The wasp’s fate seems sealed – the frog already has it in its mouth and is beginning to chew. But then he suddenly spits the wasp out, lets it go and makes no further attempts to eat. What happened? The wasp, a male of the solitary wasp Anterhynchium gibbifrons, stung the frog with its genitals and saved itself. Researchers led by Misaki Tsujii and Shinji Sugiura from Kobe University (Japan) report on this special form of defense in the journal Current Biology. They also provide a video of attack and defense.
The spiny genitals aren’t the first type of curious defensive weapon Sugiura has discovered in insects. In 2018, he reported in the journal Biology Letters about a bombardier beetle farting to free itself from a toad: Pheropsophus jessoensis emits a hot chemical when eaten by a toad, which often causes the toad to regurgitate it. The chemical is a type of gas, which is why insects equipped with it are popularly called “farting bugs” in Japan.
Two years later, Sugiura reported in the journal “Current Biology” about a water beetle that, after being eaten, stimulates its enemy’s digestion and is thus quickly transported back into the open – a sort of escape through the back door.
In the current investigation, well-fortified wasps. Similar to bees, wasps are known to use a poisonous stinger to ward off enemies – but only the female animals, because the stinger developed in the evolution of animals from the egg-laying apparatus of the female. After all, the males benefit indirectly from the protection because they imitate the warning coloring of the females and thus avoid possible attacks.
In some species from the group of solitary – i.e. not in social groups – living wasps (Eumeninae), the males have a pseudo-stinger on the abdomen or on the genitals, which is discussed among experts as a defensive weapon, as Sugiura and Tsujii write. However, there is no experimental evidence for this.
The scientist and the scientist came across the topic while studying the life cycle of A. gibbifrons and were stung by males. Although the animals did not distribute any poison when they were stung, the stings hurt, the researchers report. Their assumption that the male spines are defensive was confirmed by the observation that they were not used in mating, so the females were not injured.
To examine the phenomenon more closely, the researchers brought individual animals – there were 17 in total – together with a Japanese tree frog (Dryophytes japonica) in the laboratory and observed what happened. First, all the frogs grabbed the male wasps – but a good third (35.3 percent) then spat out the prey again. Next, the researchers removed the spines and repeated the experiment. Now defenseless, all the wasps were eaten.
Finally, the researchers used female wasps in the experiment for comparison: a good half of the frogs completely ignored the potential prey; if an attack occurred, a large number of wasps (88 percent) were spat out again. So the weapon of the females was more effective.
However, it was of no use when the water frog Pelophylax nigromaculatus attacked, as further tests showed: this frog ate both male and female animals unimpressed. “Aquatic frogs have a higher tolerance for wasp stings than tree frogs,” the researchers write.
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