Aggressiveness in two male morphs of the bulb mite Rhizoglyphus robini

We studied differences in aggressiveness between two male morphs of the bul b mite Rhizoglyphus robini (Acari: Acaridae). Heteromorphic males have a th ickened and sharply terminating third pair of legs, whereas homeomorphic ma les have unmodified legs. In another genus (Caloglyphus) of the family Acar idae, modified legs are used during fights to stab (often mortally) other m ales, 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 si milar to that previously described for the genus Caloglyphus. The presence of females increased the duration of aggressive interactions and caused ear lier initiation, but fights were also observed in the absence of females. P airs of heteromorphic males spent more time on aggressive interactions than did mixed pairs of heteromorphs and homeomorphs or pairs of homeomorphic m ales. Although the proportion of pairs involved in aggressive interactions did not differ between these three morph combinations, clasping of the oppo nent with legs, which is a typical behaviour of killer Caloglyphus males th at precedes stabbing, occurred more often between pairs of heteromorphs tha n between mixed pairs and pairs of homeomorphs. This is consistent with our other finding that mortality attributable to male aggression was only reco rded in pairs of males containing at least one heteromorphic male. We concl ude that the modified legs of heteromorphic R, robini males function as a w eapon in aggressive interactions between males and that, as previously desc ribed for the genus Caloglyphus, male morphs differ in aggressiveness.