Sensilla,punctation, reticulation,and body shape in the Hydroporinae (Coleoptera : Dytiscidae)

The purposes of this study were to illustrate reticulation, punctation, and sensilla with the scanning electron microscope, to briefly speculate on function and hydrodynamic consequences of surface structure variation, and to discuss ecological implications of body shape. The emphasis of the study was on the subfamily Hydroporinae of Dytiscidae (Coleoptera). Use of SEM revealed five basic kinds of reticulation in hydroporines: (1) completely or almost completely effaced; (2) hexagonal-pentagonal; (3) transverse; (4) asterisk or star-shaped; (5) scale-like. Four basic types of sensilla are recognized: (1) sensillum placoideum; (2) sensillum basiconicum; (3) sensillum trichoideum type 1; (4) sensillum trichoideum type 2.Surface features do not reduce drag through the “golf ball” effect. Rather, it is suggested that they are modifications for optimization of reception of stimuli and reduction of backflow within the boundary layer.Surface features (reticulation, punctation, sensilla) are too variable to be phylogenetically useful in higher dytiscid classification. The following generalizations, however, are pointed out: (1) sensilla placoidea are known only in members of Deronectes, Lioporeus, and Phreatodessus hades; (2) kidney-shaped placoid punctures are evident only in elongate species of Oreodytes; (3) species of Hydroporus s. str., Neoporus, and Heterosternuta have hexagonal-pentagonal reticulation and mostly long, sensilla trichoidea; (4) reticulation is most often effaced in taxa placed in Hygrotus, Hyphydrini, and Hydrovatini.Body shape seems to be more variable among hydroporines than in any other dytiscid subfamily. More spherical shapes decrease stability, but increase maneuverability. This is most critical in lotic situations. It is predicted that most lotic nearctic species which occupy habitats that expose them to current will tend to have rather spherical, deep body shapes.