Daytime Fireflies

2011 September 10

We caught a number of these soft-winged beetles in broad daylight on July 12, 2011. They were just about a centimeter long, and pretty easy to catch and photograph. Which implies that they aren’t accustomed to fleeing from predators. Which further implies that they are not good to eat.

These look to me like a type of firefly, family Lampyridae[1]. They had a very large, yellow-orange shield structure on their pronotum that is big enough to tuck their heads under; long, distinctly-segmented antennae; and one of them had a projecting abdomen with a rudimentary light-emitting organ:

But, note that I said the light organ was “rudimentary”. They were flying and (presumably) mating during the day, which means that flashing lights would have been pretty ineffective for attracting the attention of mates (which is what most adult fireflies use their light organs for). So, this is obviously one of the species of “diurnal fireflies” that have taken to flying in the daylight and have given up most of their ability to make flashes of light.

I think they are probably one of the species in the genus Lucidota, which are day-flying fireflies with greatly reduced (and sometimes missing) light organs. I’d even go so far as to say they are most likely Lucidota atra, the Black Firefly. Reading up on them, it looks like their larvae have a glowing tip on the abdomen, and the adults may flash a few times when they come out of their pupa, but that’s about it.

It would seem that giving off light would be a real draw for predators, but I understand that fireflies contain a lot of nasty and distasteful chemicals, and so the flashing light is probably warning coloration. This is more important for the larvae (glowworms[2]), which are predatory grubs that live in the leaf litter, where it is dark. They can’t just be brightly colored as a warning that they are toxic, like the other toxic bugs do, because that doesn’t do much good if it is so dark the predators can’t see you. So, the glowworms have hit on the approach of giving off their own light, just to be sure that they are making their point to any toad or other night-time predator that might happen by. In fact, here’s an article (pointed out to me by Carole the day that this originally posted) about determining that toads dislike glowworms, and do learn to avoid eating prey that glows

[1] I’ve been wanting to take the girls out to catch the actual light-emitting fireflies at night, except for one small detail: during firefly season, it doesn’t get dark here until 10:30 (partly because we are pretty far north, but mostly because of some foolishness with the way that our time zone is laid out. Geographically we should be smack in the middle of the Central time zone, but for political reasons practically all of Michigan is on Eastern time instead). We do have the light-emitting fireflies here, I’ve seen them from time to time. Not in large quantities, mind you, but one or two at a time. it’s just a hassle to stay up late enough at night to see them.

[2] There’s a song about glowworms[3]. Maybe you’ve heard it. A lot of performers just play it as an instrumental piece, I actually didn’t know what the words were until I looked it up just now.

[3] Not to be confused with inchworm, a completely different song. Or with Spanish Flea, for that matter (which I used to think was named “Petrified Tree” when I was very young)

3 Responses
  1. September 11, 2011

    Semi-on-topic: Here is the chemistry for firefly glowing. For me, the key part was this:

    “The very high quantum yield for this process (in alkaline solution, nearly each reacted LH2 molecule emits a photon) reflects not only efficient catalytic machinery, but also a highly favorable environment for the radiative decay of an electronic excited state.”

    I had wondered how they managed to keep blasting away with those lights as each flash must consume some chemical in the reaction. The answer is that the reaction is very efficient. The reactants must be recreated during the day while they sleep.

    Great photos as usual. I enjoyed this post because it made me want to click around and learn more stuff. Yay!

  2. Della3 permalink
    November 7, 2011

    KT Cat: Thanks for finding the answer to a question I have long-harbored as well.

  3. Lyle Laylin permalink
    March 8, 2018

    Here’s where I know “Glow Worm” from

Comments are closed.