The intromittent organs of the harvestman or “daddy-longlegs” fauna are cuticular structures often running the full length of the body. Hydraulic pressure, as well as intrinsic and extrinsic muscle, evert and may subtly alter this cuticle’s shape prior to copulation. Recent work on the North American leiobunine harvestmen taxa has suggested that aside from the primary inseminary function, the penis may deliver a mechanical stimulus and/or provide leverage for coercive access to the female pregenital chamber Accordingly, these organs are subject to morphological evolution guided by sexual selection and intersexual conflict, but it is unclear how the strength of these mechanisms of selection changes with demography, such as shifts in phenology or operational sex ratios. We studied the reproductive organs of males of five species of the Japanese curvipalpe group, collected from populations that varied in their breeding season length and observed male:female ratio. Although not all features with hypothesized utility in coercive mating were observed in species with strongly biased sex ratios, we found males of species with primarily female-biased populations had penises with reduced nuptial gift giving regions, as compared to males of species with more even operational sex ratios. Similarly, populations at high elevation or latitude tended to have males with larger intromittent structures. Even more dramatic was the morphological differentiation in secondary sex characteristics associated with these males. Ultimately our findings suggest phenology and long-term sex ratio bias are two of many factors with the potential to strongly influence evolution of primary and secondary reproductive morphology.