The spider that turns fireflies against one another

To round off this month’s Naked Genetics, it’s time to explore another quirk of evolution...

You join me at dusk in Hubei province in central eastern China. It's a part of China known best for the Three Gorges Dam, the city of Wuhan, and thousands of lakes. In summer, temperature sits at around 24-30 degrees Celsius, with a decent dash of humidity thrown in too. Very pleasant conditions for one of my favourite organisms: the firefly.

Who doesn’t love a firefly? You can look at them, you can put them in a jar and read a book. YIf you wanted to. That’s pretty much it. But there is one species in these here hills that look at these ethereal enchanting lights and sees the opportunity for an easy meal. In steps the orb-weaver spider Araneus ventricosus…

It’s an unassuming creature. Grey-brown body, a few centimetres across, scattered across much of east Asia. Visually, a bog standard spider. Every evening, it goes about the business of forming its web, and hunkering down for a night of sky fishing. But the most successful fishers use a lure…

Fireflies aren’t just flashing because they can. I certainly would, but that's probably why I haven’t been blessed with such power. No, firefly flashes are a means of communication, and obviously that includes communication between males and females. It's also apparently used to warn away bats in some species, which is pretty wild but a story for another time. Fortunately for us, its a very simple system. If you’re a female you flash once, if you're a male you flash twice. And naturally females are often inundated by many would-be suitors once they reveal themselves.

And I’m sure most of you are ahead of me by now. A firefly gets tangled in the spiders’ web. But, instead of consuming it straight away, the spider waits. Because the firefly continues to flash. But this is where it gets a little odd. Researchers at Huazhong Agricultural University took a census of the fireflies found flashing in spiderwebs. And they’re nearly all male. Now, given what we know, that doesn’t add up. Why would male fireflies dash to the location of a fellow competitor? Well, for some reason which cannot yet be fully explained, when stuck in a web and in the presence of a spider, a trapped male firefly changes its signal from two flashes to one.

Is this one last act of spite from the doomed male? If I can’t live to mate, no-one can? Is this a very unfortunate case of mixed signals? Perhaps the firefly for help is a very similar flash to that of I am a female, let's mate. Probably not to either of those.

In fact, the most prominent theory is that the spider is manipulating the fly. The researchers postulate that the spider venom actually changes the flashing pattern of the male. But just how that works is unclear, and what a cosmic miracle it would be for a random venom to be the perfect means of luring in more dinner. Behaviour alteration isn’t unheard of in the animal kingdom, that’s why rabies victims suddenly become more bitey, and why that stray cat was so friendly all of a sudden. But these are from internal parasites, not from an external source.

Also, is this technically tool use? Using an external object as a means of manipulating the world to the wielder’s benefit? That’s a remarkable leap in cognitive function for a spider the size of a golf ball.

For now, this act of master manipulation remains a mystery. But, next time you’re lucky enough to see fireflies, just remember: they aren’t always what they seem.