Re: What are the glowing, gnat-like insects seen once a year?

Well research into your glowing gnat revealed that there are in fact flies/
gnats that bioluminesce as larvae and as adults. They use the light to 
attract other insects to the light and then eat them. 

Below are websites that further describe the "glow worm" in Tennessee and 
another more scientific report about fungus gnats from scientist John 
Sivinski. Because we spend more and more of our time indoors, once familiar 
insects are rediscovered when we return to the outdoors.  I am glad you are 
curious about these glowing insects and find out more about them by visiting 
these websites.
"Glow Little Glow worm" was a popular song a few years back!
 
Glowing gnat reported in Tennessee, Tennessee Conservationist

http://www.state.tn.us/environment/tn_consv/archive/bluelights.htm^

Glowing gnats in Florida
John M. Sivinski
http://www.fcla.edu/
FlaEnt/fe81p282.html

Center for Medical, Agricultural and Veterinary Entomology
Agricultural Research Service
U.S. Department of Agriculture
Gainesville, FL 32608

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).

Key Words: Mycetophyllidae, Orfelia, fungi, prey-attraction, aposematism