October 2016

Glowing scorpion

Is it true that scorpions glow in the dark?

Answer

Flinders Ranges Scorpion (Urodacus elongatus), under normal light, normal and UV light, and full UV light.

Some animals can glow, but they do so in different ways. Fireflies produce a glow called bioluminescence using a chemical reaction. Scorpions, however, produce a glow via fluorescence.

Under ultraviolet (UV) light, chemicals in their exoskeleton (the outer shell) absorb and re-emit UV light causing the scorpion to glow or ‘fluoresce’ blue-green. An unidentified substance in a very thin layer in the cuticle of the scorpion called the hyaline layer is responsible for the production of the glowing effect when it is exposed to UV light. By shining a beam of UV light on a scorpion, you can make it glow. Natural moonlight has a similar glowing effect when reflecting off the scorpion’s exoskeleton. Newly moulted or young scorpions do not fluoresce until their exoskeleton hardens.

The reason scorpions glow is unknown but there are many suggestions. One is that it is an adaptation to avoid being eaten. Scorpions are generally solitary, nocturnal creatures. They scurry around in the dark until part of their body falls under a shadow. What we see as a dark night still has some level of light such as moonlight. One proposal is that scorpions can detect this shadow because part of the UV reaction to the light falling on their body has been interrupted. The glow would theoretically help scorpions find shelter in their dark environments. This idea suggests that the scorpion’s exoskeleton works as a large sensor sending information about light to the nervous system. This light information would supposedly be used to help the scorpion find the protection of a shaded area. In shadow, they are less vulnerable to predators. However, some arachnologists contend that the glow of the scorpion’s cuticle is not adaptive, and suggest that it may simply be an accident of evolution. There may not be a need for them to glow; instead it could just be a side-effect of having a unique cuticle. Provided such a unique characteristic is not of detriment to the animals’ survival, there is no selective pressure against its retention.

Scorpions are among the oldest known terrestrial arthropods. Scorpions sit in the class Arachnida and are related to spiders, mites and ticks. Surprisingly even some fossil scorpions have been known to fluoresce or glow!

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