Bibble-bugs

Thursday's Guardian Country Diary is about these charming little crustaceans - woodlice. They are reputed to have at least 65 colloquial names - more than any other animal in the UK fauna - and are locally known as bibble-bugs, coffin cutters, sow-bugs, tiggy-hogs, sow bugs, sink-lice and slaters.

I disturbed these while I was moving a pile of bricks under our hedge. Well over 100 were packed into the recessed face of one brick and once they were exposed to sunlight there was panic, as they fell over each other in their hurry to escape.

Eventually only this one remained - a fine specimen of Oniscus asellus, the common shiny woodlouse. I particularly like the elegant curves of its articulated armour plating and their saw-tooth outline.

Woodlouse senses are centred around the jointed antennae and these simple eyes that have only about 25 individual ocelli – probably enough to detect light and shade and largish moving objects, but 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 defensive adaptation to surviving terrestrial predators like spiders but the woodlouse’s main problem is keeping moist, because it obtains oxygen by diffusion over these 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.

Mother and child

Woodlice face a regular crisis when they need to moult in order to grow. They do this in two stages, shedding the old carapace from the hind segments first and then easing themselves out of the cladding on their front segments. You can see in this photograph that the exposed outer covering on this animal's newly exposed front segments has yet to harden. 

New Suit of Armour

The trouble with being an invertebrate with a hard external skeleton is that you have to change your suit of armour every time you grow a little bigger - which is what this woodlouse is doing. It's a risky business, leaving you exposed to predators until your new exoskeleton hardens, so this cautious crustacean is doing it in two stages. It's already slipped out of the rear half and its new shell plates have become hard and shiny. Now it's extricating itself from the head end, and here you can see that the newly exposed shell plates are still soft, with a dull surface. The animal hasn't quite removed its delicate, all-important antennae from inside their armour yet - if you double-click the image to enlarge you can just make them out inside their old translucent covering. 

This woodlouse is Oniscus asellus and you can find more about its biology here

The whole process is known as ecdysis and all invertebrates that shed their old armour in this way are grouped together as ecdysozoans. You can read all about it here.