Amazing Animals: Woodlice
Topic: Nature and Our World
This morning, I was coming out of the garage into our porch when I spotted a woodlouse on the brick wall by the front door. It was long dead of course, a desiccated shell, but it made me think of the first time I noticed something unusual about these little beasties. We hadn't been married much more than a year and we were moving into our first house in Worthing. There was a rotten tree stump in the garden, swarming with woodlice, and we were trying to clear it away. One large individual woodlouse fell on its back and froze, with its legs sticking up; it had a white patch on its underside. I bent down to look closer - it was a sort of greyish-white sack. I touched it with a tiny twiglet... A movement, a minute tear, and a horde of tiny baby woodlice started to appear one by one, hundreds, it seemed. The adult was completely motionless. Had I killed it, I wondered?
Woodlice are very ancient creatures which remind me of fossilised trilobites. There are many different species of these amazing little animals worldwide, but only 37 live in the UK. The ones most often seen in our British gardens are the common rough woodlouse (Porcellio scaber), the common shiny woodlouse (Oniscus asellus) and the common pillbug (Armadillidium vulgare). All British woodlice are vegetarians and nocturnal in habit. They like living in damp places, under rotting wood, or vegetation. They actually help to recycle waste by eating decaying wood and plants and thereby improve your garden soil! They have a head, thorax and an abdomen; two large antenna (and two tiny vestigial ones); two compound eyes; seven pairs of jointed legs (one for each of the seven thoracic segments), and an exoskeleton. They breathe through gills on their legs and also have 'pseudo-lungs' which can be seen as white patches at the back of the underside. These allow gases to diffuse in and out through a pore which can't be closed. So Woody needs a damp environment or he will soon dry out.
Woodlice are actually Crustacea and they belong to a group of Arthropods called Isopoda, a class of animals usually found living in water. Crustaceans are hard-shelled creatures - which include lobsters, crabs, shrimps, prawns, water fleas and barnacles - strange relatives for a little creature found in all our gardens. (Incidentally, trilobites were also Arthropods.)
Woodlice are usually a dark grey colour but if you find one that is half-pale, then it has just moulted. First the back half is shed and then the front half. This process allows them to grow a little before the new shell hardens. They usually live for about two years.
The 'sack' I had first seen all those years ago is, in fact, a brood pouch called a "marsupium" in which the eggs are stored for several weeks until they hatch. The new babies only have six pairs of legs and are very vulnerable to desiccation so, at this stage, they stay in their mother's pouch for several days until they moult for the first time and get their seventh pair of legs. Most woodlice breed once a year and the larger the female, the more eggs she is likely to produce.
[For further information on Woodlice, look here and here]
