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.

A self-locking mechanism of the frog-legged beetle Sagra femorata.

Insect legs play a crucial role in various modes of locomotion, including walking, jumping, swimming, and other forms of movement. The flexibility of their leg joints is critical in enabling various modes of locomotion. The frog-legged leaf beetle Sagra femorata possesses remarkably enlarged hind legs, which are considered to be a critical adaptation that enables the species to withstand external pressures. When confronted with external threats, S. femorata initiates a stress response by rapidly rotating its hind legs backward and upward to a specific angle, thereby potentially intimidating potential assailants. Based on video analysis, we identified 4 distinct phases of the hind leg rotation process in S. femorata, which were determined by the range of rotation angles (0°-168.77°). Utilizing micro-computed tomography (micro-CT) technology, we performed a 3-dimensional (3D) reconstruction and conducted relative positioning and volumetric analysis of the metacoxa and metatrochanter of S. femorata. Our analysis revealed that the metacoxa-trochanter joint is a "screw-nut" structure connected by 4 muscles, which regulate the rotation of the legs. Further testing using a 3D-printed model of the metacoxa-trochanter joint demonstrated its possession of a self-locking mechanism capable of securing the legs in specific positions to prevent excessive rotation and dislocation. It can be envisioned that this self-locking mechanism holds potential for application in bio-inspired robotics.