[For complete generic synonymy, see Liebherr and Marris 2009.]

Diagnosis. Species of the genus Mecyclothorax can be diagnosed within Psydrini by the presence of attenuate, setose male parameres and an aedeagal median lobe that is closed basally (Moore 1963). The right paramere bears a series of setae along its ventral surface and the left paramere has two to five setae at its apex (Moore 1984). The ligula of the labium is broadly truncate and bisetose apically, the paraglossae are free and elongate, and the penultimate labial palpomeres are bisetose on their anterior margin. There can be from 0–5 dorsal elytral setae, and the 7th elytral interval may be carinate or flat apically (Liebherr and Marris 2009). All Hawaiian species are brachypterous, with the flight wing vestigium varying from an elongate micropterous strap that extends past the posterior margin of the metathorax— e.g., M. montivagus (Blackburn) of Maui—to a very short vestigial flap that does not extend past the metathoracic hind margin. In association with the brachypterous condition, all Hawaiian species are characterized by a foreshortened metathorax, evidenced by a short metepisternum.