Oniscus asellus

CRUSTACEA > MALACOSTRACA > ISOPODA > Oniscidea > Crinocheta > Oniscidae

Oniscus asellus is the most abundant and frequently recorded woodlouse in Britain. It's also one of the largest species with mature specimens reaching up to 16-18mm in length. The body has a continuous outline, and is either gently curved or, probably more often, with the epimera - being the lower edges of the pereonites - noticeably splayed out giving those individuals a wider and very much flattened appearance. The colour is typically grey-brown with a shiny texture, together with a band of paler markings along the flanks and usually with some yellow flecks or patches along the back. In darker specimens the markings are usually well-defined, but it's not uncommon to find pale individuals where they are almost lost. Occasionally yellow or orange forms may be seen, but they are unusual.

The eyes are composed of numerous ocelli, and the antennal flagellum each have three distinct segments. This woodlouse has a very defined elongated telson, but lacks the pleopodal lungs that can be seen on the underside of species within the Porcellionidae and Armadillidiidae families.

They can be found in almost any damp habitat, including woodland, meadows, parks, gardens or waste ground, where they live, often in large numbers, under stones or logs or any place that provides shelter and protection from the desiccating rays of the sun. They are common and widespread across the country, but with a genetically distinct subspecies ssp.occidentalis in the extreme southwest, which is a bit smaller, higher backed and more brightly coloured, and prefers rural areas, whereas the nominate species readily colonises synanthropic sites.

The broader and flattened form of this species allows them to cling on to the substrate when disturbed and they very often stay like that for a fair while, which makes them one of the easiest woodlice to photograph.

Young Oniscus asellus are significantly smaller and less shiny than the adults and have a rough and bumpy rather than smooth texture and, as such, could easily be mistaken for immature Porcellio scaber if it wasn't for the fact that that species has two rather than three segmented flagella. The only problem is that it's often quite difficult to make out that level of detail when photographing species that may only be a few millimetres long, so for comparison I've included a couple of examples of both species on a separate juveniles and immatures page.

Updated, Sept.26 (v.3)