Scorpions emit a greenish glow at night due to photoluminescence in their cuticle. Scientists believe that the photoluminescent chemicals protect the scorpions from parasites and help them find shelter.
Scorpions are often seen moving around in the dark, and their comma-shaped tails always ready to strike give them a reputation for being dangerous stingers. As nocturnal creatures, they hunt for prey at night and seek refuge in dark places during the day. What many people don’t know is that scorpions actually glow in the dark!
Scorpions can emit a bright glow under ultraviolet light (Photo Credit: -Aastels/Shutterstock)
Glow-in-the-dark scorpions were first discovered in 1954 when they fluoresced under ultraviolet light. Since then, scientists have observed several species of scorpions with this ability, although not all scorpions glow with the same intensity. With nearly 100,000 known species of these arachnids, differences in their cuticles cause variations in their glow. Some scorpions have a faint glow, while others have a more pronounced glow.
Arachnids belong to a separate class within arthropods (Photo Credit: -SaveJungle/Shutterstock)
UV Light and Photoluminescence
We are all familiar with glowing fireflies. These small insects have specialized organs in their abdomens where they mix certain enzymes (primarily luciferase) and chemicals like luciferin to produce light. This chemical reaction takes place inside their bodies, which is why it is called bioluminescence.
However, scorpions do not use bioluminescence to glow.
Scorpions exhibit a process called photoluminescence. It occurs when their skin absorbs a certain wavelength of light that excites their organic molecules. As these molecules return to their ground state, they release energy, which produces light.
A lantern fish displaying bioluminescence (Photo Credit: 3dsam79/Shutterstock)
Specifically, scorpions demonstrate “cuticular fluorescence”. In simple terms, there are compounds in their outermost layer of skin, or exoskeleton, that absorb ultraviolet light ranging from 100-400 nanometers (nm). This excites the compounds in the exoskeleton, and when they re-emit the light, it falls within the visible range of 400-800 nm, which can be seen by humans.
The spectrum of wavelengths Credits:Peter Hermes Furian/Shutterstock
Importance of the Cuticle
The outermost layer of the cuticle, known as the epicuticle, plays a crucial role in scorpions. This thin layer, measuring 4 micrometers in thickness, contains the fluorescent compounds. The epicuticle is hard and provides protection to the scorpion. As scorpions age, they molt their cuticle, allowing for growth and better defense.
The older the scorpion, the stronger and harder its exoskeleton becomes. A recently molted scorpion will only emit a faint glow under UV light. It has been observed that the fluorescence intensity increases with age and the toughness of the exoskeleton.
The hyaline layer, which is part of the epicuticle, is remarkably durable. Fossils with remnants of this layer still attached have been discovered, and they continue to fluoresce. Sclerotization, a process of oxidative cross-linking, occurs as the cuticle hardens in all arthropods. Among the millions of arthropod species, scorpions are unique in their ability to fluoresce to such a remarkable extent.
The Cause of their Fluorescence
Research has revealed that scorpions have photoreceptors that are selectively sensitive to certain wavelengths of light. In an experiment, desert grassland scorpions were exposed to different light wavelengths, including red, green, UV, and no light. The scorpions demonstrated the highest aversion to UV light, followed by green light.
A molting scorpion (Photo Credit : -somratana/Shutterstock)
The hard outer shell of scorpions contains two compounds, β-carboline and 7-hydroxy-4-methyl coumarin, which are responsible for their fluorescence. However, scientists have discovered another compound, the phthalate ester, in shed exoskeletons, which exhibits the most intense fluorescence.
UV light is a part of the light that is bent when coming through the atmosphere. It can be found in sunlight, starlight, and moonlight. Interestingly, there is a much higher amount of UV light in the sky when the sun is setting compared to when there is no moon at night. Scorpions, being sensitive to shorter wavelengths of light, tend to be more active after the sun has gone down. They try to avoid exposure to UV light, so during the day and on nights with a full moon, when there is less UV light, they are less active. This behavior helps them decide whether to stay hidden or come out to hunt for prey.
The entire exoskeleton of scorpions acts as a collector of photons throughout the body, transforming the energy obtained from UV light of shorter wavelengths into green light of longer wavelengths. It is thought that the scorpions’ central nervous system detects and interprets this conversion and transmission of light, aiding them in locating shelter according to the intensity of the light they encounter.
The Reason Behind Their Glow
What is the significance of the glow-in-the-dark abilities of these creatures? Does it benefit them in any way, or is it just a natural occurrence caused by the chemicals in their shell?
Fluorescent substances such as pthalate esters have the ability to assist in the defense of these organisms. The stunning cyan-green luminescence they emit has the potential to distract predators. There was also a theory that the fluorescence could attract prey more effectively, but it was discovered that insects generally avoid scorpions that glow brightly.
In addition, fluorescing refers to being exposed to some form of light, and as mentioned, scorpions have a tendency to avoid UV light. However, this sensitivity to UV light might actually help scorpions search for shelter more efficiently. Researchers conducted an experiment where they placed small goggles on scorpions to block their vision and restrict their movement. The results of the experiment led the researchers to hypothesize that scorpions may be able to detect UV light by observing the fluorescence of their bodies, which helps them locate shelter by moving away from the light.
To conclude, if you compare images of scorpions under regular white light and UV light, you will notice that the entire body of a scorpion glows under UV light, with the exception of its chelicerae (jaws), the tips of its stingers (telsons), and the claws (ungues) of the movable finger (tarsus). These areas may lack the substances responsible for fluorescence. Nonetheless, UV light illuminates most of the body’s surface of these creatures in the dark. Therefore, whether you are searching for scorpions or trying to avoid them, a UV flashlight will be useful on your next field trip!