ABSTRACT
Head prognathous (Stickney 1923) and strongly retracted into prothorax, relatively short, broad and depressed (Gyrininae) or relatively short and more convex (Spanglerogyrinae); surface smooth except for tactile setae on labrum, clypeus and frons. Eyes divided, so placed that upper pair remain above the water line and lower pair below the water line as the beetle swims; eye divided by either a narrow (Spanglerogyrinae) or a broader (Gyrininae) bridge of cuticle. Antennae with eight to eleven antennomeres, antennomere 1 (scape) hemispherical, antennomere 2 (pedicel) enlarged, with scoop-shaped lobe and tactile setae, antennomeres 3-11 short, compact, forming an incrassate club which is about 1.5 times longer than antennomeres 1+2, and not as broad as 2; club without pubescence; antennae
inserted between upper and lower eye near base of the mandibles; antennal insertion exposed and visible from above. Labrum distinct and transverse, anterior margin arcuate with fringe of setae as long as the labrum (1 row of setae present in Spanglerogyrinae, three rows in Gyrininae). Clypeus distinct, frontoclypeal suture distinct, clypeus about two times (Spanglerogyrinae) to three or more times (Gyrininae) as wide as long. Mandibles short, relatively blunt, with three apical teeth; lacinia arcuate with sharp apex, inner surface with stout setae; galea two-segmented in Spanglerogyrinae, one-segmented in Gyrinini, absent in Orectochilini and Enhydrini. Maxillary palpus with four palpomeres (Williams 1938). Median gular apodeme present. Head internally with large air sacs.
