Phototropism, Bioluminescence, and the Diptera

Abstract

Many arthropods move toward or away from lights. Larvae of certain luminescent mycetophilid fungus gnats exploit this response to obtain prey. They produce mucus webs, sometimes festooned with poisonous droplets, to snare a variety of small arthropods. Their lights may also protect them from their own negatively phototropic predators and/or be used as aposematic signals. On the other hand, lights may aid hymenopterous parasitoids to locate fungus gnat hosts. The luminescence of mushrooms can attract small Diptera, and might have evolved to aid mechanical spore dispersal. Among Diptera, bioluminescence is found only in the Mycetophilidae, but the variety of light organs in fungus gnats suggests multiple evolutions of the trait. This concentration of bioluminescence may be due to the unusual, sedentary nature of prey capture (i.e., use of webs) that allows the “mimicry” of a stationary abiotic light cue, or the atypically potent defenses webs and associated chemicals might provide (i.e., an aposematic display of unpalatability).