A black light is a specialized lamp that emits long-wave ultraviolet radiation (UV-A light), which is largely invisible to the human eye. This UV-A radiation causes fluorescence, where materials absorb the high-energy light and re-emit it almost instantaneously as visible light. This unique property reveals a hidden, glowing world within many biological structures, particularly the exoskeletons of numerous arthropods.
The Science of Fluorescence and UV Light
The visible glow observed under a black light is a physical process. Fluorescence occurs when a molecule, called a fluorophore, absorbs a photon of ultraviolet light, causing an electron to jump to a higher energy level. This excited state is unstable, and the electron immediately returns to its ground state, releasing the excess energy as a photon of visible light, which is always a longer wavelength than the absorbed UV light.
This quick energy release differentiates fluorescence from phosphorescence, which involves a slower, delayed emission of light that continues after the UV source is removed. Black lights are typically UV-A lamps, emitting light in the 365 to 400 nanometer range. This wavelength is highly effective for exciting fluorophores in biological materials without the hazards of shorter-wave UV-B or UV-C light.
In arthropods, the glow is a chemical byproduct of their structure, not a conscious action. The fluorescent compounds are often nitrogen-containing molecules integrated into the cuticle or exoskeleton.
Common fluorophores include pteridines, which absorb UV light and re-emit blue light, and structural proteins like resilin. Resilin is an elastomeric protein that provides flexibility to joints and wing hinges, and it exhibits a bright blue fluorescence when exposed to UV light.
Specific Creatures That Glow Under UV Light
The most famous arthropod to glow under black light is the scorpion, an arachnid, not a true insect. All species fluoresce a brilliant blue-green color due to compounds, such as beta-carboline alkaloids, embedded in the hyaline layer of their exoskeleton. This glow is remarkably durable, with the cuticles of fossilized scorpions, dating back millions of years, retaining the ability to fluoresce.
The function of the scorpion’s glow is still debated. One theory suggests their entire cuticle acts as a large sensor to detect UV light levels. By converting invisible UV light into a visible blue-green glow, the scorpion may gauge whether it is adequately sheltered before emerging for nocturnal activity. The fluorescence is not present immediately after molting, appearing only as the new exoskeleton hardens and the chemical compounds mature.
Many true insects in the order Lepidoptera (moths and butterflies) also exhibit fluorescence, particularly in their immature stages. The larvae of species like the Black Hairstreak butterfly and the Polyphemus moth caterpillar glow a bright blue or green. Researchers often use this glow to spot the well-camouflaged caterpillars against the foliage of their host plants, which fluoresces a deep red due to chlorophyll.
The fluorescence in caterpillars may function as an aposematic signal, warning predators that the larva is chemically defended or unpalatable. Some species of katydids, which are typically green and camouflaged, also fluoresce blue under black light. The glow in these insects is concentrated in the structural components of their bodies, making them stand out when exposed to a UV source.
Other arthropods, including members of the Myriapoda class, also glow. Millipedes, like the cherry millipede, often fluoresce a bright blue or lime green from their cuticle and legs, caused by pterins and coproporphyrins. This glow may be an accidental byproduct of their nitrogenous waste products, or it could serve as a visual signal to advertise defensive chemicals like cyanide.
Some centipede species have been observed to show fluorescence, often concentrated in the terminal legs and antennae. Praying mantises are also subjects of UV photography, indicating their exoskeletons contain fluorescent compounds useful for researchers to document their anatomy.
Practical Uses for UV Light Observation
The ability of certain arthropods to fluoresce under UV light provides practical utility in pest management and ecological research. For homeowners and pest control professionals, using a black light is the most efficient way to locate scorpions, which are otherwise nearly invisible in the dark. The bright blue-green glow immediately reveals their location, allowing for easy removal or treatment.
In agricultural settings, UV light is a component of many insect traps used for monitoring and reducing populations of nocturnal flying pests like moths and flies. These traps exploit the strong natural attraction many insects have to the UV-A spectrum for navigation. Researchers are also exploring the use of high-energy UV-C light as a chemical-free method to suppress mites and other crop pests by damaging their DNA.
Ecological research has adopted black light technology as a non-invasive tool for field studies. Detecting the blue or green glow of rare butterfly larvae, such as the Black Hairstreak, at night allows scientists to survey populations with greater accuracy and speed than traditional daylight methods. This rapid identification is useful for conservation efforts, providing a clearer picture of a species’ distribution and abundance.