Have you ever rolled over a decaying log on a forest floor, and observed the teeming life found underneath?
Communities of invertebrates (animals without backbones) found amongst rotting logs play a crucial role in removing dead plant and animal material from the environment. Wood wasp and stag beetle larvae eat rotting wood, whereas beetle larvae, snails, slugs and millipedes eat soft decaying plant material.
There are fast-moving predators there too. Spiders prey on insects and woodlice. Common centipedes, brown in colour, and up to 3 cm long with 15 pairs of legs catch insects and slugs with their clawed front legs and inject lethal poison into them. Large black beetles such as devil’s coach horses and ground beetles emerge at night to catch slugs, spiders and insects with their sharp jaws.
If you have a log pile in your garden or have identified one in a wood or field, why not keep a record of the animals that are found there at various times of the year. Include vertebrates as well as the invertebrates to build up a picture of the food chains, and the diversity of life in your log pile. Good identification guides for insects, spiders, woodlice, snails and slugs can be bought from book shops or from the Field Studies Council. Bioimages offers an online guide to centipedes
The log pile environment
Small invertebrates, living under and amongst decaying logs are a community that is protected from dry air, insulated from extreme temperatures, and sheltered from predators such as birds. Exposure to below zero temperatures freezes and kills small animals like slugs, snails and woodlice. High temperatures kill small invertebrates as they reach lethal body temperatures surprisingly quickly.
The underside of a log provides a damp micro-environment. Slugs, snails, woodlice and worms do not have waterproofed skin and risk desiccation if they remain in a dry atmosphere for long.
Woodlice breathe by gills, which must be kept wet - drying out damages them. If their log is turned over, snails withdraw into their shells, slugs and millipedes crawl away, earthworms creep into their burrows, and centipedes, woodlice and earwigs scuttle off. What stimulates the animals into activity when their stones/logs are overturned? Are they reacting to light, because they prefer the dark? Or are they responding to the dryness of open air? You can investigate these questions by observing the responses of woodlice to light and humidity. First collect about 20 to 50 woodlice, preferably of the same species.
Identifying and observing woodpile dwellers: woodlice
Treat your woodlice gently and keep them in a clean margarine tub, with holes punched in the lid. Provide sliced vegetables or potato peelings that woodlice can hide in and eat. Woodlice are crustaceans, the group that contains lobsters and crabs, and we have 35 to 40 species of woodlice in the UK, and about 29 of these are native species. The Natural History Museum has a useful on-line woodlice identifier.
Place a woodlouse in a container and examine it with a magnifying glass or hand lens:
- What structures can you identify that could be the sense organs by which the animals detect cues in their environment such as light, humidity, food?
- Woodlice have a pair of antennae and a pair of eyes on the front of the head close to the bases of the antennae. Each eye consists of about 20 sub-units, resembling the compound eye of an insect
Investigation of choice of environment by woodlice
In their natural environment, woodlice do not have simple “choices”. A micro-environment may have favourable humidity but high temperature for example. To investigate choice of environment by woodlice try constructing a round choice chamber, (not square or rectangular, as corners provide extra unwanted micro-environments). Divide your box or tin into two compartments, which are connected to each other. Conditions that can be varied in each compartment include temperature, humidity, light, and food.
A key point is not to attempt to combine too many choices in one experiment. Begin by testing one option say, light, or humidity. Combinations of two choices can be tested once you have ascertained which ones the animals respond to.
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Designing and making a choice chamber
As woodlice are found under logs and in leaf litter, a reasonable proposal is that they prefer being in the dark rather than light. To test this proposal, make a simple choice chamber from a round biscuit tin, or empty ice-cream tub, with a diameter of about 20 cm.
Line the tin with smooth brown paper stuck down with a glue stick, sealing the edges with sticky tape.
Divide the choice chamber into a light and a dark part, by covering half of it with, say, a table place mat.
Stick thick cardboard to the edge of the mat with sticky tape, and fold over at right angles to form a dark enclosure in the tin.
Cut the cardboard so it fits the inside wall of the tin closely. Trim the bottom edge of the cardboard to one centimetre above the bottom of the tin, giving woodlice free access to the light and dark halves.
Illuminate the choice chamber with a table lamp fitted with an energy saver light bulb (to avoid exposing the woodlice to heat).
Place ten woodlice in the choice chamber at time zero, and count the number of animals in the light half after ten minutes.
Subtract the number in the light half from ten to obtain the number in the dark half. During the ten minutes watch the behaviour of the woodlice, and write down your observations in a notebook.
* Are they running away directly from the direction of the light source in a straight line towards the dark?
* Or are they scurrying around in random directions, making turns until they happen to reach the preferred half where they remain?
Design of choice chamber for testing light vs dark
We tried this experiment with 70 pill bugs (Armadillidium vulgare), collected from damp shady spots underneath a loose log heap. The animals were divided into groups of ten. Each group of ten was placed randomly in the choice chamber and the position of each animal recorded at the end of the ten minutes. At the end of each ten minutes the group of pill bugs was replaced by a new group. The counts of the total numbers of pill bugs in each of the light and dark halves came to 35 for each at the end of the experiment. So there was no evidence that pill bugs prefer either light or dark. Try the same experiment with woodlice collected from a dry area; the results could be different.
The design of the choice chamber can be adapted to test woodlice preference for dryness or humidity. Cover the bottom of the dry half with about one cm depth oven-dried compost, or sand. The bottom of the humid side is covered with one cm depth of damp compost or sand. The humid side of the choice chamber is covered with a transparent polythene sheet with a curtain of polythene blocking off the humid from the dry side with a one cm gap allowing free entry and exit of woodlice.
Choice chamber for testing dry vs humid atmosphere.
Place each group of ten woodlice randomly in the choice chamber and note their behaviour. Record the position of each animal at the end of each ten minute period.
When you have finished your investigations, set your woodlice free, back where they were collected.
The importance of life in a log pile
Why are we so interested in the invertebrates living in log piles? Their feeding habits recycle nutrients essential for plant growth, and they provide food for frogs, toads, moles and badgers, so overall the diversity of life is increased. Log piles provide reservoirs of beneficial invertebrates that prey on pests such as slugs and millipedes.