Lejops vittatus - has it declined or is it overlooked by lack of recording?

Lejops vittatus is a fairly easily identified species of Eristaline, but seems to be rarely recorded. It occurs in brackish environments, primarily in the ditch systems of the grazing marshes of Kent and Essex, the major coastal wetlands of Norfolk and grazing marshes of Somerset and Gwent. There is also a very old inland record that appears to be reliable (Figure 1). It is one of a group of genera whose larvae are aquatic and live deeply submerged and breathe through an extendable spiracle (rat-tailed maggots).

Figure 1. Distribution map for Lejops vittatus. Black circles = 2000 onwards, grey = 1980 to 1999 and open = pre-1980

What I had not previously relised was that the UK might be THE stronghold for this species in Europe if the GBIF map is anywhere near accurate. It otherwise seems to be coastal within the Baltic and there is a more central European record that I suspect is along the Danube (Figure 2). What this map does not tell us is that data on GBIF is somewhat patchy - there are good datasets for Germany and The Netherlands, but not for France, so the map is potentially somewhat misleading!

Figure 2. Northern European distribution of Lejops vittatus according to GBIF on 27 November 2017.

Analysis of the HRS data suggests that there has been a significant decline since the 1980s (Figure 3) but the data are very sparse, so I would treat this trend with some caution. What I think the trend tells us is that there may have been relatively little activity by skilled recorders in key areas. I have only seen this animal twice in 30+ years of recording; once in 1985 on the Somerset Levels and again at Seasalter in 1991. However, I have not looked very often so I don't think my experience is anything to go on! It is noteworthy, however, that the 1991 record arose because I was intensively surveying the ditch systems of the North Kent marshes and always had my eye open for this species. I think it is genuinely rare on the Kent coast!

Figure 3. Frescalo trend for Lejops vittatus.

Looking at the phenology histogram (Figure 4), it seems to me that this species has quite a narrow emergence window centred upon late June and early July. The very early and very late outliers in the data need to be more thoroughly investigated, although there is just a possibility that they coincide with exceptionally warm years when there might have been a partial second generation.

Figure 4. Phenology histogram for Lejops vittatus based on all records in the HRS dataset.

This one seems to me to be a potential 'target species' for Dipterists based in Somerset, South Wales, Norfolk, Essex, Kent and East Sussex. It should be identifiable from photographs, so that makes it a viable project for the many photographic recorders that live in these parts. I would add that whilst in such places it is also worth making an effort to record the Soldierflies and Horseflies - there are also some very rare and habitat-specific species that are likely to be under-recorded.