The common name for this fantastic beast is the Douglas-fir glowworm. I know, it doesn’t look like a worm. But, believe it or not, the female of the species does!

Pterotus obscuripennis is in the firefly family, and both males and females glow as larvae. When the males metamorphose, they gain wings and moose-like, antlered antennae, but lose their luminance. The females remain in larval form, and they retain their radiance. They live only on the West Coast. 

There are so many questions and few satisfactory answers about these fascinating beetles. A 1979 paper says, “the function of luminescence in beetle larvae is a mystery.” A more recent paper concludes it’s likely a defensive warning signal, but doesn’t discount a dozen other possible functions. Then there’s the fact females don’t gain wings: How do they benefit by not being able to fly, but having a glow? 

The males’ exaggerated, branched antennae have chemical sensors to help them hone in on the females, who send out pheromones from their ground-level locations. They are more visible after a rain, which is when I saw this one. Perhaps pheromones travel better in humid air? 

Fireflies across the country are in danger due to light pollution and reduced habitat. Pterotus obscuripennis would seem to be in more peril than most, because the females can’t travel very far if their habitat is damaged, but they’re not considered endangered. 

Stay curious!

See more of Karen’s work here.


Xerces Society article on fireflies: Accessed 6/15/22.  

Research paper on luminescence: Accessed 6/15/22.

A more recent (2019) paper on why larvae glow: Accessed 6/15/22.