On 13th July Chris Puddephatt captured, with some care, a huge horsefly that had found its way into his conservatory at Clashnessie (NC0530). It was nearly an inch long, with a generally dark body, but each segment of the abdomen bore an orange band with a central yellow triangle. His first photos showed the underside through the walls of a glass container (photo 1), but he was later able to get an upperside view (photo 2).
From the size and markings we were able to identify it as a female dark giant horsefly Tabanus sudeticus. This is not only the largest member of the horsefly family Tabanidae, which also includes the notorious clegs, but also the largest fly found in the British Isles. The females have a narrow gap between the eyes, in the males they are touching.
There appears to be just one previous record of this fearsome insect in Assynt, one seen by Murdo Macdonald at Culkein Drumbeg (NC1133) on 19th July 2019. However, Gwen Richards photographed another female on her car at Loch Ailsh (NC316102), about 4km east of the parish boundary, during the AFC field meeting there on 17th July 2022 (photo 3).
This seems to have been a bumper year for the species locally, since Chris saw another on the path up to Bealach a’Chornaidh on Quinag (NC2128) on 14th July, Fern Stewart had one at the campsite at Achmelvich (NC 0524) on the same day, and it was also reported from the Clashmore area (NC0331) by Eachan and Frances Mackenzie.
It is the female horseflies which should be most feared since they need a meal of blood for egg-laying, their usual targets being cattle, horses and deer. Happily for us, it is noted that ‘their very deep hum will betray their presence while flying’.
Most tabanid larvae appear to require moist conditions in which to live, such as ‘damp soil adjoining burns or lochans’, where they feed on other insects and earthworms. Few larvae of the dark giant horsefly have ever been found, but they are thought to inhabit bogs and boggy flushes.
Ian M. Evans