The ecological niche of the woodlice Porcellio scaber.
The woodlouse Porcellio scaber is native to Europe but also commonly found in New Zealand. They live in cool, dark, damp microhabitats such as in rotting wood, under rocks, in caves and leaf litter. Small insectivorous rodents and birds as well as some spiders feed on woodlice. In the rotting log from which I gathered my specimens there were also millipedes, crickets, weta and spiders living. Woodlice have physical adaptations to allow them to live in a variety of habitats, provided they are dark and damp. These include; * A mottled grey-brown colour for camouflage.
* Olfaction structures on the ends of the large antennae or covering the surface of the antennulae (smaller set of antennae). These are used to find food and other woodlice. * The excretion of nitrogenous waste as ammonia gas through their exoskeletons. This means that as well as being permeable to ammonia their exoskeletons are also permeable to water. (This means in arid conditions the woodlouse dries out and in humid conditions it absorbs a lot of water) * Up to seven sets of legs.
* Pores opening to simple lungs. The pore is unable to be closed resulting in constant water loss from the lungs. Behavioural adaptations such as:
* Freezing when exposed or attacked. (I also observed when I picked the woodlice up for the experiment that if they were accidentally rolled over on their backs, they would curl their legs up and play dead) this combined with their good camouflage makes it hard for predators to see them. * Burrowing back underground when exposed to light.
* Clustering together (a thigmokenetic response) to reduce the surface area to volume of the whole group resulting in a reduction of water loss. Woodlice are very vulnerable to desiccation. As one of the few terrestrial animals in the phylum crustacea they lack some of the structures other land dwelling arthropods have evolved. They lack the waterproof cuticle that many other arthropods have and their high surface area to volume ratio means water transpires through their exoskeleton easily. Because of this constant threat of drying out and dying, woodlice live in dark damp habitats where there is sufficient moisture in the ground and air to prevent this. It is known they can drink water through their anus and that by pressing their bodies against moist surfaces they can take in moisture but I want to investigate if woodlice can detect the difference between dry and humid air when there is no other source of water (soaked into the ground etc).
To determine what response woodlice have to different air humidity. Hypothesis:
The woodlice will prefer the more humid air and so will stop in the humid side. Method:
1. I collected twenty woodlice of similar sizes from a rotting log. 2. I placed them in plastic container with dirt from their habitat and allowed them at least 20 minutes to adjust to their new environment, as some were playing dead. 3. A choice chamber was made out of a plastic container, 14 cm by 4 cm and 8.5 cm high. It had a clear plastic lid that allowed observation and was also airtight to allow the air humidity to be controlled. 4. I marked the lid along the half way line with a permanent marker, dividing the length in half. 5. With measuring spoons, I measured out 30 ml (two tablespoons) of silica, a drying agent, and put it in a circle of thin nylon stocking. Then I drew up the edges of the fabric and twisted them together to create a bag of silica. I then taped the bag up around the neck (where no silica was) to prevent silica leaking from the bag. The purpose of the silica inside the nylon was to dry the air out around it. 6. At one end of the chamber (lengthways) I taped the bag to the wall so that it hung 2 cm above the bottom of the chamber (to prevent the woodlice climbing it). I used the tape only over the neck of the bag so it...