The morphology of arthropod intromittent organs evolves rapidly and is often species specific, phenomena widely attributed to sexual selection. Similar patterns in biomechanical properties may also exist, but practical challenges in manipulating small structures and measuring minute forces has impeded experimental biomechanical analysis. Here we describe a device that displaces a small structure while measuring its resistance, and use it to examine the biomechanics of penile flexure in the eastern North American harvestman genus Leiobunum C.L. Koch, 1839. Several Leiobunum lineages have lost primitive penis-associated nuptial-gift sacs and have gained apparent female pregenital barriers, a co-evolutionary pattern consistent with shifts from precopulatory enticement to more-antagonistic strategies. We tested for an association between losses of nuptial-gift sacs and increases in penile flexural resistance using five sacculate and five non-sacculate species. We measured three mechanical variables—resistance force, elastic efficiency and viscoelastic relaxation time—under lateral, dorsal, and ventral flexion. Our functional assumptions about sacculate and non-sacculate penes anticipated two biomechanically-defined species clusters, but three were found: a diverse sacculate group, a monophyletic non-sacculate group and an unanticipated mixed group. This work demonstrates that experimental genital biomechanics in arthropods is possible, and we discuss the functional implications of our results.