Evolution of locomotory attachment pads of hexapods.

This study shows that, in their evolution, hexapods have convergently developed two distinctly different mechanisms to attach themselves to a variety of substrates during locomotion. The first mechanism is provided by hairy surfaces and the second one by smooth flexible pads. The main similarity of both mechanisms is that the structured pad surface or particular properties of pad materials guarantee a maximum real contact with diverse substrata, regardless of their microsculpture. Ten characters of the two alternative designs were coded and analyzed together with a data matrix containing 105 additional morphological characters of different stages and body parts. The analysis demonstrates that similar structures (arolium, euplantulae, hairy tarsomeres) have evolved independently in several hexapod lineages. The evolution of flight and the associated necessity of being able to cling to vegetation or other substrates are suggested to be major triggers for the evolution of attachment structures.