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Background information

Porcellio scaber is probably the most common woodlouse in New Zealand. Although it is a native of Europe it must have arrived in this country along with the first european settlers.

Woodlice are often found in the upper layers of compost heaps, under rotting planks or logs, under sacks or stones and in other dark, damp places. They have 7 pairs of legs and although their bodies consist of three sections (head, thorax and abdomen) these are often fused together so that it is difficult to be sure where each section starts or finishes.

Woodlice belong to the biological class Crustacea. Most of the animals in this class are aquatic, and although the terrestrial species can breath with the aid of primitive 'lungs' they lack the features found in most other land dwelling arthropods.

They have no waterproof waxy cuticle on their exoskeleton and are therefore more likely to suffer from desiccation compared with other arthropods such as insects which have a well developed waxy layer.

These animals excrete their nitrogenous waste as gaseous ammonia directly through their exoskeleton (rather than as urea or uric acid), this means that their exoskeleton needs to be permeable to ammonia and is therefore also permeable to water vapour.

Gas exchange occurs in the 1st pair of anterior pleopods. These pleopods contain a pseudotrachea (a branching system of tubes) covered with a thin film of moisture to allow gas exchange over a relatively large surface area. This moist surface will also allow evaporation of water which will transpire out of the pleopods through a pore like opening.

The fact that woodlice prefer high humidity and cooler temperatures is a direct consequence of the permeability of their exoskeleton to water and the loss af water from their respiratory pleopods. These preferences are behavioural adaptations to help reduce desiccation.

Woodlice are frequently found in clumps or groups. When parts of their bodies touch a surface, woodlice slow down and will eventually stop if enough of their body comes into contact with an object or another woodlouse (thigmokinetic response). This type of behaviour will cause clumping of woodlice as they come into contact with each other as they explore their habitat. This clumping will result in an overall reduction in the surface area to volume ratio of the group (compared with individuals) and this will lead to a reduction in the rate of water loss. There is some evidence that woodlice are able to detect the odour of other woodlice and move towards them.

Moulting in woodlice occurs in two steps over a few days time. First the back half is moulted and after a few days the front half is shed. The new skin is pale and so a woodlice which has just moulted will have one half dark and the other half pale. This moulting behaviour probably helps reduce predation by allowing the woodlouse to remain relativley active and it should also reduce water losses. Woodlice often eat their moulted skins.

The P.scaber on the right has moulted within the previous 24 hours and, as shown by the lighter colouration, has replaced the front half of its exoskelton . Part of the old exoskelton can be seen beneath the left woodlouse

Woodlice can drink water by pushing the inner pair of projections of their uropods together to form a narrow tube which is then pressed against a damp surface so that water can be absorbed by capillary action. This action is readily observable in captive woodlice which have been allowed to become moderately desiccated. It is thought that they can loose excess water by pressing against dry ground.

Life cycle

 

 

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Woodlice

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