A few weeks ago I went downstairs late at night and discovered a slug on the skirting board near my front door. I proceeded to put it outside and then forgot about it. Two or three nights later, I went downstairs to discover once again a slug on the skirting board, in the exact same spot as before. I found this odd, but decided it was probably coincidence and placed the slug outside once again. However, I have discovered a slug in this exact spot late at night once every few nights since then, no less than five times. Every time I find it, I release it outside, typically in a different place to where I last set it free, yet it still comes back. Tonight I found it again, on the skirting board next to the front door and decided I needed to ask someone about it. What is going on? Is there any reason a slug might be driven to returning to the same spot each night? How is it able to find it's way back there? And how on earth does it keep getting back into the house???

Do not underestimate the humble slug! Many species of slugs make nocturnal journeys to their favourite foraging sites, returning at dawn to hide in leaf litter or damp crevices. Some woodland slugs climb to the tops of trees each night, where the algae on tree bark are more abundant and diverse. In suburban areas it's common for them to graze on the algae of rooftops, and if you look at the right time then you might be able to find the route that they use to climb your house, since they tend to follow each other along a common trail (it saves on slime).

The slug you keep finding is either on its way to a better feeding place, and you happen to keep disturbing it, or else your skirting board is a lush algal farm.

I've now consulted one of Britain's leading slug watchers, Chris du Feu, who sent the following fascinating information about these noble creatures:

From the description of the behaviour of the slug it seems likely to be a yellow slug (Limax flavus). This species feeds on fungus, lichen, mould, algae etc. and is often found in houses, typically in rooms where there is some dampness (such as under the sink in the kitchen). Although they are very widespread and very often found living in or around human habitations, they are grossly under-recorded.  This may be because they are so highly nocturnal. It is rare to see one active at any time from just before first light until after dusk. They are reputed to have a strong homing instinct although this reputation seems to be anecdotal rather than well established. Slugs of this species have markings which are individually identifiable and this will allow you to check whether your slug is the same individual returning to its base or whether it is just a good place for slugs to be and when one slug leaves another takes its place. Take a digital photograph of the slug and evict it. The next night take another photograph and see if it is the same individual.

Once you have established whether it is the same individual, you might ask what mechanism it uses to find its way home. It could be the slime trail. By removing all traces of this trail you could see whether it reduces the slugs ability to find its return route. Alternatively you could take the slug a little further away each night to try and discover the distance from which it can find its path home. Because of the individual markings the possibilities for studying this species are very great indeed.

Incidentally, this species of slug is far from being a pest. It avoids green plants in favour of other matter. I have read of an Australian who has a jar in his shower room where some of these slugs live. At might they emerge to remove all the mould that grows on the grouting between the tiles. They return to the jar before first light leaving the grout clear of mould and glistening with the dried slime trail. A labour saving, environmentally cheap solution to the tile cleaning problem.