The head of Heterogyrus milloti (Coleoptera: Gyrinidae) and its phylogenetic implications

Head structures of Heterogyrus milloti Legros, 1953 are described in detail and documented with different morphological techniques, including μ-computed tomography and computer-based 3D reconstructions. The results are compared with cephalic conditions found in other gyrinid taxa and the observed characters are interpreted and analysed phylogenetically. Nine unambiguous cephalic apomorphies support the monophyly of Gyrinidae. In addition to well-known characters like the subdivided compound eyes and highly modified antennae, this includes the very tight connection of the dorsal surface of the head with the anterior pronotal margin, the presence of a clypeofrontal gland, the separation of a lateral portion from the clypeus, and the loss of the dorsal tentorial arm. Unambiguous synapomorphies of Heterogyrus and Gyrininae sensu Miller and Bergsten (2012) are the relatively widely separated dorsal and ventral ocular subunits, the absence of tactile setae on the head capsule, the shortened mesal mandibular edge, widely separated mandibular incisivi, three rows of setae on the labrum, the enlargement of the lateral mental lobes, and the loss of the stipital muscle attached to the galea. The monophyly of Gyrininae (excl. Heterogyrus) is supported by the widened bridge between the dorsal and ventral ocular subunits, the reduced size and dorsal shift of the dorsal eye, its distinct separation from the anterior pronotal margin, the detachment of the lateral frontal ridge from the supraocular bead, the almost completely reduced setation of the antennal flagellum, and a one-segmented galea. The steep frontal side of the head, a transverse regular field of setae on the frontal region, and the fused laminatentoria are autapomorphies of Spanglerogyrus Folkerts, 1979. A field of sensilla on the interocular antennal groove is a potential cephalic autapomorphy of Heterogyrus. The cephalic characters we analysed remain ambiguous about the interrelationships among the three tribes currently recognized in Gyrininae.