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

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

The functional anatomy of the split compound eyes of whirligig beetles Dineutus mellyi (Coleoptera: Gyrinidae) was examined by advanced microscopy and microcomputed tomography. We report the first 3D visualization and analysis of the split compound eyes. On average, the dorsal and ventral eyes contain 1913 ± 44.5 facets and 3099 ± 86.2 facets, respectively. The larger area of ventral eyes ensures a higher field of vision underwater. The ommatidium of the split compound eyes is made up of laminated cornea lenses that offer protection against mechanical injuries, bullet-shaped crystalline cones that guide light to the photoreceptive regions, and screening pigments that ensure directional light passage. The photoreceptive elements, made up of eight retinular cells, exhibit a tri-tiered rhabdom structure, including the upper distal rhabdom, a clear zone that ensures maximum light passage, and an enlarged lower distal rhabdom that ensures optimal photon capture.

Whirligig beetle uses lift-based thrust for fastest insect swimming

Whirligig beetles (Gyrinidae) are the fastest-swimming insects. The one-centimeter long aquatic beetle can reach a peak acceleration of 100 m s-2 and a top velocity of 100 body lengths per second. Previous studies have suggested that drag-based thrust by their hind legs is responsible for their propulsion1,2,3,4. For the drag-based thrust to be effective, however, the leg stroke velocity must exceed the swimming velocity. Therefore, for fast-swimming whirligigs, it is unlikely that the drag-based thrust is the main source of acceleration5. Here, we demonstrate that lift-based thrust enables the rapid swimming of the whirligigs.