Xylochironomus Cranston

Description

Introduction

Medium-sized, red colored, length 6 mm, with ventral head length up to 520 ┬Ám. Head capsule golden yellow colored mid-brown gula, mentum and inner mandibular teeth. Occipital margin slightly darkened.

Dorsal Head

Frontal apotome tapered posteriorly, broad anteriorly, without frontal pit, with SIII outside lateral margin of separate clypeus; labral sclerite 1 complete, labral sclerite complete, labral sclerites 3-5 distinct.

Antenna

Antenna with 6 segments, segments 4 and 5 subequal, 3rd shorter than 2nd or 4th, 6th short. Lauterborn organs alternate. Long style at apex of segment 3. Ring organ not visible, seta absent. Blade extending slightly beyond apex of 5th segment.

Labrum

SI apically and subapically plumose, SII apically plumose, longer than SI; SIII simple, short, located between bases of SII; SIVa and b normally developed. 1 strong, plumose chaeta. Seta praemandibularis short, simple. Labral lamellae narrow, without indication of median division. Pecten epipharyngis of three indistinctly separated scales, each with 3-5 blunt teeth. 6-7, weakly serrate chaetulae laterales, 2 simple chaetulae basales displaced more anteriorly. Premandible with 2 narrow apical teeth and strong brush.

Mandible

Lacking dorsal tooth; with small apical tooth and 3 distinct inner teeth; with basal striae as in Chironomus. Pecten mandibularis reduced to 3 fine setae. Seta subdentalis inserted on ventral surface, simple, extending to innermost mandibular teeth Inner margin with 3 fine serrations. Seta interna very well developed.

Mentum

Shallowly curved, without distinction between ventromental and dorsomentum; with 8 evenly brown colored teeth on each side, the innermost pair sharply depressed relative to large 2nd. Ventromental plates separated medially by width of 4 median teeth, somewhat semi-lunar-fan-shaped, with medial corner directed posteriorly, lacking indication of dorsomentum, with smooth anterior margin; striae present as relatively uniform-lengthed ridges across much of plate. Setae submenti simple.

Body

Lateral and ventral tubules absent. Anterior parapods with dense, fine, simple claws; posterior parapod claws sparse, golden, simple. Procercus weakly pigmented, small, higher than wide, bearing 6-7 subequal anal setae.

Notes

Taxonomic Notes

Larval Xylochironomus have a distinctive 5- or 6-segmented antenna with large Lauterborn organs alternate either on an antennal segment interpreted as the 2nd, or if the latter is considered as medially divided, on the apices of the 2nd and 3rd segments. Xylochironomus resembles Polypedilum nubifer (Skuse) in this feature, although the antenna of the latter is interpreted as having 4 or 5 segments, with Lauterborn organs on a divided/undivided 2nd, and with only 2 more apical segments. The mentum of Xylochironomus, with recessed paired median teeth, and the semilunar ventromental plates are distinctive and characteristic. Resemblance to larvae of Polypedilum Kieffer is countered by the presence of a discrete clypeus (dorsal head sclerite 1) with cephalic seta S3 lying lateral to the sclerite, and the presence of this discrete sclerite 1 (in contrast to Polypedilum bearing S3 on the antero-lateral corner of a frontoclypeal apotome and lacking sclerite 1). The striate mandibular base occurs otherwise only in Chironomus and immediate relatives (Cranston, 2006).

Ecological Notes

Larval X. kakadu were found first (Cranston, 2006) in immersed and rather soft wood of undetermined identity lying in a deep rock pool in the main channel of a quite strongly flowing stream in monsoonal tropical Australia (Kakadu National Park). Field notes imply larvae were below bark but not obviously mining in heart wood, which remained firm.

The Far North Queensland site in Mossman Gorge was very similar, in being a stream deeply shaded by riparian tropical/monsoonal vegetation. Substantial immersed wood was trapped in pools between riffles. Wood texture was soft; harder wood was unsampled.

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