QuestionDeathstalker
Why Do Scorpions Glow Under UV Light? Fluorescence Explained
Scorpions glow under ultraviolet (UV) light because chemicals in their outer skin, such as beta-carboline, absorb invisible UV rays and give off visible blue-green light. This is called fluorescence. Nearly all scorpions do it, but scientists still debate whether the glow has a purpose or is simply a side effect of how the cuticle hardens.
Contents
What makes scorpions glow?
A scorpion’s body is covered by a tough outer skeleton, or cuticle. Its outermost layer, a very thin coating called the hyaline layer, contains fluorescent compounds. When UV light, which our eyes cannot see, hits these molecules, they absorb its energy and release it again as light of a longer wavelength that we can see. The result is the bright cyan glow that appears when a UV flashlight, or black light, shines on a scorpion.
Chemists identified two of the fluorescent compounds in 1999: beta-carboline and 7-hydroxy-4-methylcoumarin. Other related compounds are probably involved too, and the exact mix may vary between species. The same chemistry makes the deathstalker of North Africa and the Arizona bark scorpion of the United States glow in the same striking way.
Fluorescence is not the same as bioluminescence. A firefly produces its own light with a chemical reaction inside its body, as explained in why do fireflies glow?. A scorpion makes no light of its own. It glows only while UV light is shining on it.
The glow appears as the cuticle hardens
A scorpion’s fluorescence is linked to sclerotization, the process in which the cuticle hardens after a molt. A freshly molted scorpion, with soft pale skin, glows only weakly. Over the following days, as the new cuticle hardens and darkens, the glow becomes much brighter.
The same pattern is seen in young scorpions. Newborns riding on their mother’s back are soft and barely glow. The fluorescence grows stronger with each successive molt until the scorpion reaches adulthood. Because the compounds stay in the hardened cuticle, shed skins and preserved museum specimens still glow too.
Why do scorpions glow? The main theories
No single explanation has been proven. Researchers have tested or proposed several ideas:
| Theory | The idea | What the evidence says |
|---|---|---|
| Light detector | The whole body turns UV into blue-green light that the eyes can sense, telling the scorpion when it is exposed | Supported by behavior experiments; the leading idea |
| Moonlight sensing | The glow helps scorpions judge how bright the night is and when to stay hidden | Plausible, linked to the light-detector idea |
| Sunscreen | The compounds protect against UV damage | Possible, but most scorpions avoid sunlight anyway |
| Attracting prey | Insects are drawn to the glow | Little supporting evidence |
| Finding mates | Scorpions recognize each other by the glow | Little supporting evidence; scorpions rely mainly on touch and chemical cues |
| No purpose | A side effect of cuticle chemistry | Cannot be ruled out |
The light-detector theory is currently the best supported. Scorpion eyes are most sensitive to blue-green light, close to the color of their own glow. In 2012, a team led by biologist Douglas Gaffin published experiments showing that scorpions become less sensitive to UV light when their fluorescence is reduced. The researchers suggested that the entire cuticle may act as a simple light collector, turning faint UV from the moon and stars into a color the eyes detect well.
This would help a nocturnal animal decide whether it is exposed and should move to shelter. Scorpions often avoid hunting on bright moonlit nights, perhaps because they are easier for owls and other predators to see.
Do other animals glow under UV?
Scorpions are the best-known example, but they are far from alone. Fluorescence under UV light has been recorded in some millipedes and spiders, many corals, a number of frogs and fish, the feathers of certain parrots and the fur of platypuses, flying squirrels and some other mammals. In most of these animals, as in scorpions, the purpose is uncertain.
Some animals also have eyes that reflect light rather than fluoresce. Wolf spiders, for example, shine green in a flashlight beam because of a reflective layer behind their eyes, described in why do wolf spider eyes glow?. That is a different effect from the whole-body glow of a scorpion.
How scientists use the glow
Scorpion fluorescence was noticed in the 1950s, and since about the 1970s portable UV lamps have become a standard tool for scorpion research. On a dark night, a scorpion that is almost invisible against sand or bark lights up clearly from several meters away. This allows biologists to count populations, study activity at night and find rare species with far less effort than searching by eye.
Researchers have learned a great deal this way, including how far scorpions move, which plants and rocks they use, and how weather and moonlight affect their activity. Many new species have been found with UV lamps.
If you see a scorpion glowing under a UV light, enjoy it from a distance. Some species, including the deathstalker and the Arizona bark scorpion, can deliver painful and potentially dangerous stings, so never pick up a scorpion, and seek emergency medical help immediately if anyone is stung.
Frequently asked questions
Do all scorpions glow under UV light?
Almost all known scorpions fluoresce under ultraviolet light, although the color and brightness vary between species. Scorpions that have just molted glow only faintly until their new outer skeleton hardens.
What color do scorpions glow?
Most scorpions glow a vivid blue-green or cyan under a UV lamp. Some species look more greenish or bluish, depending on their cuticle chemistry and the light source.
Do scorpions glow in the dark without UV light?
No. Scorpions do not make their own light. They only glow when ultraviolet light shines on them, and the glow stops as soon as the light is switched off.
Do baby scorpions glow?
Newborn scorpions riding on their mother's back barely glow, because their soft outer skeleton has not yet hardened. The glow becomes stronger after each molt as the cuticle hardens.
Do dead scorpions still glow?
Yes. The fluorescent compounds stay in the cuticle after death, so preserved museum specimens and shed skins can still glow under UV light.
Can a UV flashlight help find scorpions?
Yes. Biologists have used portable UV lamps for decades to find and count scorpions at night, because the glow stands out clearly against sand, rock and plants. Scorpions should be observed without touching them.
Sources and further reading
- Stachel, S. J., Stockwell, S. A. and Van Vranken, D. L., "The fluorescence of scorpions and cataractogenesis", Chemistry & Biology 6 (1999)
- Gaffin, D. D. et al., "Scorpion fluorescence and reaction to light", Animal Behaviour 83 (2012)
- Encyclopaedia Britannica, "Scorpion"
Cite this page
“Why Do Scorpions Glow Under UV Light? Fluorescence Explained.” Animal Atlas, https://animals.bigworldlist.com/questions/why-do-scorpions-glow-under-uv/.
