| Abstract: | Gyrinidae is a group of beetles with a unique specialization of swimming on the water surface. Heterogyrus milloti Legros (Heterogyrinae) from Madagascar is a species with various preserved plesiomorphic features. The information on the morphology and biology was very limited until recently, and the thoracic anatomy remained largely unknown. Consequently, the aim of the present study is to describe external and internal thoracic features of Heterogyrus Legros in detail and to interprete them with respect to their phylogenetic and functional significance, with a special focus on the unusual flight apparatus of Gyrinidae. Characters documented with innovative techniques are compared to conditions found in other gyrinid genera and other groups of Adephaga, including characters of other body parts and larvae. A data matrix with 144 characters of adults, larvae and eggs was compiled and analysed cladistically. Gyrinidae excluding Spanglerogyrus Folkers (Heterogyrinae + Gyrininae) is supported by many apomorphies, mainly by a unique locomotor apparatus with paddle‐like middle and hind legs. The results confirm Heterogyrus as the earliest diverging branch in Gyrinidae except Spanglerogyrus, implying a sister‐group relationship between this genus and Gyrininae, a clade comprising Gyrinini, Dineutini and Orectochilini. The presence of an opening between the mesanepisternum and elytra, reduction of the lateral metafurcal arms, loss of the metathoracic M. furcacoxalis lateralis, and modifications of the head, including the dorsal shift of the upper subcomponent of the compound eyes, are synapomorphies of the three tribes. The monophyly of Gyrinini is moderately well‐supported, whereas Orectochilini is strongly supported by different characters including a highly simplified but functioning flight apparatus. A clade comprising Orectochilini and the dineutine genera is suggested by synapomorphies of adults and larvae. The monophyly of Dineutini was supported in a recent study, but not by the characters analysed here. Features of adults, larvae and eggs indicate that Gyrinidae are the sister group to the remaining adephagan families, as suggested in some earlier morphology‐based studies and recent analyses of large molecular datasets. |
