The major chelae of reproductive male crayfish contain both smooth and plumose setae and are used for the perception of female odors. A comparative and morphological analysis of setae found on the major chelae of reproductive (form I) and non-reproductive (form II) male crayfish, Orconectes rusticus was performed in order to elucidate the distribution and function of smooth and plumose setae. In particular, the distribution of setae between reproductive forms was quantified as well as putative sensory functions, based on morphological characteristics, for both smooth and plumose setae. To accomplish these goals, scanning electron microscopy, a porosity assay, anterograde labeling, and acetylated tubulin (AT) immunocytochemistry were used. We found that form I crayfish have significantly more pockets of sensory setae and individual smooth setae on their major chelae when compared to form II males. Scanning electron microscopy revealed a terminal apical pore-like structure in smooth setae that was absent in plumose setae. Congruent with this finding, smooth setae absorbed crystal violet stain and DiI which labeled putative neural fibers. Conversely, plumose setae did not show any crystal violet or DiI staining. Furthermore, smooth setae contain fiber-like processes that are AT-immunoreactive and are located below the chitinous exoskeleton of the chelae. These fibers extend from the base of the smooth setae to the pore-like structure located at the distal tip. AT-immunoreactive fibers were not present in plumose setae. These results imply that smooth setae on the major chelae are putative chemoreceptors and plumose setae may serve a mechanosensory function. Coupled with previous behavioral studies, these results suggest that a dimorphism in the major chelae, found between male reproductive forms, enhances sensory perception during reproductive behaviors.