Body colour dark-greenish, length up to 12 mm; head capsule dark brown, mentum, mandibles and apical premandibles darker, occipital margin strongly developed (c 30% of dorsal head length), black or iridescent in life, not incised laterally.
Antenna with 5 segments; segments 3-5 short, subequal or each slightly shorter than preceding, segment 3 non-annulate. Lauterborn organs v. small peg; style about as long as segment 3. Blade slightly shorter than apical antenna, spirally thickened, accessory blade long.
Labrum with all S setae simple, narrow, short, widely separated. Labral lamellae absent. Chaetae clustered, outermost short, finely serrate apically, innermost longer. 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 4-5 indistinctly-separated apical teeth, without brush; lateral spine not visible.
Mandible with apical tooth shorter than combined width of four distinct inner teeth. Seta subdentalis weakly developed, not reaching innermost tooth. Seta interna very strongly branched, with over 20 weakly serrate branches.
Mentum with two slightly smaller but non-recessed median teeth amongst 8 ventromental teeth in mid-section, with 7-8 lateral teeth scarcely diminishing in size laterad until very small outermost; ventromental plate weak. Setae submenti stout, arising anterior to base of mentum.
Maxilla with palp squat, longer than broad. Palpiger short with short sensillae aggregated as crown. Anterior lacinial chaetae forming brush, interwoven with 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 12-14 medium-sized simple claws. Procercus absent, with 5-7 long and 2 short anal setae. One seta arising anterior to bases of anal setae. Supraanal setae weak. Body setae pale, <10% of segment width.
Paraheptagyia 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.
Of seven described species of Paraheptagyia, 5 are from the Andean neotropics, one from Tasmania, another from south-east mainland Australia. Larvae of the mainland Australian species (P. tonnoiri Freeman) live in thin water films alongside waterfalls, or in splash zone of streams.