume that it was to heighten awareness and general interest for casual observers who relate better to vernacular names they can remember.

On the subject of common names, there are five British species that have long been regarded as the most frequent and widespread and they are the Common Pygmy Woodlouse [Trichoniscus pusillus agg.], Shiny Woodlouse [Oniscus asellus], Striped Woodlouse [Philoscia muscorum], Rough Woodlouse [Porcellio scaber] and the Common Pill Woodlouse [Armadillidium vulgare]. They are known by isopodologists as the 'famous five', a phrase that I believe was used by Stephen Hopkin as far back as the late 1980's when he was working on the first edition of the illustrated 'key' that we still use today. I have to say though, that whilst these species may be regarded as common, it took me quite a while to find and photograph all five, so I can only guess at how long it's going to take to add more of the less common species to this list.