Every June, fireflies light up the Smoky Mountains with their exuberant courtship displays – sure, it all seems very romantic. But a few years ago I spent some weeks with colleagues studying the predators that take advantage of these dense breeding aggregations. We knew that certain insectivores – like birds and lizards – avoid eating fireflies because they’re toxic. So we didn’t expect to find many predators enjoying this luminous feast. Little did we realize how gore-filled that summer would turn out to be!*
* You can read our Dark Side paper here: Lewis, S.M., L. Faust, and R. De Cock. 2012. The Dark Side of the Light Show: Predators of Fireflies in Great Smoky Mountains. Psyche, 634027.
We were surprised to discover lots of invertebrate predators enjoying some “light snacks” (clockwise from upper left: a wolf spider, a harvestman, a hangingfly, and an assassin bug). It remains a mystery how these predators manage to deal with firefly toxins.
We also decided to dive in for an in-depth look at the culinary habits of New World fireflies’ biggest natural enemy – a fearsome predator that just happens to be another type of firefly!
Most fireflies give up eating once they become adults. But the predatory Photuris fireflies are big, agile, and voracious. They come out at night, and they specialize in hunting other fireflies. Unable to make their own protective chemicals, they resort to attacking other fireflies then drinking their prey’s toxin-laden blood. (Check out the amazing story of these vampire fireflies told in Silent Sparks, Chapter 7: Poisonous Attractions.)
The question we set out to answer that June was: Do these predatory Photuris fireflies have particular dietary predilections? Or perhaps they find all fireflies to be equally tasty? For this study, we collected some predatory Photuris females and put each one into a quart-sized
container we got from the Kroger’s deli counter (this was a low-budget study). Because we needed to do fieldwork every night, we rigged up lights inside a tiny closet under the stairs to create a reversed day-night cycle – inside the closet, darkness fell at 11 am. Every “night” we offered each Photuris predator a choice from a rotating menu of different firefly species. For about two weeks, I woke up, grabbed some coffee, then sat inside the dark closet for an hour to record what transpired inside each container.
What did we find? When fireflies were on the menu, the predatory Photuris fireflies attacked then chowed down on lots of different kinds, although they ate not a single one of the various flies, click beetles, grasshoppers, and bugs we offered them.
Among the tastiest items were the Smokies synchronous fireflies, Photinus carolinus. Within 24 hours, the predators attacked & consumed ~75% of this prey type (shown in the graph below, left-most bar: numbers above the horizontal axis indicate how many individuals of each prey type were offered during our experiment).
While the Photuris predators – which are active at night – don’t typically encounter any day-active fireflies, when given the chance they readily consumed these (shown as white bars in the graph above).
We got a big surprise when we offered four different types of dusk-active fireflies as prey (grey bars above). If the prey belonged to either Photinus marginellus or P. macdermotti, more than 60% of them got eaten within 24 hours. Yet the Big Dipper fireflies, Photinus pyralis, rarely got eaten. These particular fireflies (along with P. brimleyi) seemed immune – 90% of them survived!
What’s the secret to the Big Dipper’s astounding success? Somehow, they were managing to evade these fearsome predators capable of taking down so many other fireflies. My in-closet observations revealed Big Dipper’s surprisingly successful anti-predator strategy. It wasn’t that they avoided attack – I watched as Photuris predators pounced on Big Dipper males just as often as they pounced on all the other fireflies.
The secret seems to be something in the Big Dipper’s blood. Like many other poisonous insects, fireflies leak droplets of whitish blood when they’re attacked, a behavior called reflex bleeding. But Big Dipper blood is unusual – rather than remaining liquid once exuded, their blood coagulates very quickly. So when a predator sinks its jaws into a Big Dipper firefly, it gets a messy mouthful of sticky blood. All glued up, the predatory Photuris needs to stop and clean off its mouthparts, in the meantime giving its prey a chance to escape.
Stuck in the closet, that summer I got quite memorably immersed in the dark side of fireflies, witness to violence, blood and gore. And I’ve never been able to look at fireflies the same way since.
Bonus: We also made this cool video, featuring original music by Raphael De Cock’s Griff Trio!