Imparipecten Freeman

Description

Introduction

Large, red coloured, up to 12mm long, with ventral head length up to 850┬Ám. Head capsule with reticulate pattern, evenly golden coloured with dark brown to black mentum, mandibular teeth and occipital margin.

Dorsal Head

Frontal apotome reticulate, anterior half parallel-sided, without frontal pit; 2 median and 3 lateral labral sclerites.

Antenna

Antenna with 6 segments, segments 2-6 subequal, 4 slightly narrowed basally. Lauterborn organs moderately developed, alternate on apices of segments 2 and 3, style subapical on segment 3. Ring organ in basal third of segment 1, seta absent. Blade extending to near apex of antenna.

Labrum

SI broad based, densely plumose; SII well developed arising from short pedestal, finely plumose apically; SIII simple, short, strongly retracted posterior to level of very small SIVa, just anterior to level of very small SIVa. 6 chaetae, innermost broad and plumose, more lateral chaetae narrower and apically finely branched. Seta premandibularis long and simple. Labral lamellae broad, comb-like undivided. Pecten epipharyngis of three broad scales, each with 6-10 apically rounded teeth. 8-10 apically tapered, simple or apically finely branched chaetulae laterales, chaetulae basales apparently absent. Premandible with 2 teeth and strong brush.

Mandible

Weak, dark, dorsal tooth present, moderately developed apical tooth and 3 inner teeth. Pecten mandibularis fine, sparse, not extending to apical mandibular tooth. Seta subdentalis inserted on ventral surface, somewhat sickle-shaped and extending to innermost mandibular teeth. Mola and inner margin weakly notched. Seta interna inserted on dorsal surface, plumose, 4-5 branched.

Mentum

Mentum with ventromental component of 6 teeth, the median large, first outer smaller, next large and partly fused at base with first of 5 dorsomental teeth; dorsomental teeth on an even slope and evenly decreasing in size laterally; all teeth similarly dark brown/black, without any paler median teeth. Ventromental plates separated medially the width of the ventromentum. Ventromental plate elongate fan-shaped, with curved anterior margin; striae relatively evenly spread across the plate, present as basally disrupted ridges running anteriorly into band of regularly spaced lappets; each stria ending with a spine just posterior to plate margin; subapical spines strongest laterally. Setae submenti simple.

Body

Lateral and ventral tubules absent. Anterior parapods with dense claws, elongate ones simple and fine, shorter ones with inner serrations; posterior parapod claws simple. Procercus pale, small, wider than high, with 7-8 subequal anal setae.

Notes

Taxonomic Notes

The larva is recognisable amongst those genera with six-segmented antennae with large Lauterborn organs placed alternately on the apices of antennal segments 2 and 3, by the all-dark mentum, pale outer mandibular tooth, and the structure of the mentum. The mentum shape, which resembles that of many species of Polypedilum with tall paired median teeth, very small first laterals, taller second lateral and thereafter evenly decreasing in size, is unknown in the generic grouping.

The larva keys unambiguously to the grouping of genera with six-segmented antennae and well-developed alternate Lauterborn organs - a monophyletic grouping (Cranston and Hardwick, 1996) that does not include Tribelos, Endochironomus or relatives. Imparipecten differs from any of the included genera in details of the pigmentation and number of teeth comprising the differentiated ventromentum (median part of the mentum) and in the pigmentation of the mandibular teeth.

Ecological Notes

All larval Imparipecten pictipes have their guts filled with distinctive wood fibres. Many specimens have been obtained from "snag" (stranded immersed wood) habitats studied as part of aquatic environmental surveys. Cranston and Hardwick's (1996) studies suggest that several different kinds of wood are used but the texture is predominantly soft ("punky") presumably following protracted immersion. The larvae are true wood-miners, forming excavated galleries in the wood and rarely living in natural crevices, for example, beneath loose bark. In wood of the optimal texture (that which can be prized apart with the fingers alone) larval numbers may reach >10 per 10cm length of wood of 2-3 cm diameter. Mature larvae of I. pictipes could be handled and reared rather easily in the laboratory without the usual high mortality encountered in many other wood-mining chironomids - successful pupation did not necessarily require a gallery within in the wood. Mature larvae have been found throughout the year and there is little evidence of any seasonality in emergence either from pupal exuvial or the few adult collections.

Imparipecten pictipes is restricted to Australia. Larvae are found in pristine upper reaches of permanently flowing waters, but tolerates moderately impacted and slower-flowing lowland reaches of larger rivers such as the Nepean and Ovens. No larvae have been collected from wood in standing waters. The distribution ranges from northern New South Wales to the Victorian alps at elevations of over 1,000m and to lower elevations in the south-east of the mainland and Tasmania.

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