Arcus as a tensegrity structure in the arolium of wasps (Hymenoptera: Vespidae).

The unfolding of the hymenopteran attachment pad (arolium) may be achieved in two ways, hydraulic and mechanical. The first was confirmed in experiments with pressure applied to more proximal leg parts and on immersion in hypotonic solutions. Presumably, this way of unfolding does not play an important role for a living hornet. Mechanical unfolding was studied experimentally with the aid of a micromanipulator pulling the tendon of the musculus retractor unguis. Ablation experiments on different parts of the arolium indicated that the arcus is the most crucial element for mechanical unfolding. The shape of the arcus in closed and open conditions was measured using a 3D measurement microscope and reconstructed by means of 3D computer graphics. The arcus coils up upon being freed from the arolium tissues, and coils up even more after immersion into a 10% aqueous solution of NaOH. Geometrical models of the arcus are proposed, from which the rotational moment of elasticity is derived. Conformations and deformations of the arcus are quantified in order to explain its role in the folding and unfolding processes of the arolium. The diversity of approaches supports the idea that the arcus is a prestressed (tensegrity) structure providing immediate, soft, and graded transmission of forces during folding and unfolding action of the arolium.