Sunday, November 15, 2009

The Armoured Brigade

Every time I lift the lid of our compost bins to dump another load of kitchen waste a horde of woodlice scurry away in all directions, so I thought I’d catch one and take a closer look under the microscope. It turned out to be the common shiny woodlouse Oniscus asellus. Woodlice are terrestrial crustaceans – relatives of crabs, prawns and lobsters but more closely related to marine isopod (meaning that all the legs are the same length) crustaceans like the one portrayed at

Michael Chinery notes in The Natural History of the Garden (Collins 1977) that woodlice have acquired at least 65 local names, including sow-bugs, tiggy-hogs, sink-lice, slaters and coffin-cutters.

Woodlouse senses are centred around the jointed antennae and simple eyes that have only about 25 individual ocelli – probably enough to detect light and shade and largish moving objects, but probably incapable of forming images with a very high degree of resolution.

The tail segment of a woodlouse is called the telson, flanked by two appendages called uropods , and its shape is often an important species identification feature.

All woodlice have only six pairs of legs in their infancy (when they’re known as mancas) and the full complement of seven pairs, visible here, only appears after their first moult, a day after they’re released from the brood pouch of their mother who carries them around. From below you can see the mouth at the head end, between the antennae .

Woodlice are omnivores but will eat other small animals if they can catch them, so have two pairs of jaws – crushers at the front and lethal-looking pointed ones behind.

The armour is an obvious adaptation to surviving terrestrial predators like spiders but the woodlouse’s main problem is keeping moist, because it obtains oxygen by diffusion over these lung plates at the tail end. Generations of schoolchildren have conducted simple experiments offering woodlice a choice of moist or dry environments but the outcome is never in doubt – in a dry environment a woodlouse will suffocate, for lack of dissolved and diffused oxygen.


  1. A fascinating group of creatures, so primitive looking. Interesting to see the closer shots. I didn't realise they were omnivores.

  2. Hi John, like so much of the low-life in gardens, woodlice really repay a close look. Somewhere I have a photo of one in mid-moult, backing out of its old skin

  3. I was picking up some windfall apples when I saw one had a deep groove underneath. In the groove were half a dozen small woodlice. They get everywhere.

  4. That looks like a male Oniscus asellus. Nice pics.

  5. Thanks for visiting Aydin, and for you kind comment. The pictures were taken through the lid of a plastic Petri dish, with the woodlouse held down with wet tissue paper, so their not especially sharp... and the animal was moving rather a lot..


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