Biomechanics of the movable pretarsal adhesive organ in ants and bees

Hymenoptera attach to smooth surfaces with a flexible pad, the arolium, bet ween the claws. Here we investigate its movement in Asian weaver ants (Oeco phylla smaragdina) and honeybees (Apis mellifera). When ants run upside down on a smooth surface, the arolium is unfolded and folded back with each step. Its extension is strictly coupled with the retr action of the claws. Experimental pull on the claw-flexor tendon revealed t hat the claw-flexor muscle not only retracts the claws, but also moves the arolium. The elicited arolium movement comprises (i) about a 90 degrees rot ation (extension) mediated by the interaction of the two rigid pretarsal sc lerites arcus and manubrium and (ii) a lateral expansion and increase in vo lume. In severed legs of O. smaragdina ants, an increase in hemolymph press ure of 15 kPa was sufficient to inflate the arolium to its full size. Apart from being actively extended, an arolium in contact also can unfold passiv ely when the leg is subject to a pull toward the body. We propose a combined mechanical-hydraulic model for arolium movement: (i) the arolium is engaged by the action of the unguitractor, which mechanicall y extends the arolium; (ii) compression of the arolium gland reservoir pump s liquid into the arolium; (iii) arolia partly in contact with the surface are unfolded passively when the legs are pulled toward the body; and (iv) t he arolium deflates and moves back to its default position by elastic recoi l of the cuticle.