The physics of slugs

At Tuesday night’s Cafe Scientifique, we had a very entertaining discussion, led by Mike Wilson from AgResearch, on that most cute and cuddly animal, the slug. Let’s face it, in terms of looks and popularity, the slug doesn’t have a lot going for it, but it’s certainly a very interesting creature.

I learnt a fair bit about its biology, but the most interesting bits for me were the physics. For example, how slugs move is a good example. They create waves that travel down their body – the bits of the slug that contact the ground then push the slug along. If you look at the photo of the underneath of the slug-on-glass, on the handout here, you’ll be able to see the wave pattern on the bottom of the slug.

Then there’s slug navigation. Slugs have homes, apparently, which they will return to after their foraging runs around dawn and dusk.  How do they navigate their way back?  I wondered whether it was scent, but, according to Mike, it might well be by magnetic field. Wow.  I knew some birds did that, but the slug? Clever little creatures.

But the physics isn’t all in the slug’s favour.  There are a few physics based weapons of mass slug destruction out there for the gardener’s use. The slug electric fence sounds an exciting one. Just two strips of wire, separated by an insulator, and wired to a battery. The sluggy crawls over them, and completes the circuit between the two, and gets a shock. That’s usually sufficient to knock them off the wire. However, Mike commented that some slugs can get stuck across the two wires and get fried. Not very nice for the poor molluscs. Some slug fences are just single-wire based – you can use the ground as the other wire – so if the slug is in contact with both the wire and the ground it will form part of a circuit and get a zap. It was suggested that we don’t even have to wire-up the fence – the slugs will get a shock off just a piece of copper wire due to electrochemical effects – in a similar way to the way you can get a small shock if you start chewing tin foil and have amalgam fillings.

But, in my experience, by far the best and most bizarre slug zapping machine is simply the yoghurt-pot filled with beer. Slugs love it, and drown happy.


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