Body pale reddish pink; head capsule medium to dark brown, squared in dorsal view.
Ecdysial lines absent. Frontoclypeal apotome present. Fine cuticular sculpturing present dorsally and ventrally but reduced or absent on about posterior half of head capsule; dorsolateral stripe dark, originating on at least anterior 2/3 of medial stripe and extending markedly anteriorly.
5-segmented, segment 3 shorter than segment 4. Lauterborn organs and style absent. Blade extending past 3rd segment; without accessory blade. Ring organ basal. 2 elongate and stout antennal setae. Area of cuticular weakness close to campaniform organ.
Chaeta media in cluster of elongate, slender, spicules with some forked apically, most laterally placed spicule blade-like, large; S I pinnate; S II simple or pinnate; S III pinnate; S I and labral lamella on semicircular plate; labral lamella with numerous spicules arising from cuticle in continuous, undivided row or divided into two groups; spicules of pecten epipharyngis simple, short, arrayed in row, grouped medially; chaetulae laterales elongate, grouped laterally; premandible with uncertain number of teeth.
Robust, triangular apically, with 3-4 uncertainly distinct teeth. Apical tooth short, without dorsal tooth. Lacking seta mandibularis, seta subdentalis or seta interna. Posterior portion of adductor apodeme without dark pigmentation.
Mentum with 8 teeth, most lateral teeth continuous with edge of ventromental plate. Ventromental plate extending anteriorly to about level of mental teeth; rounded anterolaterally, anterior edge with marked concavity near lateral teeth of mentum; no striae present; some scattered spicules present.
Palp elongate, maxilla narrow (squat), not fused to lateral edge of ventromental plate; SM1 & SM2 placed posterolaterally to ventromental plates. Ligula divided into two lobes, joined by continuous pigmentation extending posteriorly past medial cleft, each squared to somewhat circular; shagreen on lobes very fine, extending posteriorly to lateral edge of base of lobe; M appendage narrow, apically forked; medial edge of labial palpus markedly developed.
Procercus widely separated, further details unknown. Anal papillae markedly elongate, slender, highly tracheated; dorsal pair with 4-5 constrictions, ventral pair with 2 or 3 constrictions.
Xestochironomus belongs with Harrisius and Stenochironomus in a group of wood- or leaf-mining larval taxa characterised by flaccid bodies, elongate anal papillae and unusual shaped head capsules. The head is squared off in Xestochironomus in contrast to the more triangular shape in the other genera – all are wedge-shaped in lateral view. Described Xestochironomus larvae have 8 mental teeth, the others have 10-12. Perhaps the characteristic very elongate, multiply-constricted anal papillae of Xestochironomus larvae also will differentiate.
Xestochironomus is a quite speciose genus, but with species known only from the New World. They have been recorded from Florida, South Carolina, Texas and New Mexico in the USA, from the Caribbean, and from Costa Rica to Chile in the Neotropics (Andersen and Kristoffersen, 1998). Known larvae mine in relatively hard, and not highly degraded clean wood in running waters (Borkent, 1984).