Conochironomus Freeman

Description

Introduction

Medium sized to large, red coloured, up to 11 mm long, with ventral head length up to 530 ┬Ám. Head capsule evenly golden coloured to brown with darker brown gula, mentum and apical mandibular tooth. Occipital margin darkened.

Dorsal Head

Frontoclypeal apotome broad, without frontal pit; 1 labral sclerite, remaining labral area fragmented.

Antenna

Antenna with 6 segments, segments 2, 5 and 6 short, 3 and 4 subequal, with 4 often characteristically curved; Lauterborn organs moderately developed, one apical on segment 2, the other placed on opposite apex of 3rd segment. Ring organ in middle of segment 1, seta absent. Blade extending to near apex of antenna.

Labrum

SI plumose; SII densely plumose, arising from prominent tubercle; SIII simple, short; SIVa and b moderately developed. 10 plumose subequal long chaetae. Seta praemandibularis strong and simple. Labral lamellae broad, without indication of median division. Pecten epipharyngis of three narrow scales, perhaps apically notched, but not deeply toothed. 10-12 apically plumose chaetulae laterales, 2 apically branched chaetulae basales. Premandible with 3-4 teeth and strong brush.

Mandible

Weak, pale, dorsal tooth present; strong apical tooth and 2 inner teeth. Pecten mandibularis not extending to apical mandibular tooth. Seta subdentalis inserted on ventral surface, somewhat sickle-shaped and extending to inner mandibular teeth. Mola and inner margin smooth. Seta interna inserted on dorsal surface, strongly plumose, 4-branched.

Mentum

Mentum with clearly demarcated ventromental component of 4 subequal teeth, and dorsomentum of 6 teeth on each side, with 1st smaller, 2nd taller, then evenly decreasing in size laterally; all teeth similarly medium brown, without any paler median teeth. Ventromental plates separated medially by width of ventromentum plus 1-2 lateral teeth. Ventromental plate elongate fan-shaped, with curved, smooth or wavy margin; striae of mid and posterior plate present as variable-lengthed ridges running into a subapical band of regularly spaced lappets. The scanning electron microscope reveals a double series of spines between and anterior of the lappets. Setae submenti simple.

Body

Lateral and ventral tubules absent. Anterior parapods with dense, fine, simple claws; posterior parapod claws simple. Procercus weakly pigmented, small, wider than high, bearing 7 subequal anal setae.

Notes

Taxonomic Notes

Conochironomus larvae are diagnosed by the six-segmented antenna with Lauterborn organs alternate on the apices of the second and third segments, a uni-coloured brown mentum with delimited four subequal median ventromental teeth. Conochironomus fails to key beyond couplet 17 in the Chironominae key of Wiederholm (1983), since a pale dorsal mandibular tooth is present and the four median subequal-sized mental teeth protrude anterior to the remaining dorsomental teeth. Disregarding the pigmentation of the dorsal mandibular tooth and mentum, the median mental teeth pattern and length of antennal flagellum resembles that of Paratendipes but this genus differs in the pecten epipharyngis structure. Otherwise similar Stictochironomus larvae have small paired median mental teeth in the Holarctic region (Wiederholm, 1983), though Australian S. fluviatilis and S. illawara Freeman fail to conform to the Holarctic diagnosis (Cranston, unpubl.). Separation of larval Conochironomus from the larvae of these Australian "Stictochironomus", Australian Imparipecten Freeman and Afro-Australian Skusella Freeman is the subject of ongoing study (Cranston, in prep.).

Ecological Notes

Species of Conochironomus are tropical to subtropical in distribution. In Australia, the furthest southern records are from 27o30'S on subtropical North Stradbroke Island, Queensland. The absence from more southerly and central areas of Australia is likely to be genuine, since very extensive pupal exuvial collections have been made in these areas over the past eight years. On the African continent, the distribution extends from subSaharan Upper Volta to tropical/subtropical Transvaal (Makolo River, 24S, Nelspruit, 25S, A.D. Harrison pers. comm.). Outside this distribution, the genus is not recorded in well-studied Saudi Arabia, nor does it figure in Indian publications on Chironomidae. Other areas that would link the recorded Afrotropical and Australian distributions include parts of the southern Oriental regions. The genus is known (under incorrect name) in Indonesia and exuviae and undescribed adults are known from the Oriental region. On present information, Conochironomus exhibits a vicariant tropical/subtropical Australian-Oriental-Afrotropical distribution.

Conochironomus larvae live in sediments in lakes and rivers. In West Africa, Hare and Carter (1986, 1987) found larval C. acutistilus abundant in near-shore areas of less than 1 m depth in the small, dilute (conductivity c. 20uS cm-1) Lake Opi, Nigeria. Details of depth distributions, population densities and co-occurring species in Opi Lake are provided by Hare and Carter (1986). Adult emergence took place in all months of the year with the exception of August and September, the period of maximum lake depth. At the beginning of the rainy season (June 1980) the mean (+/-SE) abundance of larvae (total of all instars) decreased with increasing water depth: 0.5m 1030+/-400; 1.0m 130+/-60; 2.0m 25+/-10; 2.5m 0. The ratio of numbers of instars (4th:3rd:2nd) collected on this date was 1:2.2:2.2.

In Australia, all four species of Conochironomus have been found in lentic and seasonally lotic habitats in the northern third of the continent. During recessional flow, as rivers become a series of pools, Conochironomus can be an important faunal component. There are few Australian wet season records from smaller temporary streams (Magela Creek, Rock Hole Creek) in which no dry season above-surface water is present for up to six months. The mechanism for survival in such perennial streams is unknown but dispersal from more perennial waters seems likely, since so few intermittent waters of arid Australia support the larvae.

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