Small larvae, up to 4 mm long.
Antenna with 6 segments; basal segment shorter than flagellum; segment 3 shorter than segments 2 and 4, segment 6 minute, not hair-like. Ring organ in middle of basal segment. Blade shorter than flagellum. Lauterborn organs weak; style well developed.
Labrum with SI with apical teeth, other S setae simple. Labral lamellae weakly sclerotized between SI. Chaetae c 6, simple; 2 spinulae. Pecten epipharyngis consisting of 3 simple, short spines. Seta premandibularis simple. Chaetulae laterales c. 7 pairs simple; chaetulae basales bifid apically. Premandible with 2 teeth; brush absent.
Mandible with apical tooth slightly shorter than combined width of 3 inner teeth. Seta interna of 7 branches, plumose on inner side.
Mentum with two large median teeth and 5 pairs of gradually diminishing lateral teeth. Ventromental plates double, distinctly connected to outer margin of first lateral teeth, anterior plate about half as wide as posterior plate. Seta submenti situated slightly below base of outer mental tooth; beard absent.
Maxilla with weak pecten galearis. Anterior lacinial chaeta shorter and broader than other lacinial chaetae.
Body with no distinct body setae. Procercus longer than wide, with 7 anal setae. Anal tubules shorter than posterior parapods. Short claws of anterior parapods with teeth, longer claws apparently smooth.
Aagaardia was described for a species sivertseni Aagaard, originally described from Norway, tentatively placed in Eukiefferiella (Aagaard, 1979), and then transferred to Zalutschia (Cranston and Oliver, 1988). Discovery of the immature stages showed that both placements were incorrect and a new genus was warranted (Sæther, 2000).
Four species are recognised: A. sivertseni from Norway and Finland, A. protensa from Finland, A. triangulata from Turkey and A. longicalcis from Canada. Immature stages are poorly known, but may occur in both lakes and rivers and perhaps especially in bogs.