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Thanks are due to the following organisations for permission to record: Somerset Wildlife Trust for West Coker Fen and the Wildlife and Wetlands Trust for the Huntspill River and Steart Marshes sites. The latter two site visits were organised by Phil Thorpe, Natural England, as part of the activities of the Recorders of the Avalon Marshes (RoAM).

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Ball, S.G., "Key to the British Scathophagidae", Dipterists Forum, February 2014

Falk, S.J., "A review of the scarce and threatened flies of Great Britain (Part 1)", Nature Conservancy Council, Peterborough, 1991

British Scathophagidae https://scathophagidae.myspecies.info/scathophagid-checklist/conisternum-decipiens

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A mating pair of C. decipiens taken on 24th November 2020 from West Coker Fen, Somerset

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On three occasions, twice at West Coker Fen on 24th November and once at Steart Marshes on 9th November, pairs of C. decipiens were found to be mating inside my pooter and were still joined post mortem. Rather than risk damage by trying to separate them, I took the opportunity to pin and mount them as a pair.

It would appear that C. decipiens is highly sexually active at this time of year, presumably shortly followed by egg-laying.

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Conisternum decipiens in cop Coll 24Nov20 West Coker Fen

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Month of occurrence of C. decipiens from NBN (21 records) and iRecord (20 records) together with the three records reported here (MA)

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This figure shows the month of occurrence for C. decipiens for all records which have an identifiable month, from NBN (21 records) and from iRecord (20 records) together with the three records reported here (MA). The number of records is limited but the data suggest that the species may be bivoltine with a first generation peaking in May and a second generation centred around October. A bivoltine distribution is also suggested in the data presented on the British Scathophagidae website.

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NBN Easy Map for C. decipiens, 42 records, generated 19.xi.2021

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The National Biodiversity Network (NBN) database has 42 records of C. decipiens showing a scattered distribution across southern England and Wales, south of a line from Lancaster to Scarborough. iRecord has a further 21 records similarly distributed. The species is designated Notable (Falk, 1991).

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C. decipiens has recently been found at three sites in Somerset in late October and into November.
24.xi.2020: two females and three males (including two mating pairs), from West Coker Fen, an inland Somerset Wildlife Trust site with flower-rich meadows bordered by streams (ST505137, VC5)
26.x.2021: six males from vegetation alongside the Huntspill River at a coastal location where it joins the River Parrott just short of where the latter meets the Severn Estuary (ST294456, VC6)
9.xi.2021: fourteen females and seven males (including one mating pair) from vegetation surrounding freshwater pools at the WWT Steart Marshes, a coastal wetland site near the outflow of the River Parrott into the Severn Estuary (ST255440, VC6)

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C. decipiens taken at the Huntspill River site on 26th October 2021

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A medium-sized dung fly (ca. 5-6mm) with head, thorax and abdomen mostly grey, femora grey, all tibiae and tarsi reddish-yellow. The frons is reddish yellow, arista bare, palpi yellow and proboscis shining black. The key feature which distinguishes it from the closely related C. obscura (Fallén, 1819) is the presence in decipiens of two intra-alar bristles, absent in obscura (Ball, 2014).

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Conisternum decipiens m Coll 26Oct21 Huntspill River

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The dung fly Conisternum decipiens (Haliday in Curtis, 1832) (Diptera: Scathophagidae) is uncommon and recorded from damp habitats across southern England and Wales. We present new records from late October and November at two coastal sites and one inland fen in Somerset. Evidence of mating activity in November is presented. Records suggest that the species may be bivoltine with a second generation centred around October.