Karyotype of the Membracid Umbonia crassicornis is 9 II + XO; 2n = 20 in female, 2n = 19 in male. It is presumably a fusion derivative from the basic Membracid 10 II + XO Accordingly, one of the autosomal pairs of U. crassicornis is notably large. All chromosomes are metacentric; and, except for the X chromosome, euchromatic. In the male, chiasma frequency is only 9.40 per nucleus. Chiasmata are confined to distal halves of the arms. Such a karyotype forms a conservative recombination apparatus that may have helped the species in its rapid adaptation to the Puerto Rican habitat, which presumably does not differ much from the habitat of the mother population.