Species of Pentatomidae are cytogenetically characterized by the presence of holokinetic chromosomes, a pre-reductional type of meiosis, and a great constancy not only in chromosome number (2n = 14 in 85% of the 250 species analyzed) but also in the sex chromosome determining system (XY/XX).
Edessa meditabunda and E. rufomarginata males have 2n = 14 = 12 + XY, and both species present small telomeric positively heteropycnotic bands which are DAPI and CMA bright. In E. meditabunda the NOR region is clearly revealed at the telomeric region of the largest autosomal pair by silver staining and CMA banding. Meiotic behaviour of both species follows the general pattern of the order: autosomes divide pre-reductionally, sex chromosomes are achiasmatic and divide postreductionally, and at both metaphase plates the autosomes become arranged in a circle with the sex chromosomes lying at its center. In E. meditabunda, however, the larger sex chromosome is generally observed at metaphase I forming part of the ring of autosomal bivalents. Bivalents with two chiasmata are frequently observed in E. meditabunda and E. rufomarginata; mean chiasma frequency (6.45 and 6.26, respectively) differ significantly between both species, but differences between populations within each species are not significant.
The metaphase plate arrangement of autosomes and sex chromosomes is rather constant in Heteroptera. However, our results in E. meditabunda together with previous reports in other species of the order led us to suggest that the metaphase plate arrangement is more liable to variation at the first meiotic division than at the second one, when it is almost constant. The presence of ring bivalents in both species here analyzed constitutes further evidence against the previous statement of only one chiasma per bivalent in Heteroptera.