Isopods - Woodlice and Pillbugs

Open up a compost heap and you’ll see a scatter of little grey “armadillos”! They are called Isopods, commonly known as slaters, woodlice or pillbugs. These are related to aquatic crustaceans such as shrimps and because of this, they are less well adapted to terrestrial life than insects. Isopods are vulnerable to desiccation, as unlike insects, they do not have a waterproof waxy outer layer.

There are 2 types of these “armadillos” and they are often found together. One is rounded in cross-section and rolls up into a tight ball when disturbed – very much like the mammalian armadillo. Their common name is “pillbug” and they are pictured below. The other sort is the woodlouse or slater which is more flattened and cannot roll up into a ball. They just scuttle off fast when disturbed.

Isopods are wingless and have seven pairs of legs on their underside. They can be grey or brown and are around 1 cm long when not curled up. A pair of antennae and eyes on the head help isopods obtain information about their environment.

They are generally nocturnal and shun the light when we uncover a nice dark, moist compost heap, scuttling for the darker regions once the heap is disturbed. Curling up is a defence reaction in pillbugs against predators and drought. During the day and during dry periods, they hide under rocks, logs and dung pats and a favourite place to find them is under a house brick where they collect in the concave section if the brick is placed with this side next to the soil. You will often find the two types – pillbugs and woodlice – living together under the brick so if you are collecting them, take a note of which ones curl up when poked and which ones don’t.

Isopods are omnivorous and feed on decaying organic matter such as dead vegetation. They do a great job in my compost heap. Isopods also feed on carrion. A bat specialist at the Australian Museum trusts only slaters to delicately clean the flesh off his precious bat skeletons. Isopods can sometimes be a pest on seedlings but they are mainly “goodie” decomposers.

Isopods have maternal behaviour in that they protect their young in brood pouches underneath their bodies for up to 8 weeks after hatching.

Isopods are prey to other predators living in the soil such as spiders, lizards, centipedes and beetles. Scorpions that I have kept as pets do not seem to like them and this may be because they have an offensive taste to some predators.

Woodlice and Old Wives Tales
Woodlice indoors bring bad luck. Any food touched by woodlice is poisoned. Poke live woodlice down the neck of a sick cow and it will be restored to health. Swallow live woodlice and be cured of tuberculosis and liver disease. The cure might be worse than the disease!

Isopod Defence Reactions
The picture above shows how pillbugs curl up into a tight ball as a defence reaction to disturbance. Pillbugs and slaters will also seek moister enviroments under stones and logs and pillbugs, at least, curl up in dry conditions to protect their vulnerable underside against desiccation. This drought reaction occurs as isopods lose body water more easily than insects as they don’t have the waxy skin of an insect which helps slow down evaporation. Also their respiratory surfaces open by a simple pore near the first pair of legs which, unlike the spiracles of insects, can’t be closed and so water will continually be lost.

Isopods excrete their waste products not as solution in water, but as a gas, such as ammonia. This is one reason why the skin has to remain permeable to gases, including water vapour.