Polypedilum Kieffer

Cranston, Peter S., 2007, The Chironomidae Larvae Associated With The Tsunami-Impacted Waterbodies Of The Coastal Plain Of Southwestern Thailand, Raffles Bulletin of Zoology 55 (2), pp. 231-244 : 241-243

Polypedilum Kieffer

( Figs. 48–59 )

This most species-rich genus of Chironominae is very common in tropical waters, including those of Thailand. The larvae of the genus require taxonomic study: there are many names, many species and different names have been used for the same organisms in different parts of the range. The current subgeneric system does not work for all larvae.

Features that can assist in discriminating between larvae:

Mentum ( Figs. 48, 51–53 ): relative heights of teeth and evenness of size of increasingly lateral teeth; ventromental plates: relative dimensions, inner contour, medial separation and number of striae;

Mandible ( Figs. 58–59 ): number of inner teeth, extent of pigment and size of outer tooth;

Dorsal surface of head ( Fig. 57 ): shape of anterior margin of frontoclypeal apotome, depth of any hyaline anterior submargin thereof, position of cephalic seta S3 relative to anterior and lateral margins;

Antenna ( Figs. 54–56 ): relative and absolute lengths of segments and blade.

Polypedilum leei Freeman is easily-recognisable amongst the regional species by the mentum ( Fig. 48 ) with all teeth more or less equal in size combined with a five-toothed premandible. The presence in the anterior abdomen of a sclerotised ‘crop’ ( Fig. 49 , arrow) is highly characteristic, perhaps even unique. Larvae of P. leei occurred in several site recovering from tsunami impact (314–1,847 µS. cm−1 conductivity, 157–927 ppm), including three sequential samples from one site and also in the dilute, unimpacted pond (25–42 µS. cm−1 conductivity, dissolved solids 13–21 ppm) in which P. nubifer (see below) occurred. A pharate larva and a pharate female pupa confirms the identity and association.

Polypedilum ‘Thai sp. 1’ has a mentum with relatively even tooth height, although the first laterals are lower than the medians and 2 nd laterals, but not excessively so ( Fig. 51 ). The premandible has the typical two apical teeth of most congeners, but unlike P. leei . The larva resembles Polypedilum ‘B1’ figured in an Australian guide ( Cranston, 2000), with a ratio of the ventromental plate width to height near to three (108 × 38 µm) and with a distinctive rather elevated cluster of outer (5 th –7 th) mental teeth. However, the curvature of the ventromental plate of P. ‘B1’ is not evident in the Thai specimen, and conspecificity is unlikely. The single larva of this type occurred in an unusual pond which had water chemistry of 1,571 µS. cm−1 and dissolved solids 786 ppm on the one occasion in which the larva was found, but although unimpacted. showed increasing conductivity with the passage of time.

Polypedilum nubifer Skuse , the type species of Polypedilum , has a typical mentum ( Fig. 52 ) but is atypical for the genus in having the Lauterborn organs alternate on an antenna apparently composed of four segments ( Fig. 55 ). Although larvae of P. nubifer are common in eutrophic and disturbed habitats almost globally, in post-tsunami surveys the species was found only in two consecutive samples from one pond of conductivity 25–42 µS. cm−1, dissolved solids 13–21 ppm.

Polypedilum (Pentapedilum) ‘ K1’‘. This larva is unusual amongst surveyed Polypedilum in that the first lateral mental teeth are very small, each almost adpressed to a median tooth ( Fig. 53 ), the ventromental plate width-height ratio is ca. 2.0, the antennal 3 rd segment is half the length of the 4 th ( Fig. 56 ) and the mandible has three distinct inner teeth ( Fig. 58 ). In all these features the larva is identical to Polypedilum (Pentapedilum) ‘K1’ from northern Australia, which may belong to P. nodosum (Johannsen) described from Indonesia. A larva of this type occurred on consecutive sampling dates in a pond recovering from impact (to conductivity of 515, then 314 µS. cm−1, and to dissolved solids of 259 and 157 ppm respectively).

The most common, widespread and salinity-tolerant species of Polypedilum is one that falls within a complex of species scarcely distinguishable in the larval stage. This complex includes the widespread P. convexum (Johannsen) , the recently-described P. australotropicus Cranston , and several Australian taxa distinguishable only with difficulty in the larval stage. The occipital margin is pale, the frontoclypeus shows only a modest pale area anteriorly ( Fig. 57 ) and the antenna has segments 3 and 4 more or less similar in length ( Fig. 54 ). The mandible has two free inner teeth ( Fig. 59 ) (note that the innermost ‘tooth’ is not free-standing as seen in Fig. 58 but is fused into the mola). Given the apparent lack of conspecificity with previously recognised species, the species here is termed Polypedilum ‘Thai 2’.

Figures

Figs. 48–59. Thai post-tsunami Chironomidae. Polypedilum leei: 48, mentum, ventromental plate; 49, anterior thorax; 50, premandible. Mentum, ventromental plate: 51, Polypedilum ‘Thai sp 1’; 52, Polypedilum. nubifer; 53, Polypedilum (Pentapedilum) ‘K1’. Antenna: 54, Polypedilum ‘Thai 2’; 55, P. nubifer; 56, P. ‘K1’. 57, Dorsal head sclerites, P. ‘Thai 2’. Mandible; 58, P. ‘K1’, 59, P. ‘Thai 2’. Abbreviations: S3, cephalic seta 3; sc1, labral sclerite 1; vmp, ventromental plate.