1. Introduction

Remains of chironomid larvae in lake sediments have been widely used for reconstructing the past limnology of lakes. Recently, fossil chironomid research has received additional momentum by the development of statistical inference models linking the distribution of chironomid assemblages to climate or water chemistry variables and fossil chironomids have now been used to quantitatively reconstruct parameters such as temperature, or the past salinity or nutrient conditions in lakes. 

Larval remains of chironomids, especially the heavily sklerotized head capsule, are well preserved in lake sediments. Using existing keys for living larvae chironomid head capsules are identifiable to genus- or species group-level in many cases. However, the remains of one subgroup of chironomids, the tribe Tanytarsini, are exceptionally difficult to identify based on larval head capsules. For the head capsules of most chironomid larvae the mentum and ventromental plates provide the most useful characters for identification. However, within the Tanytarsini these characters are very uniform. Previously, subfossils of most Tantarsini have either been lumped into a single taxonomic unit or they have been been classified into different categories based on head capsule characteristics such as the presence or absence of a spur on the antennal pedestals, the length of the antennal pedestals, and the number and arrangement of teeth on the larval mandible and premandible. However, since the larvae of only a few Tanytarsini have been described in the scientific literature it is difficult to relate this parataxonomical classification scheme to the modern taxonomy and systematics of the Chironomidae. 

Since the Tanytarsini include a number of valuable environmental indicator species it would be useful to achieve a classification scheme for fossil specimens which is compatible with the modern taxonomy of the tribe. In the following we present photographs and descriptions of the head capsules of a range of European Tanytarsini species. We focus on head capsule features which are visible and preserved in fossil material and on the three genera Micropsectra, Paratanytarsus and Tanytarsus. We also present a classification scheme for subfossil head capsules of the examined species, which we compare with available classifications for fossil Tanytarsini. At present about 40% of the described Micropsectra, 15% of the described Paratanytarsus and 45% of the described Tanytarsus species of the European chironomid fauna are included in this document. Within the following months we will hopefully be able to obtain associated larvae from further species to provide a more comprehensive compilation. For a few additional Tanytarsini genera photographs of the larval head capsules of selected species are also available and these can be found in the alphabetical species list at the end of this document.

2. Methods

Except where otherwise indicated, species identification has been based on associated pupae or adults and photographs are of fourth-instar larvae. The reader is cautioned that the described features may be somewhat different in lower-instar larvae. Only a low number of specimens per species have been examined for this study and it is difficult to assess the within-species variability of characters such as the head capsule pigmentation. However, the presented classification scheme is based on features which - based on our experience in identifying fossil material - tends to be fairly uniform for a given taxon. The terminology of mandible teeth used within this document is given in Figure 1 below.

3. Results

The examined material indicates that fourth instar headcapsules of European Micropsectra can be separated into at least five categories recognizable on fossil material. Larval Paratanytarsus have been separated into two and Tanytarsus into eight classes. Six classes include a single species only, whereas the rest of the categories presently include two to sixteen species. However, this will obviously change once additional Micropsectra, Paratanytarsus and Tanytarsus species are incorporated into the classification. 

The proposed classification scheme is largely compatible with other described classifications based solely on subfossil material (see 3.4 below). However, it is based on a few selected characters only (i.e. features which are visible and well preserved in fossil material) and the reader is cautioned that the proposed classes do not always agree with the species groups defined for Paratanytarsus, Micropsectra, and Tanytarsus in modern chironomid systematics. For example, due to the mandible morphology we have classified Tanytarsus mancospinosus and Tanytarsus gracilentus together with species from the Tanytarsus lugens-group to produce the "fossil" category Tanytarsus lugens-type. However, in modern chironomid systematics Tanytarsus mancospinosus is considered to belong to the mendax group (Ekrem, 2004) and Tanytarsus gracilentus to the norvegicus group (Reiss and Fittkau, 1971). Similar discrepancies exist for Micropsectra and will probably also be found for Paratanytarsus once additional species are examined.

3.1. Classification scheme for European Micropsectra based on the examined material. 

Generic diagnosis for Micropsectra is given in Wiederholm (1983). In fossil fourth-instar larvae the combination of a short and sharp, a long medium-sized and sharp or a long straight and blunt spur, bifid premandibles and a reduced to medium-sized postoccipital plate will often separate Micropsectra from other Tanytarsini. M. seguyi does not feature a spur. However, in contrast to Paratanytarsus, the fourth instar larva of M. seguyi has long antennal pedestals and a well developed postoccipital plate.

A. M. radialis-type: Short antennal pedestal, short sharp spur present, postoccipital plate strongly reduced to a thin band following the postoccipital margin.

M. radialis

B. M. aristata-type: Long antennal pedestal, long, straight and blunt spur present

M. aristata

C. M. bidentata-type: Long antennal pedestal, medium-sized, straight and sharp spur present

M. bidentata

D. "M. insignilobus"-type: Long to medium-sized antennal pedestal, short sharp spur present, postoccipital plate well developed, except for M. groenlandica and M. atrofasciata, which have somewhat reduced postoccipital plates (but still larger than in M. radialis). Note that M. insignilobus has not yet been examined. However, this type of Micropsectra head capsule is called M. insignilobus-type in many published fossil data sets and we therefore retain this name until we are able to examine associated larval material of M. insignilobus.

