An insect that harvests and modifies plant resin to snare its prey adds to a growing body of evidence that suggests we may be underestimating the cunning of invertebrate animals.
The assassin bug (Pahabengkakia piliceps), found across Thailand and China, smears its legs with plant resin, but not just for the reason you might think. The smell of the sap lures their prey to the optimal attack position, greatly improving the odds of the bug’s success in hunting.
“We empirically demonstrate how an invertebrate predator adapts to the colony defense of social insects via tool use behavior,” writes a team of researchers, led by entomologists Zhaoyang Chen and Li Tian of China Agricultural University, in a new paper.
“Our findings offer a new model for studying the adaptive functions and underlying mechanisms of tool use behaviors in animals.”
In recent years, we’ve been discovering that tool use in non-human animals is probably more common than we once thought – and it’s not limited to vertebrates.
Insects such as bees and ants use tools to make their lives easier; and a 2023 study on Australian assassin bugs found that these intrepid invertebrates use resin to make themselves sticky, facilitating greater success at prey capture.
P. piliceps is a different species in a different region of the world, and its lifestyle is a lot more specialized than the assassin bugs of Western Australia. It relies on very particular prey: six species of stingless bee. It builds its own nest not far from the hives of stingless bees, and hangs around the entrance to the bees’ hives to prey upon them.
The bees only possess degenerate stingers, which means they need to get creative with their defensive measures. They coat the entrances to their hives with droplets of resin; when interlopers trying to infiltrate the hive get caught in the resin, guard bees come rushing out to attack and neutralize the threat.
So here’s what P. piliceps does. It dips its legs in the resin deposited by the worker bees, applying an even coating to its front and middle legs. Then, it sits by the hive entrance, and captures the guard bee that comes scurrying over to see what the fuss is all about. The assassin bug then retreats with its secured prey to a nearby safe location.
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It gets even more interesting, though. Even though resin is all around the entrance to the hive, the guard bee makes a beeline (heh) for the waiting assassin bug. Something else seemed to be at play.
To investigate, the researchers conducted field experiments in which they examined the stickiness of the resin and the role it played. They smeared the hindlegs and butts of some of the assassin bugs with bee resin and watched what happened.
Even assassin bugs with resin smeared on their butts and hindlegs – not the legs they use for grabbing – showed more hunting success than assassin bugs with no resin. This suggested that the stickiness played only a partial role in successful predation, unlike their Australian relatives.
So the researchers conducted experiments to find out why. They hypothesized that, when resin dries, it emits volatile compounds at a lower rate, and that by smearing it on their legs, the assassin bugs increase the volatile release rate.
Chen and colleagues placed some resin in a ventilated environment, and measured the emission rate before and after smearing. Sure enough, the volatile emission rate was higher after smearing.
This suggests that, by manipulating the resin, the bugs are signaling to the bees that they need to come and deal with a problem. Then, they just become a bigger problem.
This, the researchers say, constitutes tool use.
“To be classified as tool use, a behavior must satisfy three criteria:” they write in their paper, “1) exerting control over a manipulable external object closely linked to a specific goal; 2) altering the user’s or another organism’s physical properties through a dynamic mechanical interaction; or 3) mediating the flow of information between the user and the environment or other organisms in the environment.”
The application of resin – an external object – alters the physical properties of the assassin bug’s body to achieve a specific goal. That fits the specified criteria. It’s interesting, though, that the way the tool is used differs from the way assassin bugs in Australia use a very similar tool. P. piliceps‘ bee specialization may play a role in this; but it will need further investigation.
“While the role of food specialization in the evolution of resin use warrants future multi-species comparative studies within a phylogenetic framework,” the researchers write, “this stingless bee-specialized assassin bug system offers a valuable model for investigating adaptive evolution and tool-use behaviors, with fewer ethical constraints than vertebrate-based studies.”
The research has been published in PNAS.
