Thursday, 31 May 2012

Why do scorpions fluoresce and other such trivia...

Scorpions - everyone's favourite!
I have to admit that I find scorpions a bit creepy. Not only do they have too many legs to begin with, but some of them seem to accelerate from stationary to far too fast in no time at all. And, of course, some of them (a tiny minority, it's true) can be really rather nasty when they're pushed to it. However, despite the slight wariness they inspire in me, I do find them absolutely fascinating creatures. One of the big things that puzzled me about them was their bizarre fluorescence under ultra-violet light. If you've never been it, it's well well finding someone who's got a fluorescent light and taking them to the bush at night. You'll be amazed not only at how the beasties glow, but by quite how many of them there are! In many savanna habitats you'll see them every 3 or 4 metres. Hopefully enough to convince you never to walk in the dark without shoes! Why they should do this has been a mystery to me, but some new research published this year by Gaffin et al (find it here, but you have to pay...) has, perhaps, started to unravel the mystery.

In normal light...

And fluorescing under ultra-violet
The first thing to know is that scorpions don't like light. They're mostly nocturnal, and hide under rocks or in burrows during the day. Despite their defences, rather a lot of things like to eat scorpions and they're safer hiding away during the day. Shine a torch on them at night and they'll often head for the nearest hole too (which is why you don't realise quite how many there are around until you look with a fluorescent lamp!). Now, scorpions have eyes that can obviously sense light. In fact, most (all?) have eight eyes - a big pair on the top of the head and three pairs around the side. These eyes in the centre are, in fact, one of the most sensitive light gathering tools in nature and they can see perfectly well for hunting using only starlight. In fact, none of the eyes give clear vision, but they certainly do allow them to sense light and dark. However, the new research has shown that even if you block all these eyes, scorpions can still respond to light by running away. It seems that some species have light receptors in their tails, but by shining lights of different colours Gaffin et al have discovered that some eyes are particularly sensitive to the blue-green colours given off by a fluorescing scorpion. And as shining even faint ultra-violet light makes the scorpions themselves fluoresce blue-green, they are very sensitive to UV (ultra-violet). Fluorescence is a pretty good way of measuring light: as you'll know if you've ever tried making a room really dark when there's strong sun outside, you have to shut out even the tiniest bits before you make a very noticeable impact. Fluorescence, on the the hand, because it starts at much lower intensities is much more graduated. So what the scorpions may have evolved is  a 'whole-body' light measuring device which is exactly what you want if you want to scuttle backwards into a crack or hole whilst keeping your eyes (and pincers and sting) all facing the way of a predator. As the sun is the only significant natural source of UV, scorpions have a very sensitive way to detect when they start to find a suitable hole without needing to look.

I'm not sure all the details are quite worked out yet, but I can see that there's some sense to this, and it's the best answer I've seen yet!

Eight legs in a circle are perfect for vibration detection
As well as fluorescing, of course, scorpions have a number of other handy tricks that I think are rather neat. Firstly, they have structures called pectens on the underside of their body. (Strangely, I don't have any close-ups of the underside that show them, but they're small to medium-sized hairbrush-like structures under the body, near the legs.) These they use to detect chemical signals - they're bigger in males than females and males certainly seem to use them to follow pheromone signals from females - though they also probably help when hunting prey. Pretty neat.

And the other useful trick they have for helping hunt is a phenomenal ability to sense and locate vibrations. As you have no doubt noticed, it's possible for us to hear something and look in the right direction to see what caused it - we do that by measuring the difference in time it takes for the sound waves to reach our left and right ears, but we're not very good at it really. Owls have perfected the ability and can use these aural queues alone to hunt in total darkness. But for scorpions it's the legs that do the job - and they've got eight to use to give extremely precise distance and direction sensitivity. It's estimated that using very sensitive hairs on the legs they can pick up a vibration of less than one millionth of a millimetre and use the difference in timing it takes for a vibration to travel from one leg to another to pinpoint the source so they can turn and run exactly the right distance and direction to catch their prey, despite very poor vision. Now that is truly awesome!

Main reference:
Gaffin, D., Bumm, L., Taylor, M., Popokina, N., & Mann, S. (2012). Scorpion fluorescence and reaction to light Animal Behaviour, 83 (2), 429-436 DOI: 10.1016/j.anbehav.2011.11.014

No comments:

Post a Comment