Gerris lateralis and Caenis rivulorum
Possible new county records from Minsmere

Gerris lateralis is a small pondskater identifiable by the laterally placed patches of silvery hairs on the underside of the abdomen. The exact status of this species has always proved a problem. When doing the initial survey of existing freshwater records back in 1988 I found only one record for the county. This dated from 1953 in a pond at Leavenheath  just north of the Suffolk-Essex border.

Although I have since recorded the species from just over the border in Essex, the pond at the Leavenheath location does not appear to have been a typical habitat for the species even back in 1953 and is clearly not suitable now.  As no other specimens have been found elsewhere in the county in the last 16 years of surveying I had considered that it might well be absent.

It is therefore gratifying that I am able to report positively identifying this species from a well vegetated drainage ditch on the RSPB reserve at Minsmere in May 2003. Only one specimen was taken, but due to the large number of sites covered in only a short preliminary visit this is not surprising.

G. lateralis has a very scattered distribution in the British Isles, and although it has been found as far south as Berkshire it is recorded more often in northern Britain. However with only 135 records from 112 10km squares appearing  in the 2003 atlas of aquatic bugs (Huxley, 2003) it is nowhere common.

It is particularly associated with habitats where very shallow water occurs amongst tall emergent vegetation such as Carex. Minsmere seems to have plenty of suitable habitat although whether other populations will be found in Suffolk, particularly in marshy areas on the coastal strip remains to be found out.

As always any additional records are very welcome and I will also be pleased to hear of any sites which may have held G. lateralis in the past or any which may be worthwhile areas to search for the species.

Another first for my records on that trip to Minsmere was a single larval Caenis rivulorum.This mayfly has no previous records held on the database of the Freshwater Invertebrate Survey of Suffolk and so, as far as I am aware, this is a new county record. Hopefully I will hear to the contrary and somebody will point me to previously located but unpublished records for the species. The habitat at the reserve, a weedy drainage ditch, had a rather uncharacteristically muddy bottom as opposed to the stony bed usually preferred by the species and it may be that main populations might be located in larger streams nearby. Certainly this is a record that will need to be confirmed in future visits. Whatever the case the species recorded from the reserve are an indication that it will repay further work this coming season.

Reference

Huxley, T. (2003).    Provisional Atlas of the British Aquatic Bugs (Hemiptera, Heteroptera).  Huntingdon: Biological Records Centre.

Adrian Chalkley

Freshwater Invertebrate recorder

e-mail: Adrian@boxvalley.co.uk