M. atrofasciata M. contracta M. groenlandica M. junci M. pharetrophora

E. M. seguyi-type: Long antennal pedestal, spur absent, postoccipital plate well developed, triangular with rounded anterior margin, not trapezoidal as in many Tanytarsus

M. seguyi

Recognized West Palaearctic Micropsectra species not examined within this study:  M. apposita, M. attenuata, M. auvergnensis, M. bodanica, M. borealis, M. clastrieri, M. fusca, M. insignilobus, M. lacustris, M. lindebergi, M. lindrothi, M. notescens, M. recurvata, M. roseiventris, M. subnitens

3.2. Classification scheme for European Paratanytarsus based on the examined material.

Generic diagnosis for Paratanytarsus is given in Wiederholm (1983). In fossil fourth-instar larvae the combination of a short antennal pedestals, the absence of a spur, the strongly reduced postoccipital plate and the strongly incised ventral postoccipital margin will often separate Paratanytarsus from other Tanytarsini.

Recognized West Palaearctic Paratanytarsus species not examined within this study: P. abiskoensis, P. bausellus, P. boemicus, P. chlorogyne, P. confusus, P. dimorphis, P. hyperboreus, P. inopertus, P. inquilinus, P. intricatus, P. laccophilus, P. laetipes, P. lauterborni, P. luteola, P. mediterraneus, P. natvigi, P. pseudotenellulus, P. setosimanus, P. tenellulus, P. tenuis

3.3 Classification scheme for European Tanytarsus based on the examined material.

Generic diagnosis for Tanytarsus is given in Wiederholm (1983). In fossil fourth-instar larvae the medium-sized to long antennal pedestals and the large, often trapezoidal postoccipital plate will often separate Tanytarsus from other Tanytarsini. The examined Tanytarsus species may feature a large, backwards-curving, a medium-sized straight and blunt or a short and blunt spur on the antennal pedestal. However, none of them features a short and sharp spur as can be found in many Micropsectra larvae. 

Recognized West Palaearctic Tanytarsus species not examined within this study: T. aberrans, T. aculeatus, T. appeninicus, T. cretensis, T. curticornis, T. debilis, T. decipiens, T. dispar, T. excavatus, T. fennicus, T. glabrescens, T. heusdensis, T. innarensis, T. latiforceps, T. longitarsis, T. miriforceps, T. multipunctatus, T. niger, T. nigricollis, T. norvegicus, T. palletaris, T. pallidicornis, T. quadridentatus, T. recurvatus, T. sinuatus, T. smolandicus, T. sylvaticus, T. tika, T. veralli)

3.4. Comparison of the proposed classification scheme with existing classifications

4. Acknowledgements

Slides of reared and associated Tanytarsini larvae were made available for this study from a range of sources. We would like to thank all the involved scientists and institutions for their support. These include G.A. Halvorsen, Ø.A. Schnell, and E. Stur, the Museum of Zoology, Bergen, and the Zoological Staatssammlung Munich. The first author's visit at the Museum of Zoology, Bergen, was supported by a Swiss National Science Foundation fellowship for prospective researchers (Fellowship 81BE-66224) and the Norwegian Research Council Strategic University Project ‘Norwegian Palaeoenvironments and Climate’ (NORPEC).

5. References

Brooks, S.J., 2000. Late-glacial fossil midge stratigraphies (Insecta: Diptera: Chironomidae) from the Swiss Alps. Palaeogeography, Palaeoclimatology, Palaeoecology 159: 261-279.

Brooks, S.J., 2004. Diagnosis of Tanytarsini. Unpublished. 3 pp.

Heiri, O., 2001. Holocene palaeolimnology of Swiss mountain lakes reconstructed using subfossil chironomid remains: past climate and prehistoric human impact on lake ecosystems. PhD, University of Bern, Bern, 113 pp.

Hofmann, W., 1971. Zur Taxonomie und Palökologie subfossiler Chironomiden (Dipt.) in Seesedimenten. Arch. Hydrobiol., Beih. 6: 1-50.

Lotter, A.F., H.J.B. Birks, W. Hofmann & A. Marchetto, 1997. Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages as quantitative indicators for the reconstruction of past environmental conditions in the Alps. I. Climate. J. Paleolimnol. 18: 395-420.

Reiss F. and E.J. Fittkau, 1971. Taxonomie und Ökologie europäisch verbreiteter Tanytarsus-Arten (Chironomidae, Diptera). Arch. Hydrobiol./Suppl. 40: 75-200.

Wiederholm, T. (ed.), 1983. Chironomidae of the Holarctic region. Keys and diagnoses. Part I. Larvae. Ent. Scand. Supplement 19: 1-457

Appendix: List of examined species

Micropsectra

Micropsectra aristata

Micropsectra atrofasciata

Micropsectra bidentata

Micropsectra contracta

Micropsectra groenlandica

Micropsectra junci

Micropsectra pharetrophora

Micropsectra radialis

Micropsectra seguyi

Paratanytarsus

Paratanytarsus austriacus

Paratanytarsus bituberculatus

Paratanytarsus penicillatus

Tanytarsus

Tanytarsus bathophilus

Tanytarsus brundini

Tanytarsus buchonius

Tanytarsus chinyensis

Tanytarsus dibranchius

Tanytarsus ejuncidus

Tanytarsus eminulus

Tanytarsus formosanus (no photographs available)

Tanytarsus gibbosiceps

Tanytarsus gracilentus (no photographs available)

Tanytarsus gregarius/inaequalis (no photographs available)

Tanytarsus lactescens

Tanytarsus lapponicus

Tanytarsus lestagei

Tanytarsus lugens (no photograph available)

Tanytarsus mancospinosus

Tanytarsus medius *

Tanytarsus mendax

Tanytarsus nemorosus

Tanytarsus occultus

Tanytarsus pseudolestagei

Tanytarsus signatus

Tanytarsus striatulus (no photograph available)

Tanytarsus telmaticus

Tanytarsus usmaensis

Tanytarsus volgensis

Tanytarsus spec. C (fossil specimen)

Other Tanytarsini

Constempellina brevicosta

Rheotanytarsus pentapoda

Cladotanytarsus pallidus