I recently received a question about the use of technology in ecological research, which led to a discussion about blacklights. I wrote a column 5 years ago about a nighttime adventure in a southwestern desert involving black lights. It answered some of the reader’s questions and bears repeating.

When Randy Babb handed us flashlights in the middle of the night in the Sonoran Desert we were not surprised. It was summer. The desert sands had cooled down to the mid-90s. We figured we would have to watch for rattlesnakes. But when we turned the lights on, they emitted a purple glow, not a beam of white light. Randy said he had a surprise for us. We were going to look for scorpions.

Randy, a naturalist with the Arizona Game and Fish Department, knows more about southwestern wildlife than anybody else I can think of. He, my son, my grandson and I were on a nighttime excursion into the desert. Our ultraviolet flashlights (blacklights) were the only illumination within 25 miles, the distance to the nearest town. We had barely stepped away from Randy’s Land Rover before we saw the first one—a glowing scorpion a hundred feet away. The eerie bluish glow looked alien beneath a creosote bush. We walked up to the large scorpion and peered down at it. Our presence did not seem to concern it. It never moved. After 15 minutes of wandering around in the desert night with our blacklights, we had located more than a dozen glowing scorpions. Each was beneath the cover of a bush of some sort, minding its own business.

The explanation for why scorpions glow in the dark when you shine a UV light on them is not completely understood. Scorpions have hard shells like crawfish, and the fluorescent property is in the shell or cuticle. When scorpions shed their skin, which they do periodically, they do not glow until the cuticle grows back. The persistence of the fluorescent molecules is measured not in days or years, because even scorpions preserved for decades in museums continue to glow. In fact, a few fossil scorpions millions of years old have been found in which the cuticle continues to glow. Part of the physical explanation for the phenomenon is that the short wavelength UV light is invisible to humans, but when it reflects off of fluorescent molecules it assumes a longer wavelength that we are able to see.

Whether glowing in response to UV light has a purpose for the scorpion is even less clear, although scientists always enjoy speculating. Hypotheses include dispelling or confusing predators, protecting the scorpion from sunlight or serving as some kind of scorpion signal for other scorpions. Maybe it is just something our technology revealed and it serves no purpose. Whatever the biological function of fluorescent properties, if there is one, it has nothing to do with people.

Other animals also glow under UV light. Kurt Buhlmann and I were once using blacklights in a forest as we looked for glowing specks of fluorescent powder he had placed on baby turtles during a field experiment. The powder gradually falls off as the turtles move through the leaf litter allowing us to follow their shining trails. Kurt and I were surprised when we saw a bluish glow on the forest floor. Much larger than any of the powder flecks we were tracking, it turned out to be a cherry millipede, more than 2 inches long, ambling across the forest floor. It glowed like the scorpions.

Special lighting and technological manipulations have revealed many other animals that display colors otherwise invisible to the naked eye—or at least that are not the colors we normally see. Not surprisingly, people interpret the natural world based on what we can perceive. Using a UV flashlight to observe fluorescence in scorpions reminds us that life on earth has many facets we have not yet revealed.

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