Body colour greenish, length 8.5 mm; head capsule brown, mentum, mandibles and apical premandibles very dark, occipital margin moderately developed (c 15% of dorsal head length), black, not incised laterally, length 580 mm.
Antenna with 5 segments; segments 3-5 short, subequal or each slightly shorter than preceding, segment 3 non-annulate. Lauterborn organs very small peg; style about as long as segment 3. Blade as long as apical antenna, spirally thickened, accessory blade long.
Labrum with all S setae simple, narrow, widely separated. Labral lamellae absent. Chaetae clustered, outermost 2-3 spine-like, some inner ones finely serrate apically. 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 one basal and one broad apical tooth, 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 recessed median teeth amongst 8 ventromental teeth in mid-section, with 6 lateral teeth diminishing in size beneath quite well-developed triangular ventromental plate. Setae submenti stout, arising close to base of mentum.
Maxilla with palp squat, longer than broad. Palpiger short with short sensillae aggregated as crown. Anterior lacinial chaetae forming strong dense brush, interwoven with hypopharyngeal brushes.
Body with anterior and posterior parapods separate; anterior parapod crowned with dense cluster of bifid and trifid small claws; posterior parapod with apical crown of 12-14 medium-sized simple claws. Procercus near absent, with 6 subequal lengthed anal setae, slightly shorter than posterior parapod, and no more than half length of segment 10. One seta arising anterior to bases of anal setae. Supraanal setae weak. Body setae pale, no longer than 10% of segment width.
Heptagyia 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.
Heptagyia annulipes Philippi was recognised by Brundin (1966) for adults and associated pupae from a remnant adult in Philippi collection in Santiago from central Chile. Modern understanding comes from Brundin's redescription based on both sexes of adult, and associated pupae (and exuviae) from localities further south in Chile, and from Bariloche in Patagonian Argentina. The larval diagnosis and photographs presented here are the first prepared, derived from a pharate larva (containing the characteristic pupa) from Peulla, one of the localities from which Brundin rediscovered the genus and species.