Ceratinostoma ostiorum
A large, robust, dark grey, sea coast species. The whole fly is covered in dense dark grey dusting, with some brown areas on top of the thorax. Frons also rather dark blackish-brown. Only on the rather stout proboscis does the black ground colour show. Palps yellow-brown, conspicuously long and armed with rather stout, short black bristles. Legs entirely grey dusted, but the undersides of the tarsi are covered in a thick felty layer of yellowish-brown hairs. Wing length: ♂ 4.5 - 6.7 - 7.7 mm (16); ♀ 5.1 - 6.4 - 6.9 mm (6).
Widespread around the British coast and Collin (1958) reports it from "Inverness and the Isle of Arran down to the south coast of England". Irwin (1974) reports it from Northern Ireland (Strangford Loch). I have found it on rocky shores in Northumberland and on estuarine breakwaters in North Wales and Devon. In these types of location it can be extremely challenging to catch because it is very alert and difficult to approach closely and has a habit of flying very low and swiftly just above the water surface where you are more likely to get a wet net than a specimen! However, during the Dipterists Summer Field Meeting in 2006, it was swept in abundance from salt marsh in Pagham Harbour. According to Smith (1989) it breeds in thick, moist rotten seaweed on the sea shore.
World distribution: Palaearctic: Belgium, France, Germany, Iceland, Ireland, Lithuania, Netherlands, Norway, Poland; Nearctic: Canada, USA.