Monsoon time in southern Arizona brings moisture to the dry desert and a burst in biological activity, including ants, butterflies and all sorts of beetles. This week, we visited Madera Canyon in the Santa Rita Mountains just south of Tucson (a traditional nighttime collecting spot for entomologists). There, we found 3 genera of bioluminescent beetles (Pleotomus in the family Lampyridae and Stenophrixothrix and Distremocephalus in the family Phengodidae). I’ve seen 2 more luminescent beetle genera there, Zarhipis (family Phengodidae) and Vesperelater (family Elateridae, a striking click beetle with two super-bright glowing spots on its pronotum). This beetle is Stenophrixothrix fuscus (inset, photographed in daylight and below in a darkroom to visualize it’s luminescent lamps). Stenophrixothrix fuscus has 8 pairs of lamps (technically photic organs) down its side and headlamps embedded anteriorly in its head capsule, shining through its translucent exoskeleton. The function of bioluminescence in these beetles is uncertain, hypotheses (nearly all remain untested) are aposematism, kin density recognition, prey luring, and mimicry. A nice article reviewing the form and possible functions of luminescence in beetle larvae is by Sivinski (1981). The light-generating organs contain the biochemical source of luminescence, which is luciferase, luciferin, ATP and oxygen. The photic organs are surrounded by a network of tracheae to supply air to the oxygen-hungry light reaction (the tracheae in the photo appear as fuzzy edges of the lateral light organs).

Here are 2 movies: the first shows S. fuscus in LED light that is quickly turned off to reveal the beetle illuminated by its own light. The second shows S. fuscus dimming its posterior 8 photic organs, the headlamps always seem to stay on, and then turning them back on (anterior to posterior) after I blew a puff of air on its back. [They’re large movies > 45 MB, and they might take awhile to download, but they’re in HD so you can watch on your flat-screen TV at home!]