Epomis beetle larvae look delicious to frogs. They’re snack-size, like little protein packs. If a frog is nearby, a larva will even wiggle its antennae and mandibles alluringly.
But when the frog makes its move, the beetle turns the tables. It jumps onto the amphibian’s head and bites down. Then it drinks its would-be predator’s fluids out like a froggy Capri Sun.
We tend to think of food chains moving in one direction: Bigger eats smaller. But nature is often not so neat. All around the world, and maybe even in your backyard, arthropods are bodying vertebrates and gobbling them up.
Jose Valdez, soon to be a postdoctoral researcher at the German Centre for Integrative Biodiversity Research, identified hundreds of examples of this phenomenon in the scientific literature, which he detailed in a review published in July in Global Ecology and Biogeography. He and others who study the topic think that once the initial gasp of shock is past, it’s important to understand what eats what.
Dr. Valdez became interested in these role reversals during his doctoral research, after watching a gang of water beetles devour a rare tadpole. The larvae were known predators, whereas the adults were widely considered to be scavengers. But Dr. Valdez developed a hunch, borne out by further research, that they were actively hunting vertebrates, too.
He got a similar feeling when, while reading the news or surfing YouTube, he saw other bugs punching above their weight: a huntsman spider savoring a pygmy possum, a praying mantis chewing off a gecko’s face.
“Maybe this is not so rare,” Dr. Valdez remembered thinking.
Dr. Valdez found 1,300 similar examples, which he gathered into a searchable database. The entries cover 89 countries and involve many types of arthropod predator: storied vertebrate-hunters like scorpions and spiders, along with less well-documented cases such as dragonflies and centipedes.
It is a formidable catalog of invertebrate vengeance: A spider snares a songbird in its web, giant water bugs wrestle snakes into submission, fire ants team up to overrun a baby alligator. “Every time I would read a new one I was like, ‘Oh my goodness,’” Dr. Valdez said.
There are few full-fledged studies on the topic; Dr. Valdez built on the work of Martin Nyffeler, a conservation biologist at the University of Basel in Switzerland who has documented spiders eating everything from fish to bats. Another large contribution came from a 1982 literature review by Sharon McCormick and Gary Polis. Many of the matchups that Dr. Valdez added to his database were originally described in brief observational notes by scientists who hadn’t set out to study the subject.
After witnessing his water beetle-tadpole smackdown, Dr. Valdez, too, had written it up as a note. But treating these instances as one-offs might be obscuring a larger ecological significance, he said: “We should see what kind of effect this is having on food webs.”
There could also be conservation implications, said Dr. Valdez. He points to the case of the Devils Hole pupfish. Scientists had trouble breeding the rare species in a lab to save them until they realized that diving beetles — accidentally imported from the pupfish’s habitat — were eating many of the larvae.
It is difficult to investigate what arthropods eat, said Eric Nordberg, a wildlife ecologist at James Cook University in Queensland who has also studied the topic but was not involved with the new paper. If you want to learn more about what a vertebrate eats, “you can flush the stomach contents or look at preserved specimens,” he said. But invertebrates lack stomachs, so “you need to be in the right place at the right time.”
These moments of serendipity are becoming increasingly common, said Gil Wizen, one of the entomologists who discovered the unique behavior of the Epomis beetles. He credited the prevalence of smartphones, as well as scientists and the public becoming “more alert to these interactions,” he said.
Even with the new database, however, he didn’t think scientists have seen it all. “Without doubt there are more arthropods out there hunting vertebrates,” he said. “Nature is more fluid than we think.”