Mimicry between adult rove beetles and assassin bug nymphs with unequal defenses: antagonistic or mutualistic?

Defensive mimicry encompasses a continuum ranging from Batesian to Müllerian mimicry. Batesian mimicry involves antagonistic interactions between undefended and defended species, whereas Müllerian mimicry represents mutualistic interactions between species with comparable levels of defense. When mimicry occurs between species with unequal defensive abilities, it is termed quasi-Batesian mimicry, though whether such interactions are antagonistic or mutualistic remains debated. Despite their common occurrence in nature, few quasi-Batesian mimicry systems have been experimentally studied. Here, we investigated the mimetic interaction between two chemically defended insect species, the rove beetle <i>Paederus fuscipes</i> Curtis, 1826 (Coleoptera: Staphylinidae) and the assassin bug <i>Sirthenea flavipes</i> (Stål, 1855) (Hemiptera: Reduviidae), through behavioral assays with their potential predator, the pond frog <i>Pelophylax nigromaculatus</i> (Hallowell, 1861) (Anura: Ranidae), which naturally co-occurs with these insects in Japan. Adult <i>P</i>. <i>fuscipes</i> resemble <i>S</i>. <i>flavipes</i> nymphs in their conspicuous reddish-orange and black coloration. Under laboratory conditions, 45.8% of pond frogs rejected <i>P</i>. <i>fuscipes</i> adults, whereas 70.8% rejected <i>S</i>. <i>flavipes</i> nymphs, suggesting that the assassin bug nymphs are better defended. Prior exposure to <i>S.</i> <i>flavipes</i> increased frog rejection of <i>P. fuscipes</i>, whereas exposure to <i>P.</i> <i>fuscipes</i> slightly reduced rejection of <i>S. flavipes</i>. These results indicate that adult <i>P</i>. <i>fuscipes</i> may gain protective benefits from mimicry of <i>S</i>. <i>flavipes</i> nymphs, while the latter may incur a small cost.