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We studied differences in aggressiveness between two male morphs of the bulb mite Rhizoglyphus robini (Acari: Acaridae). Heteromorphic males have a thickened and sharply terminating third pair of legs, whereas homeomorphic males have unmodified legs. In another genus (Caloglyphus) of the family Acaridae, modified legs are used during fights to stab (often mortally) other males, but fights have not so far been observed in the genus Rhizoglyphus.

In this study we showed that in Rhizoglyphus robini males fight in a way similar to that previously described for the genus Caloglyphus. The presence of females increased the duration of aggressive interactions and caused earlier initiation, but fights were also observed in the absence of females. Pairs of heteromorphic males spent more time on aggressive interactions than did mixed pairs of heteromorphs and homeomorphs or pairs of homeomorphic males. Although the proportion of pairs involved in aggressive interactions did not differ between these three morph combinations, clasping of the opponent with legs, which is a typical behaviour of killer Caloglyphus males that precedes stabbing, occurred more often between pairs of heteromorphs than between mixed pairs and pairs of homeomorphs. This is consistent with our other finding that mortality attributable to male aggression was only recorded in pairs of males containing at least one heteromorphic male. We conclude that the modified legs of heteromorphic R. robini males function as a weapon in aggressive interactions between males and that, as previously described for the genus Caloglyphus, male morphs differ in aggressiveness.