Structure and mechanics of the tarsal chain in the hornet, Vespa crabro (Hymenoptera: Vespidae): implications on the attachment mechanism

Abstract

Two combined mechanisms on the hornet tarsus are adapted to attachment to the substrate: a friction-based (claws and spines) and an adhesion-based one (arolium). There are two ranges of substrate roughness optimal for attachment, either very smooth or very rough. There is an intermediate range of substrate grains of small but non-zero size, where both of these mechanisms fail. The optimal size of substrate grains for hornet grasping was 50–100 μm. Maximal hold to the substrate was achieved when surface irregularities were clamped between the claws of opposite legs. In such a position, the insect could withstand an external force which was almost 25 times larger than its own weight. The tarsal chain is an important part of the entire attachment mechanism. The articulations in the kinematic chain of tibia-tarsus-pretarsus are monocondylar. Three tarsal muscles and one head of the claw retractor muscle originate in the tibia. On pull to the retractor tendon, the tarsus bends in a plane. All elements of the tarsal kinematic chain have one active degree of freedom. The distance between the intertarsomeric articulation point and the tendon of the claw retractor (75–194 μm) corresponds to an efficiency of 1° per 1–3 μm of pulling distance travelled by the tendon. The claw turns about 1° per 4.3–5.0 μm of pulling distance travelled by the unguitractor. The arolium turns forward and downward simultaneously with flexion of the claws. The kinematic chain of the arolium lacks real condylar joints except the joint at the base of the manubrium. Other components are tied by flexible transmissions of the membranous cuticle. The walking hornet rests on distal tarsomeres of extended tarsi. If the retractor tendon inside the tarsus is fixed, passive extension of the tarsomeres might be replaced by claw flexion. Tarsal chain rigidity, measured with the force tester, increased when the retractor tendon was tightened. Probably, pull to the tendon compresses the tarsomeres, increasing friction within contacting areas of rippled surfaces surrounding condyles within articulations.