Arizona Bark ScorpionCited field reference
Biology

Why bark scorpions glow under UV light

Direct answer

Compounds in the scorpion's outer cuticle absorb ultraviolet light around 395 nm and re-emit it as visible cyan-green light near 500 nm, so a UV flashlight makes a bark scorpion shine from yards away.[1][2] Nearly every scorpion species does this, and a freshly molted scorpion does not until its new cuticle hardens.[2]

By Devon "The Scorpion Guy" at ArizonaBarkScorpion.orgPublished 2026-09-12Last reviewed 2026-09-12Editorial policy
Excitation and emission of scorpion cuticle fluorescence A wavelength bar from 350 to 650 nanometers. Ultraviolet excitation peaks near 395 nanometers; the visible glow is emitted near 500 nanometers, a cyan-green. 350 nm500 nm650 nm UV excitation ~395 nm Emission ~500 nm (cyan-green) Figures from Agnew et al. 2026 (JIPM). Freshly molted scorpions do not fluoresce until the new cuticle hardens (ASU Ask A Biologist).
Figure 1. Excitation and emission wavelengths of scorpion cuticle fluorescence.[1]
Arizona bark scorpion under UV light on a block wall, holding and feeding on a captured insect
What a 395 nm flashlight shows: the cuticle fluoresces cyan-green against a block wall that stays dark. Field photograph, Phoenix area. Photo: Devon "The Scorpion Guy", arizonabarkscorpion.org. Field observation, Phoenix area.

What makes them glow?

Fluorescent compounds in the hyaline layer of the exocuticle. The 2026 JIPM review names a beta-carboline and 4-methyl-7-hydroxycoumarin among the candidates and gives peak excitation near 395 nm with emission near 500 nm.[1] Arizona State University's Ask A Biologist is more cautious, calling the fluorescing substance not fully identified.[2] The glow is a property of the cuticle itself: a shed exoskeleton glows too, and preserved specimens keep glowing for decades.[2]

Why did scorpions evolve it?

Nobody knows for certain. Hypotheses include detecting moonlight and starlight over the whole body surface (the "whole body as an eye" idea reported by National Geographic), attracting prey, sun protection, and a side effect of cuticle chemistry with no function at all.[3][2] The question is open.

Do all scorpions glow?

Nearly all species tested do, so fluorescence does not identify a bark scorpion and says nothing about how dangerous a scorpion is.[2][1] The exceptions are recently molted individuals, which stay dark for several days while the new cuticle hardens, and very young scorplings.[2]

How do people use this?

Pest-management professionals and homeowners walk the property after dark with a UV flashlight (365 to 395 nm) and look for the glow along block walls, foundations, and baseboards.[4][5] Extension guidance says scorpions "can easily be located from a distance of several yards."[6] Two cautions: never look into the UV source, and remember that the molt blind spot means a sweep undercounts juveniles.[2]

Does UV hurt them?

Prolonged UV exposure does bleach the fluorescence in specimens, and some studies suggest scorpions avoid UV light, which fits the light-detection hypothesis.[3] A few seconds under a flashlight is not known to harm them.

What is not knownThe exact identity of the fluorophores and the evolutionary function of the glow.[2]
Cite this pageDevon "The Scorpion Guy" at ArizonaBarkScorpion.org. "Why Arizona Bark Scorpions Glow Under UV (Blacklight)." 2026-09-12. https://arizonabarkscorpion.org/uv-fluorescence/

Related

Sources for this page

  1. Agnew J, Bowers K, Bundy S, Romero A. Distribution, biology, and management of medically relevant bark scorpions, with emphasis on Centruroides sculpturatus. Journal of Integrated Pest Management 17(1), 2026. academic.oup.com/jipm/article/17/1/pmag029/8724371Tier 1
  2. Arizona State University, Ask A Biologist. Ultraviolet Scorpions. askabiologist.asu.edu/ultraviolet-scorpionTier 2
  3. National Geographic. Why do scorpions glow in the dark (and could their whole bodies be one big eye)? www.nationalgeographic.com/science/article/why-do-scorpions-glow-in-the-dark-and-could-their-whole-bodies-be-one-big-eyeTier 2
  4. University of Arizona ACIS Community IPM. Scorpions (Public Health IPM). acis.cals.arizona.edu/community-ipm/public-health-ipm/scorpionsTier 1
  5. Gouge DH, Li S, Bibbs C, Nair S. Scorpions of the Desert Southwest United States. University of Arizona Cooperative Extension, publication AZ1768, 2018. acis.cals.arizona.edu/community-ipm/community-ipm-output/publications/publications-view/scorpions-of-the-desert-southwest-united-statesTier 1
  6. Schalau J. Scorpions. University of Arizona Cooperative Extension, Backyard Gardener #184, June 13, 2024. www.extension.arizona.edu/sites/extension.arizona.edu/files/attachment/Scorpions.pdfTier 1