Larval Morphology of the Australian Endemic Diving Beetle Genus <i>Hyderodes</i> Hope, 1838 (Coleoptera: Dytiscidae)

ABSTRACT All larval instars of the two species that compose the Australian endemic diving beetle genus Hyderodes Hope, 1838, that is, Hyderodes shuckardi Hope, 1838 and Hyderodes crassus Sharp, 1882, are described and illustrated in detail including morphometric and chaetotaxic analyses. Within the subfamily Dytiscinae, the larvae of the tribe Dytiscini (i.e., Hyderodes + Dytiscus Linnaeus, 1758) are characterized by the presence of a row of natatory setae on external margin of urogomphus, the antennomere 1 subdivided into two articles in instars II–III, and the antennomeres 2 and 3 subdivided into two articles (although weakly in Hyderodes ) in instar I. Various primary chaetotaxy features also characterize both the tribe Dytiscini and the genus Hyderodes within this subfamily Dytiscinae. Interestingly, several of the features considered here as characteristic of Dytiscini are shared with members of the subfamily Cybistrinae, suggesting a close phylogenetic relationship between these two groups. All instars of Hyderodes differ from those of Dytiscus in lacking a subdivision on the first maxillary palpomere, the presence of ventral spinulae on claws, and by an almost (instar I) or completely (instars II–III) sclerotized abdominal segment VII. A key is presented to separate these genera. Comments on the biology and distribution of the two Hyderodes species are provided.