Corixoidea is the largest nepomorphan group and their members are commonly called water boatmen.
The superfamily Corixoidea comprises about 400 species, which are included in Corixidae and Micronectidae.
Previous cytogenetic studies on 27 species of Corixoidea have shown a noticeable karyotypic uniformity with a diploid chromosome number of 24, a pair of m chromosomes, and an XY/XX sex chromosome system (male/female) (Table I) (Ueshima, 1979; Ituarte & Papeschi, 2003, 2004).
So far, from the 27 cytogenetically studied species of Corixoidea, two belong to Micronectidae and the remaining 25 are included in Corixidae.
Furthermore, the m chromosome pair characteristic of Corixoidea could lose its particular meiotic behaviour and become a regular autosomal pair, contributing thus to the increase in the diploid chromosome number of the species.
From all these results, we suggest that Corixidae is a cytogenetically homogenous family, and an ancestral karyotype of Corixoidea should be 2n= 24= 20+2m+XY.
From a cytogenetic point of view Nepomorpha may be regarded as containing five groups: Nepoidea, Ochteroidea, Naucoroidea, Corixoidea, and Notonectoidea.
The Corixoidea and two other superfamilies, Naucoroidea and Notonectoidea, present a pair of m chromosomes and show variable sex chromosome systems (XY, X0 and [X.
n]0 of the Corixoidea, Naucoroidea and Nepoidea most probably originated during the evolution through the loss of the Y chromosome (X0) and in some species fragmentation of the original X chromosome ([X.
Achiasmatic male meiosis in Tenagobia (Fuscagobia) fuscata (Heteroptera, Corixoidea, Micronectidae).