What is Bioluminescence?
Bioluminescence (literally “living light,” or “the production and emission of light by a living organism as the result of a chemoluminescence reaction during which chemical energy is converted to light energy”) consists of two types: a) luminescence, and b) phosphorescence. In the former, the release of light lasts less than 10 nanoseconds while the latter lasts longer than 10 nanoseconds, typically 100 nanoseconds.
While some insects are blind (i.e., all larvae and termites), various other insect species have three (like humans), or even four or five, color receptors and can produce the entire range of visible light color: red, orange, yellow, green, blue, indigo, and violet. Notably, the click beetle can even produce different colors (Wood 2005), but the range of insect vision seems limited from yellow to ultraviolet. “They cannot see red or orange,” according to the University of Florida.
In the case of fireflies, visible spectrum monochromatic light is produced inside the podocytes “via a chemical reaction consisting of Luciferin (a substrate) combined with Luciferase (an enzyme), ATP (adenosine triphosphate) and oxygen,” according to “Firefly Facts” at Ohio State University. It is thought that they use this bioluminescence as a warning to predators and as an intra-species form of courting/mating where the males flash flying above to on looking females in the grass below (Sivinski, 1981). Interestingly, the female Photuris versicolor attracts male fireflies in order to eat them.
The following, though not an example of bioluminescence, is the iridescent Blue Morpho of Costa Rica which has color vision (and can see blue) and it is highly territorial, so it may use the light scattering (i.e., structural color) properties of its wing as a way to preserve distance or spacing of its species, given the darkness in the deep undergrowth of a tropical canopy; its striking color might also be useful for mate identification, but it is not bioluminescent, merely an optical phenomenon of multiple reflected light depending upon the observation angle.
As for sand scorpions, technically they are not bioluminescent (and which would require oxygen), since they only fluoresce under external exposure to UV light at 365 nm. Little is known about the unidentified substance (although it may be a flavin or flavones) in the hyaline layer of the cuticle which causes it. Speculation ranges from 1) a form of sun block, signaling dangerous levels of radiation (which seems unlikely, given that they are nocturnal today); 2) assists somehow in feeding/attracting prey; 3) useful in attracting a mate; and 4) perhaps best of all, it “plays no biological role” at all, according to Fasel et al. (1997). Given that scorpions are now land-based “crabs” from the middle Silurian more than 400 million years ago, then their blue-green fluorescence is quite possibly an artifact of a biological adaptation from the deep seas (N. Richardson, personal communication, 20 May 2008).
A. Fasel; P.-A. Muller, P. Suppan, and E. Vauthey, “Photoluminescence of the African Scorpion ‘Pandinus Imperator’,” Journal of Photochemistry and Photobiology 39 (1997): 96-98.
“Firefly Facts” at http://iris.biosci.ohio-state.edu/projects/FFiles/frfact.html (accessed 20 May 2008).
John Sivinski, “The Nature and Possible Functions of Luminescence in Coleoptera Larvae,” The Coleopterists Bulletin 35 (no. 2, June 1981):167-179.
University of Florida, Principles of Entomology ENY 3005, “Insect Vision and Communication” at http://basicentomology.ifas.ufl.edu/lab6.pdf (accessed 21 May 2008).
K. V. Wood, Y. A. Lam, W. D. McElroy, and H. H. Seliger, “Bioluminescent Click Beetles Revisited,” Journal of Bioluminescence and Chemiluminescence 4 (no. 1, 2005): 31-39.