True flies

Wet, acid, species-rich mires are found across Devon. These include the valley mires on Dartmoor (many of which are within Rhos pasture mosaics), the spring line mires of the Blackdown Hills and smaller areas of valley mires which form part of a heathland mosaic in areas such as the East Devon Pebblebed Heaths.

These damp, vegetated areas provide perfect habitat for flies and support at least 15 species which are nationally scarce or threatened. Many of these sites are protected as SSSIs or nature reserves. However, their management tends to focus on species that are better known and easier to monitor. Key sites on Dartmoor include Raybarrow Pool, Foxtor Mire, Emsworthy and Blackslade Mire. More details and a list of the 15 species can be found in the LNRS valley mires species profile for flies, see Find out more below.

Current pressures for these flies include loss of open habitat to willow due to lack of grazing and increased nutrient levels due to atmospheric nitrogen deposition and occasionally through agricultural fertilizer run-off. Climate change is a particular challenge for this group because species are at the southern edge of their range and as flies will be badly affected by droughts if these result in mires drying out, even for short periods. See heath, bog, mire and species-rich pasture for more details on valley mires.

The Devon globetail hoverfly is a dainty black and yellow-banded hoverfly that is known from just six sites in the UK. Four of these sites, Beaford Moor SSSI and Common Moor (East Putford) SSSI in north Devon, a site on the flanks of Cosdon Hill on Dartmoor (near South Zeal, Okehampton), and Gotleigh Moor on the border with Somerset in the Blackdown Hills.  The remaining two sites are nearby moors in mid-Cornwall. Although the hoverfly is found elsewhere in north-western Europe, it’s considered rare throughout its range and is considered vulnerable to extinction, making every population significant.  

The species is very hard to identify unless it is examined under a microscope as there are a number of other very similar species.  

The Devon globetail hoverfly lives in wet, acid spring-line mires and wet species-rich rush pastures where it seems to prefer areas with tussocky purple moor grass and wet runnels with some small shrubs and trees. Adults feed on heath tormentil flowers and larvae are likely to feed on aphids. Threats may include heavy cattle grazing, scrub invasion and winter burning, although more research into its precise habitat requirements is needed. Given the hoverfly’s preference for very wet conditions, climate change may also be a threat as it continues to cause more frequent and more severe summer droughts. 

For more detailed information see the LNRS profile for Devon globetail hoverfly in Find Out More.

Wet woodlands are found across Devon but are particularly frequent on the heavy, wet, clay soils of the Culm Measures that occupy the land between Dartmoor, Exmoor and the Atlantic Coast.  Other key areas are the edges of Dartmoor National Park, where they often occur together with Rhos pastures, and the spring lines of the Blackdown Hills. 

Devon’s wet woodlands support an exceptional range and abundance of fly species. One wet woodland in west Devon has recently been estimated to produce 38 million flies per hectare per year. They support many nationally rare or scarce flies, including four listed as Devon Species of Conservation Concern. Three are considered vulnerable to extinction in Britain (Macrocera longibrachiata, Neoempheria winnertzi, and Rhaphium fascipes) and one is near threatened (Coenosia pudorosa).  See the Devon Species of Conservation Concern spreadsheet.

Other species in this group include Egle parvaeformis which is near threatened and four nationally scarce species, the liverwort snipefly (Spania nigra), the sooty blacklet hoverfly (Cheilosia carbonaria), the rust fly (Chyliza vittata) and the lesser housefly (Fannia speciosa).  See Find out more for a link to the more detailed wet woodland fly group profile and a list of all species in this group.

Flies in this group are strongly dependent on conditions relating to water within the wet woodlands – water quality, quantity, flows etc. In Devon key impacts include land drainage and pollution (mainly from fertiliser run-off). Climate change, especially the increasing frequency and severity of summer droughts, poses a major threat to this group as fly larvae are dependent on continuously moist conditions.

