placed the Corixidae (Corixoidea) as the second group (second branch) in the arrangement of relationships system.
Comprehensive studies of characters of the labium in the nepomorphans and in several individual species of the Corixoidea were conducted by Parsons [22, 48]and previously by Griffith , Bentwitz , and Puchkova .
The construction of the labial segments of most nepomorphans was presented by Brozek ; moreover, details of morphological characters of the labium in the Corixoidea (Nepomorpha) were also investigated by Brozzek (2014 in press).
Solely in the Corixoidea, the labium is more or less triangular, much shorter and dorsally bears a deep stylet groove flanked by a series of sclerotized transverse bands .
The one pair of prioprereceptive sensilla (COS) is restricted to the dorsal side of the second and fourth segments of the labium in most nepomorphan species, except for the Corixoidea (these sensilla have not been found).
In the Corixoidea, the triangular labial tip has several PAS2 and PES sensilla, unevenly spread.
The evidence of such evolutionary changes may be the clear difference in the structure of the labium between the Corixoidea (short and triangular, almost unsegmented labium) and the remaining taxa of the Nepomorpha (tubular, segmented labium of varying lengths, with several characteristic elements in particular families or species).
A similar shape, however, distinctly smaller, can be observed in the Corixoidea (Brozek, submitted, 2013).
In the future a cladistic analysis should be conducted, taking into account selected characters and states determined for these taxa, possibly including also the analysis of the data that are presently missing (i.e., on the Corixoidea and Potamocoridae).