Fireflies / Glowworms

A reader posted a comment that requested, “Tell us more about glowworms!!”  So, Clare…  here goes…

GlowwormFireflies aren’t flies at all; they are beetles.  Like all beetles, they undergo complete metamorphosis – a 4-stage lifecycle.  Eggs are laid in the soil in summer.  Larvae hatch and are active on and underground until the following spring when they will pupate.  Because the larvae, like the adults, are luminous, they are called glowworms.  Sometimes eggs, and adult wingless females (of some species – others have winged females) will also be called glowworms.  Adults emerge in summer to mate and lay more eggs.

Winter Fireflies - by Seabrooke LeckiClassified under the Lampyridae family, there are around 200 species of fireflies in North American, and over 2,000 worldwide.  Most of the U.S. fireflies seem to be found east of Kansas.  Interestingly, there are some species in this family that are diurnal, rather than nocturnal and therefore are not bioluminescent.  The photo here is from Seabrooke’s Flickr site.  She wrote about these diurnal fireless fireflies in her blog in March.  Sometimes called Winter Fireflies, these critters overwinter as adults rather than as larvae and are often seen on the bark of trees on winter days – if the sun shines warmly enough.

In species that are bioluminescent, there are special light-producing organs in the abdomen that contain “photocytes” – light-producing cells.

The light is created when oxygen combines with a substance called luciferin in the presence of the enzyme luciferase, in special cells called photocytes. – Dan and Susan Mahr

Or, if you want a really scientific explanation, check the link below called Nitric Oxide and Firefly Flashing!

Here’s a great photo from Jenn Orth’s Flickr site showing an adult’s abdomen where the light is produced:

Firefly by Jenn Forman Orth

The flashing patterns of the various species make for interesting studies.  Scientists have found that yes, fireflies use their flashing lights to attract mates… But in some species, the females have developed the ability to mimic the flash pattern of other species and thus lure unsuspecting males.  The poor deceived fellows become lunch.

We won’t be seeing adults for a few months around here where I live… But boy, when summer comes, the fields are just a-glitter with them.  In the meantime, I’ll keep my eyes on the ground for glowworms!

Learn more:

Update 5/12/2013:  A reader let me know that the term glowworm means different things in different parts of the world.  Scroll down to see John’s comment and learn more, and be sure to visit his website, too:

17 thoughts on “Fireflies / Glowworms

  1. Thanks Jennifer.

    I’m somewhat familiar with fireflies, but not the “glowworm” variety. So I’m somewhat more curious about them.

    Am I correct that the glowworm in your hand would be a larviform adult female? Do you know what species or family it would be?

    How visible is the bioluminescence? Steady or flashing like a firefly? Bioluminescence makes sense (to me) as a strategy for a winged animal. They get to fly about in the dark relatively safe from predators while they attract mates, the flash and somewhat erratic flight would keep them safe while still broadcasting their location to a male/female. For a relatively sedate larva or larviform female it would seem, at first blush, to be a poor strategy for survival.

    Now I’m curious.

    • Have recently had a glow worm encounter. Saw wee little “eyes” in the grass, mulch, at night. Presumed was a tiny creature whose eyes were reflecting in the lamplight, as the eyes blinked…, ran inside for a flashlight…discovered an insect, much to my surprise. Thought it might be a glow worm, although it doesn’t look like a worm, looks exactly like the above photo. So, yes, it is a glow worm, but worm is not a very accurate description. Definitely brown cammo beetle color and shape. Was fun to discover, saw quite a few. Putting the light on it took away the mystery and charm. But still fun.

  2. Hi Jennifer – I just found your site through Seabrooke’s site. Being an entomologist, it was this insect post that first attracted my attention, but I’m impressed by the breadth of other natural history subjects you cover. I’ll be adding you to my blogroll.

    I hope you don’t mind my addressing Clare’s questions about bioluminescence and its relation to predation, since I have the answers at my fingertips. The flash (which are pulses rather than steady light) of the larva and larviform females may function as warning in the same way that the bright red/black coloration of the adult males functions to warn would-be predators not to eat them. Many fireflies sequester foul-tasting defensive steroids called lucibufagins that cause vomiting – predators will quickly learn to associate the flash with the taste and avoid them.

    The glowworm in the hand is a larva – the larviform female still has a typical adult appearance only without the wings – that is, the thorax (the segment behind the head) is sheild-like as in the adult male, the antennae are long and many segmented, and the legs are longer with distinct “feet” (tarsi).


  3. You’ve now sent us on a glow worm quest. Any advice on where would be the best place to look? Fields? Damp areas? Woods? We have plenty of fireflies in the summer — the fields dance with their lights — but we haven’t seen the larvae. Very excited to see our first!

  4. K&R – well, the eggs are usually laid in moist areas – damp leaf litter, edges of streams. i found mine right along the path at audubon on the sides of the ponds – right where we see the adults in summer. happy hunting!

  5. Hi Jennifer, I live in Washington State and have never seen a firefly. It is the last thing on my bucket list. I can not get anywhere in June or early July. Is there a type of firefly I can see at some other time of the year? Thanks for the information. Judi

    • I passed your question off to one of the other naturalists at Audubon and he came back with this response:

      There is a national citizen science program called Firefly Watch that keeps tracks of first and last firefly sightings and the species that are out there. Their website is and they probably have the best resources to answer the question.

      I haven’t had time to check the site myself, but hope to soon. Let me know if it helps.

  6. I just want say I found a glow worm n Wisconsin and I lived here on and off for 36yrs and never saw one till now I was fascinated by it so I put it n a container I was gonna call a school to c what it was but Yr site helped but I typed r there glow worms n wis and Yr site came up, I didn’t c anything about being n wis but I think they do lol thanks for the site

  7. I like you article very much and I thought it might interest your readers to know that the term glow-worm refers to different things in different parts of the world. Here in the UK glow-worm refers to the species Lampyris noctiluca which, like related species have females that look very similar to the larvae and do not fly, yet emit a bright steady glow. Conversely in Australasia glow-worm refers to a type of gnat that creates blue light and famously illuminates underground caves with an eerie light. I have more information on my site about the definitions and other details about European fireflies and glow-worms.

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