Flies can also be at risk from lack of grazing. Wet woodlands are often permanently fenced off from adjacent wet grasslands because it’s thought that this will benefit their wildlife. However, research in Devon shows that the fly population is likely to be more diverse and richer in scarce species when there is some cattle grazing.

See the Wet Woodlands page for more details.

Read more on Dead wood invertebrate group page. (TO ADD)

A nationally rare fly that is dependent on the smooth-gastered furrow bee (Lasioglossum parvulum). It belongs to a group of flies known as satellite flies because they shadow their host bees to their nesting sites. The fly lays its eggs in the bee’s nest and the larvae feed on the pollen and nectar the bee collects.    

This fly has only been recorded seven times in the UK since 1960, all in southern England and southern Wales. In Devon it has only been found at Halsdon Nature Reserve, where a vast colony of furrow bees nests in the soft, eroding banks of the River Torridge.    

The fly’s main threats relate to the furrow bee and include loss of the riverbank due to high flows and flooding, and, if cattle are prevented from accessing the river’s edge,  too much stabilization and excess shading from trees.   

Read more on the north and south Devon coastal invertebrate group page.

Read more on the Sidmouth to Dorset coastal wildbelt invertebrate group page.

Read more on the Start Point to Bolt Tail coastal wildbelt invertebrate group page.

The saltmarsh and tidal mud flats in the ‘Bight of Dawlish’ at the mouth of the Exe Estuary support at least three threatened or near threatened flies that are found in few other areas nationally. There are three species listed as Devon Species of Conservation Concern.

The lagoon snout cranefly (Geranomyia bezzii) lives on tidal mudflats where its larvae feed on green algae. The weak-flying adults shelter in low-growing bushes such as sea purslane, just above the highwater mark. It’s only known to be in five other UK sites (on the south and east coasts), however these records are old. It is assessed as vulnerable to extinction in Britain.

The only known UK location of the housefly (Neolimnophora maritima) is Dawlish Warren, although it hasn’t been seen since 2011 (in 1939 it was seen at Blakeney Point in Norfolk). Its precise habitat needs are unclear, but it appears to prefer the margin between foredune and tidal flat.  This fly is near threatened.

Muscidideicus praetextatus is mainly associated with the north Norfolk coast and Dawlish Warren, although it is also known from a few other sites around the British coast. It’s restricted to saltmarshes, although adults may be found on nearby dunes or estuarine mud. Again, its precise habitat needs are unknown but given its very restricted distribution are likely to be highly specialised. The fly is vulnerable to extinction. 

The Bight of Dawlish is affected by rising sea levels as a result of climate change. The saltmarsh is showing signs of stress as it is squeezed between the sea and the artificial sea wall that protects the spit and its golf course.  

The tidal flats have also been put under pressure by coastal defence work on the sand dune system. For example, in 2018, the Environment Agency carried out major works to restructure the dune spit at its narrowest part.  

Read more on the sand dune invertebrate group page.

The least cigar-gall fly is a small endangered fly. It’s dependent on freshwater and tidal reed beds where its larvae develop in galls on the stems of common reed. The galls provide the only known home for an even smaller fly, Cryptonevra consimilis, which is also endangered. The fly overwinters in the dead reed stems. 

In Devon, the least cigar-gall fly is found in a small patch of reedbed in the Axmouth-Lyme Regis Undercliffs SSSI and two Devon Wildlife Trust sites at the top of the Exe Estuary, the Exe Reed Bed and the Old Sludge Beds. Cryptonevra consimilis has also been found at Springhead Fen near Axmouth, suggesting that the least cigar-gall fly is there too.     

Nationally, the fly is only found in nine other sites across East Anglia, the New Forest, Kent, Dorset and Cornwall. Due to the small size of the least cigar-gall fly and its dependant, it can be hard and time-consuming to monitor the species. 

The fly’s freshwater sites in Devon are at risk from willow invasion shading out the common reed. Threats to the tidal reedbeds at the head of the Exe estuary are less immediate but in the longer term may include rising sea levels as a result of climate change.