Redescription of Diesingia megastoma (Diesing, 1836) Sambon, 1922, a pentastomid parasite from the South American terrapin Hydromedusa tectifera Cope.

Slide-mounted material of the pentastomid parasite Diesingia megastoma (Diesing, 1836) Sambon, 1922 from the South American chelonian Hydromedusa tectifera Cope is reviewed and the perfunctory, often omissive, description of the species is amended. The strong morphological similarities between D. megastoma and the crocodilian and chelonian pentastome genera of the family Sebekiidae Sambon, 1922, Alofia Giglioli, 1922, Selfia Riley, 1994, Sebekia Sambon, 1922, Agema Riley, Hill & Huchzermeyer, 1997, Leiperia Sambon, 1922 and Pelonia Junker & Boomker, 2002, clearly place Diesingia Sambon, 1922 within the same family. However, the unique combination of its main diagnostic criteria makes Diesingia a distinct genus. The absence of an elaborate, flared cirrus-tip in D. megastoma distinguishes it from Leiperia, while emphasizing its similarity to the remaining genera mentioned above. D. megastoma resembles Alofia in that it possesses smooth, flat-topped hooks and an anteriorly open oral cadre with an oesophageal peg. The copulatory spicules of Diesingia, however, lack the double-hooked collar, typical for Alofia and Selfia. Unlike the peg-like extension of the fulcra of the hooks of Sebekia, that of D. megastoma is cowl-like and carries spines only on the anterior fulcra. Moreover, the hooks of Sebekia are usually convex and spinose and the ovoid oral cadre is closed anteriorly. Diesingia differs from Pelonia through the latter's smooth but dorsally convex and extension-free hooks. The copulatory spicules of Pelonia and Agema are reminiscent of the basic build found in Sebekia, whereas in D. megastoma the short, ventral extension of the cowry shell-shaped base of the copulatory spicules has been transformed into a structure resembling the collembolan fulcrum. The latter is connected to the base via a joint, a configuration which is unique in the Sebekiidae.