Small to medium-sized larvae, up to 4.5 mm long.
Antenna short, with 5 segments, terminal 3 segments subequal or 5th longer than 3,4;. Blade longer than flagellum. Ring organ distinct at mid-segment. Subsidiary style on subapex of 2. Lauterborn organs absent.
SI, SII and SIII simple, lanceolate; SIV short. Two pectinate labral chaetae. Labral lamellae absent. Pecten epipharyngis of 3 subequal, apically rounded scales. Chaetulae laterales and basales short. Premandible with 5 teeth; brush absent.
Mandible with apical tooth shorter than combined width of 4 inner teeth, a strong mola is present, on which is placed a short seta subdentalis short. Seta interna of three short, simple branches.
Mentum with double median tooth and 5 pairs of lateral teeth. Ventromental plate very short and weak, not extending to outer lateral tooth; beard absent.
Anterior and posterior parapods, procerci, anal setae and anal tubules all absent. Body without distinct setae.
Eretmoptera was described from adult females of a brachypterous species found in South Georgia (subAntarctic south Atlantic Ocean). Another species was added to the genus from the marine shore of Point Lobos, California, but the species is too poorly known to confirm its congenericity.
The immature stages were described by Cranston (1985) based on a suspected introduction from South Georgia to Signy Island, in the Arctic South Shetland Islands. Larvae were associated with plant material at a site where a translocation experiment had taken place. The species is parthenogenetic according to evidence to hand.