Adhesive properties of the arolium of a lantern-fly, Lycorma delicatula (Auchenorrhyncha, Fulgoridae)

The arolium in Lycorma delicatula is shaped as a truncated pyramid, tapering proximally. The base or the terminal area is corrugated, forming parasagittal wrinkles (period 1.5–5.0 μm), which are supported from inside by cuticular dendrites. Side faces of the arolium are made up of sclerotized dorsolateral plates. When claws slip on a smooth substrate and pronate, the dorsolateral plates diverge and expand the sticky terminal area. The real contact area with the glass plate was recognized by light reflection on its periphery. This area was measured and shown to be smaller when the leg was pressed perpendicularly to the substrate (0.02 mm2) than when it was sheared in a direction parallel to the substrate (0.05 mm2). Attachment forces were measured with the aid of dynamometric platforms during pulling of active insects from horizontal or vertical glass surfaces. Normal adhesive force (about 9–12 mN) was much less than friction force during sliding with velocity of 6–17 mm/s (50–100 mN); however, when expressed in tenacity per unit contact area the difference was less pronounced: 170 and 375–625 mN/mm2, respectively. Sliding of the arolium during shear displacement was shown to be oscillatory in frame-by-frame video analysis. Relaxative oscillations consisted of periodical sticks-slips of the arolium along the glass surface.