Overcoming Drag at the Water-Air Interface Constrains Body Size in Whirligig Beetles

Abstract

Whirligig beetles (Coleoptera: Gyrinidae) are among the best swimmers of all aquatic insects. They live mostly at the water’s surface and their capacity to swim fast is key to their survival. We present a minimal model for the viscous and wave drags they face at the water’s surface and compare them to their thrust capacity. The swimming speed accessible is thus derived according to size. An optimal size range for swimming at the water’s surface is observed. These results are in line with the evolutionary trajectories of gyrinids which evolved into lineages whose members are a few milimeter’s long to those with larger-sized genera being tens of millimeters in length. The size of these beetles appears strongly constrained by the fluid mechanical laws ruling locomotion and adaptation to the water-air interface.

Functional Anatomy of Split Compound Eyes of the Whirligig Beetles Dineutus mellyi (Coleoptera: Gyrinidae)

“…Whirligig beetles (Coleoptera: Gyrinidae) are among the aquatic beetles that spend most of their time gyrating on the water surface [1,2]. This behavior is attributed to hunting and feeding behaviors, whereby they prey on insects trapped in water [3].…”

Cryptic lineages, cryptic barriers: historical seascapes and oceanic fronts drive genetic diversity in supralittoral rockpool beetles (Coleoptera: Hydraenidae)

“…This is the case with many organisms inhabiting extreme environments such as caves (Zhang & Li, 2014), hypersaline waters (Arribas et al, 2013), mountain summits (Adams et al, 2014) or the intertidal zone (Pfingstl et al, 2019;2021). Ecomorphological constraints also operate on aquatic organisms more generally, however, particularly through their hydrodynamics, something which increases the likelihood of cryptic species complexes being present in such habitats (Céréginho et al, 2018;Domenici et al, 2007;Jami et al, 2021;Knowlton, 1993). Whilst lineage divergence without population isolation certainly does occur, particularly in marine scenarios (e.g., Bowen et al, 2013;Faria et al, 2021), speciation is thought much more likely to occur when there is an external barrier to gene flow, at least initially (Barraclough, 2019;Haak et al, 2019;Stock et al, 2019).…”