Bombardier vs. assassin: Mimetic interactions via a shared enemy

Newswise — Creatures employ diverse tactics to protect themselves from their natural adversaries. Species that possess effective defense mechanisms often exhibit prominent body hues, either resembling other well-protected or defenseless creatures, thereby establishing mimetic associations. Certain bombardier beetles expel poisonous substances at a scorching 100°C, deterring foes like frogs, and many of them display conspicuous colors as a warning signal to discourage potential threats. In the same locale in Japan, an assassin bug called Sirthenea flavipes displays a striking body coloration resembling that of the coexisting bombardier beetle Pheropsophus occipitalis jessoensis (Fig. 1). The assassin bug possesses a proboscis capable of inflicting intense agony upon humans. Despite the effective defenses possessed by both insects, the nature of the mimetic relationship between the bombardier beetle and the assassin bug remains unclear.

Entomologists Shinji Sugiura from Kobe University and Masakazu Hayashi from the Hoshizaki Green Foundation conducted a study revealing that the bombardier beetle P. occipitalis jessoensis possesses a more robust defense mechanism against a mutual predator when compared to the assassin bug S. flavipes. Furthermore, their research, which is featured in the 6 June 2023 edition of PeerJ, demonstrates that both the bombardier beetle and the assassin bug reap advantages from their mimetic relationship in the presence of the shared predator.

In central Japan, the pond frog Pelophylax nigromaculatus shares its habitat with the bombardier beetle and the assassin bug. The pond frog, renowned as a predator of diverse insects, has the potential to prey upon the bombardier beetle and the assassin bug in natural surroundings. The scientists observed the conduct of pond frogs towards bombardier beetles and assassin bugs in controlled laboratory conditions (watch video). Among the frogs, all of them declined bombardier beetles, while 75% rejected assassin bugs (Fig. 2). This suggests that the bombardier beetle possesses better defenses against frogs compared to the assassin bug. Furthermore, the researchers presented either a bombardier beetle or an assassin bug to a frog that had encountered the other insect previously. Frogs that had previously encountered one species showed reduced aggression towards the other species (Fig. 3). Specifically, a previous encounter with assassin bugs lowered the attack rate of frogs on bombardier beetles from 75.0% to 21.7% (Fig. 3). Likewise, a previous encounter with bombardier beetles reduced the attack rate of frogs on assassin bugs from 91.3% to 40.0% (Fig. 3). Hence, the mimetic interaction between the bombardier beetle and the assassin bug may be mutually beneficial.