Porcellio scaber - some background information
In most of the suggested investigations you will be looking at behavioural adaptations of the woodlice to changes in its habitat. The information given below is designed to give background data which needs to be considered in experimental design and also to suggest some additional areas for investigations.
Two other areas you may like to also look at are graphs showing the results from some of my student's investigations and the equipment used in these investigations.
Desiccation
Woodlice lack a waxy cuticle on their exoskeleton. This means that they are much more prone to desiccation (drying out due to excess water loss) than other members of the arthropoda such as insects and spiders. As a result of this, much of their behaviour is concerned with avoiding desiccation. The absence of wax or cutin means that the exoskeleton is permeable to water and so woodlice will rapidly desiccate in dry conditions. Conversely, in excessively humid conditions it may be possible for woodlice to absorb water vapour by diffusion through their exoskeleton. The observation that the behaviour of woodlice is quite different if they have previously been living in very humid conditions compared to woodlice which have been living in low humidity provides some support for this hypothesis.
Woodlice have primitive lung-like structures in two of their pleopods. These show up as two white patches on the lower side of their abdomen. (don't confuse this with a swollen yellow structure present in females - this is a brood pouch!). The "lungs" open to the outside via a simple pore, this again means that water loss from their respiratory surfaces is likely to be greater than is the case in most insects.
Woodlice will try to reduce water loss by moving away from hot and/or dry areas and will therefore tend to congregate in more humid, cooler regions. It might be expected that because dark areas tend to be cooler then the woodlice would also prefer dark conditions over brighter areas. The fact the woodlice are more active at night is partially to avoid predators but this nocturnal behaviour will also result in a greatly reduced water loss.
Reduction of surface area is another mechanism which helps reduce water loss. It is not possible for an individual slater to reduce its surface area, however grouping of slaters (clumping behaviour) will cause a reduction in the exposed surface area and help reduce water loss from the clump. The woodlice in the centre of the group will have a lower humidity gradient and therefore loose water vapour at a much reduced rate when compared with an isolated woodlouse. Even the woodlice on the outside of the group will loose water vapour at a lower rate compared with an isolated woodlouse. Hence clumping is a behavioural adaptation to reduce water loss. It might be expected that woodlice would be more likely to show clumping behaviour in less humid conditions.
Desiccation experiments could involve:
 | Measuring changes in weights of groups of woodlice in dry and/or humid conditions (you will need at least 20 woodlice and a balance accurate to 0.001g.) |
| You could also use choice chamber experiments to find preferred conditions of humidity. You will need to find some way of controlling and measuring humidity. |
| You might like to compare the effects of pre-exposure to humid conditions, compared with low humidity conditions, on behaviour. |
Alternation
Alternation is shown when a forced turn is followed by a turn in the opposite direction at the next barrier. For example if a woodlouse encounters a barrier which forces it to turn left , then if it next encounters another barrier where it has a choice of turning either left or right, a right turn would indicate alternation has occurred.
Alternation would result in the woodlice crossing an open (or hot, or low humidity) region, containing a large number of obstacles, more rapidly than if alternation did not occur. If alternation always occurred regardless of distance travelled between turns then it could result in the woodlouse spending a longer time in the exposed conditions - this might occur when the exposed region had few obstacles. Obviously there should be a maximum distance or time after which alternation behaviour would no longer be an advantage.
Alternation could be investigated by using channels which force a turn and then after a variable distance allow a choice of turn.
Light/heat behaviour
Because high levels of light and temperature tend to be associated with low humidity, then one would expect that woodlice would avoid these conditions. Conversely, if the temperature is too low then the woodlice may become so sluggish that they would be unable to avoid predators. Clearly there must be an optimum temperature.
To avoid desiccation in high temperatures many observers have reported that the speed of movement and also the rate of turning both increase as temperature is increased. These behaviors may result in the woodlouse spending less time in these unfavourable conditions and therefore increase its chances of survival.
It may be an advantage for woodlice if they are able to tell the difference between light from the extremes of the spectrum - which coloured light is more likely to be found in 'cooler' regions?
Investigating these factors could be done by using either:
| A series of choice chamber experiments |
| Setting up a long tube with a gradual gradient in conditions from one end to the other. |
| Use a flaskwith a line ruled on the bottom, placed in a water bath. Count the number of times the line is crossed in a 2 minute interval |
A worthwhile reference is:
Stephen Sutton, 1989, Woodlice, Ginn
