Sometimes I think the biological literature needs more natural history reports. Sometimes I get to read such reports and I’m not sure that I wanted to know what they had to tell me.
To wit: there are two species of beetle, Epomis circumscriptus and E. dejeani, whose larvae eat frogs (and other amphibians). They do so in a rather unusual way: they don’t kill them and then chew them up, but rather attach to them like a leech and suck them dry. And they actively lure in their prey by twitching their antennae and mouthparts (in a manner that I assume mimics the movement of smaller insects like ants; at least, that’s what it looks like to me – see video S1 at this link). When the frog tries to chomp them—and frogs can move really fast—the beetle larva moves even faster and latches on to the frog’s mouth or belly and just doesn’t let go. In fact, it has double-pronged mouthparts for doing so. The frog can’t get it off, and is just gradually sucked dry.
What is even more cool/horrifying is how resilient the beetles are. Many beetle larvae are soft-bodied; these, instead, are tough enough to endure the frog’s pawing at them and trying to swallow them. One, in fact, does get completely swallowed. And it stays alive, squirming in the frog’s belly for two hours, before the frog pukes it up. And then it eats the frog.
There’s a video of this (it’s video S4). I watched it twice in a row, and sat there with my mouth hanging open, unsure whether to shout “holy shit that’s cool!” or just whimper. I just watched it again and I still can’t decide.
Okay, deep breaths; serious science blogging time.
I am curious to know why the frogs (apparently) eventually stop trying to dislodge the larvae and just let themselves be drained. Are the larvae injecting them with some toxin (I’ve never heard of a beetle doing that)? Or just weakening them to the point where they can’t fight back? Or, in nature, do they usually drop off before the host dies?
Also, does this really constitute “role reversal” as the paper defines it? It seems to me that the beetle larvae are mimicking some other potential frog prey, and amphibians typically look for movement when hunting, whereas the larvae are only moving their mandibles and antennae. Although some of the frog species tested eat close relatives of Epomis beetles, perhaps this is not a case of a typical prey becoming a predator (if the ancestors of these two species are sit-and-wait predators that normally eat other creatures) but rather just of the origin of a new predatory behaviour. But right now there’s probably not enough information to evaluate either hypothesis, and regardless, I don’t mean to detract from this truly cool discovery. (Yes, I’ve come down on the side of amazement rather than horror in the end.)
Wizen, G., & Gasith, A. (2011). An Unprecedented Role Reversal: Ground Beetle Larvae (Coleoptera: Carabidae) Lure Amphibians and Prey upon Them PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0025161
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