Structure and function of the elastic organ in the tibia of a tenebrionid beetle

Many insects have a pair of claws on the tip of each foot (tarsus and pretarsus). The movement of the pretarsal claws is mediated by a long apodeme that originates from the claw retractor muscles in the femur. It is generally accepted that the pulling of the apodeme by the muscles flexes the claws to engage with a rough surface of a substrate, and the flexed claws return to their initial position by passive elastic forces within the tarso-pretarsal joint. We found that each tibia of the tenebrionid beetle Zophobas atratus had a chordal elastic organ that tied the apodeme to the distal end of the tibia and assisted the pulled apodeme to return smoothly. The elastic body of the elastic organ consists of a bundle of more than 1000 thin fibrils (0.3–1.5 μm in diameter) with a hairy yarn-shaped structure made by assemblies of intricately interwoven microfibers. Both ends of the fibrillar elastic body were supported by clusters of columnar cells. Ablation of the elastic organ often disturbed the rapid and smooth return of claws from a flexed position when the tarsal segments were forced to curve in order to increase the friction between the apodeme and surrounding tissues in the segments. The result suggests that rapid claw disengagement is an important step in each cycle of leg movements, and the elastic organ may have evolved to assist the reliable detachment of claws that engage tightly with the substrate when climbing or traversing inverted surfaces.