Diptera Survey in Glen Affric in 2008

In 2008, as part of our ongoing project to document the biological diversity in the Glen Affric National Nature Reserve, we commissioned Graham Rotheray to carry out a survey of the Diptera, or two-winged flies, occurring in the glen. Graham is the Curator for Insects at the National Museums for Scotland in Edinburgh, and a member of the Malloch Society who had previously done some survey work for us on the saproxylic, or dead wood-dependent, insects associated with aspen in Affric in 2001. This new survey was the first organised effort to identify the Diptera in the glen, and as with other recent surveys there, we’re very grateful to Forestry Commission Scotland (FCS) for covering half of the costs.

Between May and September Graham spent eight days in Affric, utilising a variety of methods to locate either Diptera larvae or adult flies, and his visits were timed to coincide with periods when Diptera abundance and diversity was likely to be high. The survey techniques ranged from searching underneath the bark of dead standing trees (snags) or fallen logs for larvae or puparia (the cases or cocoons within which larvae pupate into their adult forms), to sweep-netting of ground vegetation and tree branches to catch adult flies.

From the specimens that he collected, Graham identified a total of 134 species (see the list on our web site), with hoverflies being particularly well-represented amongst these. Few craneflies were seen during the survey, but that reflects a national trend in 2008, as it was generally a good year throughout the country for hoverflies, but a poor one for craneflies. Populations of insects such as Diptera tend to fluctuate from year to year, partly in response to annual weather variations, but also for reasons that are still not fully understood by biologists.

I spent a couple of days out with Graham during his survey, and learned a lot from his experience and knowledge, both of the insects themselves, and about their habitat requirements and preferences. As the third largest remnant of the Caledonian Forest, and with a lot of old trees, Affric provides a good habitat for the saproxylic insects whose larvae live in dead wood. On one of the days when I accompanied him, Graham found one species of saproxylic fly (Lonchaea ragnari) that was first recorded in the UK in 1996, and is still only known from a handful of sites in the Highlands. It breeds in the decayed wood of large dead birch trees, and because of its rarity, has been added to the Biodiversity Action Plan (BAP) list as a priority for conservation action.

Another deadwood-dependent species found in the survey is a hoverfly (Callicera rufa) that is particularly associated with the Caledonian Forest, as it breeds in water-filled rot holes in Scots pine trees and stumps. Graham had found this species in a distinctive pine during a previous visit to Affric in 1990, and the larvae were present again in the same tree during this survey, 18 years later, indicating that old pines provide a habitat for these hoverflies over a long period of time. The adults of this species, which are bumblebee mimics, are rarely encountered, and Graham demonstrated the unglamorous technique necessary to locate the larvae. Reaching his arm elbow-deep into the water-filled hole where a large branch left the trunk of an old pine, he pulled up a mass of brown sludge which contained several squirming larvae of the hoverfly – not everyone’s idea of a good day’s work! This species was formerly listed as a Red Data Book (RDB) species, but has recently been downgraded in status, because of an expansion of its range, due to the felling of conifer plantations providing an increased habitat in the form of abundant tree stumps.

One specimen collected during the survey appeared to be of a species (Sphaerophoria bankowskae) not recorded in Scotland before, with the only other UK record being from Essex in 1990. If this provisional identification is confirmed, it will be one of the most significant results of the survey.

In terms of habitats, the richest areas for Diptera were the wet grasslands and meadows, especially near Dog Falls at the eastern end of the glen, and Graham commented that the patchy mixture of those habitats, along with fens and bogs, makes Affric a special place for Diptera. His report also emphasises the glen’s importance for saproxylic insects and recommends that additional surveys be carried out for these in future, as it is likely that there is a greater range of species present than those found during the 2008 survey.

Alan Watson Featherstone

Pages about Diptera on this site

• Diptera Survey in Glen Affric 2008
• The Malloch Society survey of Glen Affric, Glen Cannich and Corrimony in May 2001
• Diptera occurring in Glen Affric
• Insects recorded on Dundreggan
• Highland biting midge species profile

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