External reproductive organs of spiders are often species-specific and are important taxonomic characters in species identification. One explanation of this phenomenon, the mechanical isolation hypothesis was tested in two Nuctenea (Araneidae) species. According to this hypothesis, a lock-and-key isolating mechanism should lead to character displacement (mechanical, ecological, or behavioral) between sympatric species with similar genitalia in order to prevent costly interspecific copulation attempts. Homologous external genitalia of male and female Nuctenea sclopetaria and N. patagiata were measured, The measurement means and variances were statistically compared to determine if character displacement was occurring between areas of sympatry and allopatry. Differences in both mean and variance were observed, but the number that differed between regions of sympatry and allopatry was not greater than the number that differed between adjacent regions of sympatry. Thus, these species failed to demonstrate the character displacement predicted by the mechanical isolation hypothesis. Data collected in the study were also used to test a hypothesis that spider genital features tend to be less variable than overall I size features, a hypothesis supported by an earlier study of the primitive spider Hypochilus. F-tests of coefficients of variation showed measurements of the Nuctenea genitalia to be more variable than first femur length. Differences in environmental pressures and genital complexity between Nuctenea and Hypochilus may explain this disparity. / Master of Science