ISOPODA | Woodlice

ISOPODA is one of the primary orders that make up the taxonomic class MALACOSTRACA, which in turn is the largest division of 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 or so orders. My main interest here is with the isopods of the suborder Oniscidea which are collectively known as woodlice, although there are two semi-terrestrial coastal species that are more commonly referenced as slaters. A further suborder, Asellota is also relevant as it contains five species of aquatic waterlice.

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It may come as a surprise to learn that apart from the slaters and waterlice noted above, there are over 60 species of woodlice on the British List. However, many are regarded as non-native or introduced species that have only been found in garden centres and hothouses, and once these are extracted the true number of native or naturalised terrestrial species is reduced to around 40, but that's still a good number of species given that many people think that a woodlouse is just a woodlouse without appreciating that there are different varieties.

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They are taxonomically separated 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.

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Woodlice are characterised by having seven pairs of more-or-less identical legs and a somewhat flattened body that consists of twelve segments. The body comprises 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. The colour, which can be quite variable in some woodlice, together with the texture of the body are discussed within the individual species accounts.

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Other important features include the number and shape of the segments forming the flagellum at the end of the antennae. For example, where 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. And, with the pygmy woodlice, the eye or lack of, together with the body sculpture are also specific characters that need to be checked in order to be sure of the genus.

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In nearly all cases, species can be distinguished with reasonable certainty from photos for most recording purposes, but there are some where close examination would be required for positive identification.

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Interestingly, most of the woodlice you're likely to encounter have been given common names, which is rather unusual as 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 the same treatment when comparable species such as centipedes and millipedes, don't have them, but can only assume that it was done to heighten awareness and general interest for casual observers who relate better to vernacular names they can remember.

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On the subject of common names it would be remiss of me not to mention that there are five British species that have long been regarded as the most frequent and widespread; they are the Common Pygmy Woodlouse [Trichoniscus pusillus], Shiny Woodlouse [Oniscus asellus], Striped Woodlouse [Philoscia muscorum], Rough Woodlouse [Porcellio scaber] and the Common Pill Woodlouse [Armadillidium vulgare]. They used to be known as 'the famous five', a term first 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. However, whilst these species may be regarded as common, I have to say that it took me quite a while to find and photograph all five, as they're not found everywhere. And it's exactly the same with all the other species, because if you're not looking in the right area or habitat, you're simply not going to find them. This became increasingly apparent with two particular species I wanted to photograph before updating this section that I just couldn't find locally, but during a day exploring a couple of disused quarries near the coast on Portland I managed to see both, plus another new species that I would be unlikely to find closer to home.

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Many woodlice are rare or localised, so once you start looking more closely at the distribution records you find that the actual number that could be found locally is reduced. At a push, including a few scarce species, I have a possible target list of 28, although realistically it's more like 20. To date I've found 16 as featured below.

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