PHOTOTROPISM, BIOLUMINESCENCE, AND THE DIPTERA

Many arthropods move toward or away from lights. Larvae of certain lum inescent mycetophilid fungus gnats exploit this response to obtain pre y. They produce mucus webs, sometimes festooned with poisonous droplet s, to snare a variety of small arthropods. Their lights may also prote ct them from their own negatively phototropic predators and/or be used as aposematic signals. On the other hand, lights may aid hymenopterou s parasitoids to locate fungus gnat hosts. The luminescence of mushroo ms 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 sugges ts multiple evolutions of the trait. This concentration of bioluminesc ence may be due to the unusual, sedentary nature of prey capture (i.e. , use of webs) that allows the ''mimicry'' of a stationary abiotic lig ht cue, or the atypically potent defenses webs and associated chemical s might provide (i.e., an aposematic display of unpalatability).