SUMMARY
To evaluate the adaptive value of the widening of the bilobed tarsi that has paralleled the tremendous radiation of the staphylinid genus Stenus, the performance of slender versus wide tarsi has been evaluated in two different contexts: (i) locomotion on the surface of water, and (ii) climbing on vertical (plant) surfaces. Contact angle measurements at the underside of the tarsi have revealed that, irrespective of tarsus width, all the investigated species are well supported by the surface of water while walking on it. The main selective demands driving the widening of the tarsi in several lineages have instead come from their firm attachment to smooth plant surfaces. This is suggested by measurements of the maximum vertical pulling forces exerted by intact and manipulated individuals on various rough and smooth surfaces. Species with widened tarsi associated with considerably more tenet setae attain significantly higher pulling forces,particularly on smooth surfaces. The tarsal setae are of greater importance on smooth surfaces, but the claws seem to be more important on rough substrata. On substrata that combine the attributes of rough and smooth surfaces, both claws and tenent setae add significantly to the pulling forces exerted,suggesting a functional synergism. The contribution of the present study to our understanding of insect tarsal attachment to surfaces with a variety of textures is discussed.
