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The NBN atlas has 56 records for Gasterophilus intestinalis, with the largest cluster in North Wales close to our location. The most recent North Wales record is from Anglesey in 2015. This species is almost certainly more common than the data suggests, with the very short adult flight period restricting the number of records.

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The males we encountered were very obviously hilltopping, noisily buzzing around, hassling each other and competing for the best perch on the rocks. Perched males were also "singing" producing a sound like that of a low-pitched mosquito. On closer inspection, singing males were clearly vibrating theirs wings rapidly, presumably to produce the sound.

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Gasterophilus intestinalis lifecycle

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Some Gasterophilus intestinalis larvae in the collections at the Natural History Museum, London.

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Mature larvae of Gasterophilus intestinalis in situ, attached to the stomach lining of a horse. Complete with bits of dried grass from the horse's last meal!

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Gasterophilus intestinalis (male)
There were between 10 and 20 flies buzzing around and settling on the exposed rocks at the summit of Gyrn Wigau. They were very obvious and easy to spot.

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The route we walked is shown in red, and our lunch stop where we spotted the flies is circled in green. It was a perfect summer's day with blue skies and very little wind.
The area was typical open Welsh mountain pasture, grazed by sheep and feral ponies. The summit was of exposed rocks, with the strata emerging almost vertically in thin slabs a few meters high.

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Date: 28th August 2021
Location: Gyrn Wigau, a small peak with an exposed rocky summit; approx. 3 km east of Bethesda town centre.
Latitude = 53°11'13.48"N
Longitude = 4°0'52.07"W
Grid Ref = SH65416748