Body colour dark-greenish brown, length up to 10 mm; head capsule brown to black, mentum, mandibles and apical premandibles darker, occipital margin moderately developed (c 10% of dorsal head length), weakly incised laterally.
Antenna with 5 segments; segments 3-5 short, each slightly shorter than preceding, segment 3 non-annulate. Lauterborn organs v. small peg; style about as long as segment 3. Blade shorter than apical antenna, accessory blade long. RIng organs basal and apical.
Labrum with all S setae simple, narrow, short. Labral lamellae absent. Chaetae clustered, short. Pecten epipharyngis consisting of 5 elongate, non-overlapping, scales. 3 pairs of stout, simple, chaetulae laterales, 2 pairs of simple chaetulae basales. Ungula broadly U-shaped; basal sclerite divided into two sclerotised rods supporting hyaline area. Premandible apically broadened with 2 indistinctly-separated apical teeth, with short but dense brush.
Mandible with apical tooth shorter than first of four inner teeth; with hyaline lobe on external surface opposite inner teeth. Seta subdentalis weakly developed, not reaching innermost tooth. Seta interna very strongly branched, with over 20 weakly serrate branches.
Mentum with three subequal median teeth, with very small 1st lateral followed by larger 2nd lateral - these all forming the ventromentum, then 5 (dorso-)lateral teeth diminishing in size laterad; ventromental plate triangular, covering outer lateral teeth. Setae submenti very stout, arising at base of mentum.
Maxilla with palp squat longer than broad. Palpiger rectangular, with short sensillae aggregated as crown. Anterior lacinial chaetae forming stubby brush, meeting the well-developed hypopharyngeal brushes.
Body with anterior and posterior parapods separate; anterior parapod crowned with cluster of bifid and trifid small claws; posterior parapod with apical crown of 30-40 medium-sized simple claws. Procercus squat, with 4-6 anal setae as long as posterior parapod. One seta arises on base of procercus. Supraanal setae weak. Body setae pale, perhaps variable between species, up to 15% of segment width.
Maoridiamesa belongs to the Diamesinae tribe Heptagiini – the tribe is austral and well-defined in all stages. However it is not possible yet to distinguish the larvae to genus level with any confidence: too few have been reared and it is unclear which characters are reliable. Note that the ‘distinctive’ collar (occipital margin of the head) apparently is a feature only of final instar larvae.
Apart from living in New Zealand, as does Lobodiamesa, it is unclear how the larval Maoridiamesa can be differentiated from relatives without substantial revisionary taxonomic studies. Lobodiamesa has a pale head and body, and pale mentum without median teeth, in contrast to Maoridiamesa with red to dark black head and multitoothed median part of the mentum. The lateral notch in the ‘neck’ (dark collar of the head) may be distinctive but it is unknown if the feature applies to all Maoridiamesa larvae. In South America, Reissmesa and Limaya larvae have these dorsolateral emarginations on each side of the moderately developed collar. The hyaline lobe on the outer mandible of at least one species of Maoridiamesa (likely harrisi Brundin) may be discriminatory.
Five species of Maoridiamesa are described mostly from fast running waters, including those of glacial origin, in New Zealand (Brundin, 1966). At least M. stouti exhibits a hygropetric lifestyle in more lowland rivers (Boothroyd, 1998). Maoridiamesa intermedia is known from alpine streams southern North Island including on Mt. Ruapehu, North Island, and on the South Island the Canterbury area and the Fox Glacier plus Westland and Southland Rivers. Maoridiamesa glacialis is known only from Fox Glacier and Maoridiamesa insularis from Campbell Island (640 km south of South Island).
Type locality of the commonest species, Maoridiamesa harrisi: Ohakune, Wellington, May-Nov. 1919-23, T. R. Harris, J. W. Campbell (holotype lent to Dr. Pagast lost during the Second World War, the rest is retained in BMNH, according to Freeman).
Known also pre-Brundin from Cass, Canterbury, Dec. 1924, A. Tonnoir; Hilltorp, Canterbury, Tonnoir (Freeman).
Brundin added new material. From NORTH ISLAND: Mangawhero Stream, coming from Mt. Ruapehu, Ohakune, Wellington, 4.x.61, Brundin (this stream is probably the birthplace of the type material); Mangaturuturu Stream, from Mt. Ruapehu, Wellington, Brundin; Waimarino Stream, near Erna, from Mt. Ruapehu, 6.x.61, Brundin; Whakapapaiti Stream, from Mt. Ruapehu, Wellington, 7.x.61, Brundin.
From SOUTH ISLAND: Ribbonwood Stream, Canterbury, 7.xii.58, V. Stout; Hennah Stream, Canterbury, 4 exuviae, 2 larvae, 16.xi.58, V. Stout; small stream running into Lake Lyndon, just below Porter's Pass, Canterbury, 13.xii.59, V. Stout; Lyndon Outlet Stream, Canterbury, 23.xi.58, Stout; Pearson Inlet Stream, Canterbury, 7.xii.58, Stout; Simois Stream, near Lake Coleridge, Canterbury, 16.xi.58, V. Stout; Porter River, 2335 ft, Canterbury, 11.x.61, L. Brundin; tributary to Porter River, 2000 ft, Canterbury, 13.xii. 59, V. Stout; Broken River, 2165 ft, Canterbury, 11.x.61, L. Brundin; Grey River, Westland, 22.vui.61, Stout; Mikonui River, Westland, 12.x.61, L. Brundin; Wanganui River, Westland, 12.x.61, L. Brundin; Fox River, just below Fox Glacier, Westland, 14.x.61, L. Brundin; Bullock Creek, south of Cook River, Westland, 14.x.61, L. Brundin; Waikukupa River, Westland, 15.x.61, L. Brundin; Eglinton River, Southland, 20.x.61, L. Brundin; Upukerora River, Southland, 22.x.61, L. Brundin.