Modest to large-sized. With 2 pairs of widely separated eyes. With strong dorsal ‘ecdysial’ bands. Thoracic segments swollen even in pre-pharate pupa, remaining body flaccid. Dorsal surface of head. Frontoclypeolabral apotome present, lateral labral sclerites indistinct, anterior labrum rugulose.
Antenna with 5 segments, segment 3 half length of segment 4. Lauterborn organs small, no longer than segment 3; style short, originating subapically on segment 2. Ring organ in basal 1/3 of segment 1, strong antennal seta inserted distal to midpoint of segment. Blade broad, extending no further than apex of 3rd antennal segment, accessory blade longer than blade (NZ species) or detected (Australian species).
S1 seta pectinate / weakly plumose, bases separate; SII seta simple, SIII simple or weakly pectinate, somewhat displaced laterally towards SIV; SIV quite well developed. Chaeta media numerous bunched spines. Seta premandibularis simple. Labral lamellae represented by 2-3 fine spines. Chaetulae and spinulae simple, bunched, long. Pecten epipharyngis comprising 3 scales each with 2-4 spines. Premandible with 3 teeth and strong brush.
Broad, triangular, distal half black. Dorsal tooth absent; with apical tooth and 2 inner teeth and extended mola. Pecten mandibularis and seta interna absent. Seta subdentalis simple, slender, short.
Very worn in many specimens, with 10 teeth, central block of 6, and 2 outermost laterals curved so penultimate overlies the dorsal, outermost (in uncrushed head); ventromental plates transverse extensions to lateral mental teeth, fused to gula/maxilla, with few to many spicules and spines, with several weak and recurved striae. Setae submenti long, simple.
Maxilla narrow, with elongate palp.
Slender, becoming narrower anally. Anterior parapods fused with broad crown of small claws. Posterior parapods very short, with claws directed backwards. Anal setae seated directly on body; procercus absent. Lateral and ventral tubules absent. Anal tubules large, parallel to longitudinal axis of body, projecting backwards, with 2 constrictions.
Harrisius belongs with Xestochironomus and Stenochironomus in a group of wood- or leaf-mining larval taxa characterised by flaccid bodies, elongate anal papillae and unusual-shaped head capsules with robust, triangular mandibles and strong apodemes. The head tapers anteriorly and is wedge-shaped in lateral view. The mentum of Harrisius larvae has 10 teeth, as in Stenochironomus and in contrast to the clear 8 of Xestochironomus, but the position of the outermost tooth in Harrisius prevents easy observation. Borkent (1984) suggests that the spicules of the pecten ephipharyngis have wide bases in Harrisius in contrast to being slender in Stenochironomus, and this is confirmed. Aside from on grounds of geography (Xestochironomus is solely New World), may be recognised by the characteristic very elongate and multiply-constricted anal papillae.
In Australiasia, Harrisius may be separated best from Stenochironomus by the very short 3rd antennal segment in the former, in contrast to this segment being subequal in length to the 4th in Stenochironomus.
Harrisius is known only from Australia and New Zealand, with one described species in each - H. montanus (Skuse) in Australia, H. pallidus Freeman from New Zealand. The larvae are wood-miners, or at least all have wood fibres in the gut. Freeman (1961) pointed out variability in adult morphology across the Australian continent-wide range, and to an extent this is reflected in immature stage variability – there may be cryptic species present.