When long-wave ultraviolet light is shone on a scorpion in the dark, the animal glows a distinct blue-green. Seen in almost all known scorpion species, this trait has become one of the most practical ways of finding scorpions during night-time fieldwork. How the glow arises is largely understood; why it exists is still debated.
How does the glow arise?
The phenomenon is called fluorescence. Some molecules absorb short-wavelength, high-energy light and, after losing a small part of the energy as heat, emit light at a longer wavelength. The scorpion cuticle takes in ultraviolet light that is invisible to the eye and gives it back as blue-green light in the visible range. When the light source is switched off, the glow stops immediately; this is what distinguishes it from phosphorescence, in which light is stored and released later.
In the laboratory, another side of the same physics is put to use. The absorption of ultraviolet light by proteins is one of the basic ways of monitoring the amount of protein in a sample; this measurement was described in the article on 280 nm. The glow of a scorpion, by contrast, results not from absorption but from the re-emission of the absorbed energy.
Which compounds are responsible?
The fluorescent property is concentrated in one of the outer layers of the exoskeleton, the thin, transparent hyaline layer. Among the substances isolated from this layer, a compound from the beta-carboline group and a coumarin derivative called 7-hydroxy-4-methylcoumarin have been identified. Other fluorescent components are thought to be present in the layer as well; the picture is not yet complete.
The light that triggers the glow is long-wave ultraviolet, just beyond the violet visible to the eye. Fieldwork mostly uses lamps emitting between 365 and 395 nanometres. Short-wave ultraviolet can also produce the glow, but it is not preferred for this purpose because it is more harmful to both the eyes and the skin.
The fact that the compounds sit in the cuticle also explains why the glow does not depend on the animal being alive. Dead specimens or those preserved in collections can go on glowing for a long time; in specimens stored in alcohol, the fluorescent compounds have been reported to leach into the liquid, making the alcohol itself glow faintly. The layers of the exoskeleton and the general body plan are described in the article on body structure.
Relationship with moulting
A scorpion replaces its exoskeleton at intervals in order to grow. The cuticle of a freshly moulted individual is soft and pale; at this stage it shows no distinct glow under ultraviolet light. As the cuticle hardens and darkens, the fluorescence becomes increasingly pronounced. The intensity of the glow has also been observed to vary with age. This observation suggests that the fluorescent compounds accumulate or form as the cuticle hardens; research into the details of the mechanism is ongoing.
The same observation also has a practical use. In closed-cycle breeding, a brief ultraviolet check can help to identify individuals that have recently moulted; these individuals are left undisturbed until their cuticle has hardened. The process itself is covered in the article on moulting.
Does the glow have a function?
There is not yet a definitive answer to whether fluorescence provides any benefit to the scorpion. Behind the debate lies the scorpion's visual system. A pair of median eyes sits in the middle of the carapace, and lateral eyes, whose number varies by species, sit along the front edges. These eyes are sensitive to changes in light intensity rather than to detailed images and are adapted to working in very low light. The main explanations proposed in the literature are as follows:
| Hypothesis | What does it propose? | Status |
|---|---|---|
| Light sensor | The cuticle converts ultraviolet into blue-green light, to which the eyes are more sensitive; the animal senses ambient light with its whole body surface and avoids being exposed | The most debated explanation, partly supported by behavioural experiments |
| Sun protection | The fluorescent compounds protect tissues by absorbing harmful ultraviolet light | Limited evidence; questioned for a nocturnal animal |
| Prey attraction | The glow attracts insects that are sensitive to ultraviolet | Weak experimental support |
| Intraspecific communication | Individuals recognize each other by their glow | No direct evidence |
| By-product | The glow is a non-functional consequence of cuticle chemistry | A possibility that has not been ruled out |
The light-sensor hypothesis draws support from observations showing that scorpions become more inclined to seek shelter under ultraviolet light. According to this view, even at night moonlight contains a weak ultraviolet component, and the whole body may act as a large sensor telling the animal whether it is exposed. Even so, the findings are not strong enough to settle on a single explanation; several factors may also be at work together.
Testing the hypotheses directly is not easy, because there is no practical way to remove the fluorescence without damaging the cuticle. In some experiments the glow has been reduced by prolonged ultraviolet exposure, but it has been argued that this treatment may also affect other properties of the animal. This is why the results should be read with caution.
What does the glow not indicate?
- It does not indicate species identity. The shade and intensity of the glow can vary with species, age and the time elapsed since the last moult; identification is based on morphological criteria.
- It says nothing about the effect of the venom. Medically significant and insignificant species glow in a similar way.
- It is not related to the venom. What glows are the substances in the exoskeleton; the venom glands and their secretions play no part in this phenomenon.
For field researchers, the ultraviolet torch has been a standard tool for decades; a glow that can be spotted several metres away in the dark has made compiling species inventories much easier. Before such lamps became widespread, scorpions were mostly found by searching under stones and bark during the day; night surveys made it possible to count active individuals in their natural habitat and sped up the compilation of regional species lists. The distribution of the species found in our country is summarized in the article on scorpion species in Türkiye. When working with such lamps, the light should never be pointed directly at anyone's eyes. If a scorpion is spotted at home or in the garden, it should not be touched; in the event of a sting, stay calm, keep the stung area still and call 112.
