By Kevin Scott
Man’s highly developed color sense is a biological luxury- inestimably precious to him as an intellectual and spiritual being…. -Aldous Huxley inThe Doors of Perception
Aldous Huxley concisely described man’s fascination with things that shimmer and glow. The earliest admiration of the luminous was probably lightning at night, wildfires, the sun or a full moon.
To be sure, these things were revered and their causes unknown to the point that gods and goddesses were invoked, whose attributes described these phenomena.
Fast forward several millennia to the advent of artificial ultraviolet light and another curious phenomenon is observed: the fluorescence of scorpions under a black light. After the initial curiosity one comes to wonder what the purpose of this could be and how works.
Wild california scorpion while illumated with a blacklight
Light in the ultraviolet wavelengths is certainly not abundant at night (although the proportion of ultraviolet to white light is higher at night), so why the glow? The wavelengths in question abound during the day, but scorpions are decisively nocturnal animals that have well developed sensory mechanisms, allowing them to efficiently hunt and mate in low levels of light. The chart below describes some of these adaptations (Blass and Gaffin 2008).
Although all of these adaptations allow scorpions to capture prey, navigate and reproduce in low-light conditions, they do have several photosensory organs as well.
Emperor Scorpion, showing both fluorescing color and normal color
Scorpions have lateral and median eyes that are capable of detecting light magnitude changes and image formations, respectively (Gaffin, et al. 2011). The median eyes are most sensitive to wavelengths around 500 nm (green) and secondarily sensitive to light in the 350-400 nm (ultraviolet) range (Machan 1968, Fleissner & Fleissner 2011).
In addition, scorpions have a metasomal element that is sensitive to light in the green area of the visible spectrum (Zwicky 1968, 1970; Rao & Rao 1973).
These technical details may at first seem superfluous until one thinks about the fact that when one views a scorpion under a black light, the black light and the green glow are in the same areas of the light spectrum as those to which the scorpion’s eyes are most sensitive; this is probably not a coincidence.
But this still does not explain the function of the fluorescence. Although the exact purpose is not known, there are a few hypotheses. There is the obvious possibility that no function is served at all.
Gary Polis suggested that fluorescence could act as a lure to draw prey in, but subsequent tests of this hypothesis show that insects will actually avoid scorpions that are fluorescing (Polis 1979; Kloock 2005).
One other theory suggests that fluorescence may play a role in courtship behavior, allowing one scorpion to tell whether a near by scorpion is of the same species and/or of the opposite sex (Kloock 2008). This would allow, from a distance, a scorpion to decide to approach (or be approached by) another that is either of the same sex or of a different species – either would be futile in a mating attempt. A negative photoresponse is observed in scorpions suggesting that the cuticle may serve as a photodetection device, however, it is not clear that fluorescence plays any role in this. One study showed that prolonged exposure to ultraviolet light caused a decrease in fluorescence (Kloock 2009).
Although the exact function has yet to be elucidated, mechanism by which fluorescence occurs in scorpions is relatively well understood. The scorpion’s exoskeleton is made from chitin, like other invertebrates. The compounds that fluoresce are found near the surface of the cuticle and relatively recently two molecules (4-methyl-7-hydroxycoumarin and beta carboline) have been isolated, both of which fluoresce in the presence of ultraviolet light (Stachel et al. 1999; Frost et al. 2001). Interestingly, Polis points out in his book The Biology of Scorpions that scorpions that have newly undergone ecdysis do not exhibit total fluorescence until 48 hours thereafter.
Fluorescence is observed in many life forms. Some of their functions are understood and some remain a mystery. Significant progress has been made with respect to that of scorpions, but more work is needed to fully understand the function thereof. For anyone who is interested in a deeper understanding of any of the topics discussed here, please explore some of the books and papers referenced below.
Blass, G. R. C & Gaffin, D. D. 2008. Light wavelength biases of scorpions. Animal Behaviour, 76, 365-73.
Gaffin, D. D., Bumm, L. A., Taylor, M. S., Popokina, N. V., & Mann, S. 2011. Scorpion fluorescence and reaction to light. Animal Behaviour, 83, 429-36.
Fleissner, G. & Fleissner, G. 2001. Night vision in desert scorpions. In: Scorpions 2001; In Memoriam Gary A Polis (Ed. by V. Fet & P. A. Selden), pp. 317-324. Burnham Beeches, Bucks: British Arachnological Society.
Frost, L. M., Butler, D. R., O’Dell, B. & Fet, V. 2001. A coumarin as a fluorescent compound in scorpion cuticle. In: Scorpions 2001; In Memoriam Gary A Polis (Ed. by V. Fet & P. A. Selden), pp. 363-368. Burnham Beeches, Bucks: British
Kloock, C. T. 2005. Aerial insects avoid fluorescing scorpions. Euscorpius, 21, 1-7.
Kloock, C. T. 2009. Reducing scorpion fluorescence via prolonged exposure to ultraviolet light. Journal of Arachnology, 37, 368-370.
Machan, L. 1968. Spectral sensitivity of scorpion eyes and the possible role of shielding pigment effect. Journal of Experimental Biology, 49, 95-105.
Polis, G. A. 1979. Prey and feeding phenology of the desert sand scorpion Paruroctonus mesaensis (Scorpionida: Vaejovidae). Journal of Zoology, 188, 333-346.
Rao, G. & Rao, K. P. 1973. A metasomatic neuronal photoreceptor in the scorpion. Journal of Experimental Biology, 58, 189-196.
Stachel, S. J., Stockwell, S. A. & Van Vranken, D. L. 1999. The fluorescence of scorpions and cataractogenesis. Chemical Biology, 6, 531-539
Zwicky, K. T. 1968. A light response in the tail of Urodacus, a scorpion. Life Sciences, 7, 257-262.