Gelastocoridae (Kirkaldy, 1897) has over 70 species in two subfamilies: Nerthrinae (Kirkaldy, 1898), genus Nerthra (Say, 1832) and Gelastocorinae (Champion, 1901), genus Gelastocoris (Kirkaldy, 1897). The position of the cephalic gland openings, presence and shape of the gular bridge and the number and position of abdominal spiracles, are presented as new morphologic diagnostic characteristics that support the argument used by Todd for the segregation of the subfamilies. The characteristics are described and illustrated on the basis of specimens from Laguna Iberá, Corrientes, Argentina.