Stenus palposus – a rove beetle
Stenus palposus Zetterstedt, 1838
Family: Staphylinidae
This insect belongs to a very large family of beetles in which the wing cases are shortened to expose a flexible, segmented abdomen. These are the rove beetles (Staphylinidae) with 671 species recorded from Ireland (including Pselaphidae). Rove beetles are notorious for their smallness and for the difficulty in identifying many of the smaller and more obscure species.
There are 18 subfamilies present in the British Isles. Stenus palposus belongs to subfamily Steninae with 53 species in Ireland. There are two genera, Stenus (52 species) and Dianous (1 species).
The Steninae are minor celebrities in the scientific world because of their startling ability to skim across the surface film of water at speed. This is a useful ability in beetles which live very close to water and are in constant danger of falling in. Some live in very hazardous places, Dianous coerulescens and Stenus guynemeri, occurring in moss in the middle of waterfalls! The trick to their unique form of ’jet’ propulsion lies in having two small glands in the tail which release a surfactant (detergent) on to water behind the animal. This reduces the surface tension to the rear and propels the animal rapidly forward on the surface film. Stenus and Dianous which have fallen into the water can be seem whirling around in circles before setting off to a nearby bank by a minor adjustment of their tails (Figure 1).
In brief
- Formerly found on sandy shorelines of Lough Neagh but unknown elsewhere in the British Isles. Last recorded in 1983 and now believed extinct
- Confined to fine sand habitats on river or lake margins where it feeds on springtails (Collembola)
- Adults have only been observed in May and June
- This is a UK Priority Species which has its sole British Isles site in Northern Ireland and it has declined
- Sand extraction, consequent beach erosion, pollution and disturbance are factors which are likely to have caused the decline of the species.
Species description
The Steninae have a rather characteristic shape designed to let them clamber up and down stems near water in search of food – mainly springtails or small flies. This includes a very large pair of eyes to home in on prey (Figure 1) and a long, extendible pharynx (mouth) which can be shot out like the tongue of a chameleon to snatch springtails and other agile species (Figure 2).
The Irish Steninae are unusual in that two species found here do not occur in Britain. These are Stenus palposus and Stenus glabellus Thomson. Stenus glabellus occurs in a very restricted geographical area of County Westmeath on quaking bog (scragh). Stenus palposus is confined to Lough Neagh where it has occurred on several of the larger sandy beaches at the northern end.
Life cycle
Adults have been observed in May and June running swiftly on the surface of damp sand on a beach near Moyola Waterfoot. Its breeding habits and seasonality are otherwise unknown, as are the exact reasons for its disappearance.
Similar species
Most of the Stenus species are closely similar, making identification difficult. Stenus palposus is distinct from the other British and Irish species in having clumps of silvery hairs on the elytra which can be seen as indistinct whitish areas (Figure 3). These should not be confused with the round red spots found on two of the Irish Stenus (as in Figures 1 and 2) and in Dianous.
How to see this species
Probably extinct, but it was formerly found by closely examining rove beetles found running on beaches between Moyola Waterfoot and Toome on sunny days in May and June. Relevant access permissions should always be sought prior to visiting any sites.
Current status
In the British Isles it is not recorded outside Lough Neagh. Since 1970 there have been four records, all from a sandy beach at Moyola Far Waterfoot in County Londonderry. These were made in 1977 (Anderson, 1979), 1978, 1980 and 1983. It is now believed to be extinct.
Historically it was known to Johnson and Halbert (1902) who refer to an ill-defined location between Toome and Ballinderry Waterfoot. This locality may very well have been the same as the Moyola Waterfoot site. Halbert (1910) also refers to a record for Shane’s Castle, Antrim, and Janson (1924) recorded it at Shane’s Castle in 1923. Halbert (1910) gave another site at what he called Toome Spit, a locality nowadays occupied by a sand extraction company north of the Moyola Waterfoot site.
Why is this species a priority in Northern Ireland?
- Listed as a UK Priority species
- It has declined and has its sole British Isles site in Northern Ireland.
It is a flagship species for Lough Neagh fine sand beaches which have a rich and diverse beetle fauna and which are threatened by sand extraction, disturbance and pollution.
Threats/Causes of decline
Sand extraction, consequent beach erosion, pollution and disturbance
Conservation of this species
Current action
There is a UK Species Action Plan which was published in 1995.
- A survey of Lough Neagh beaches was carried out in 1996, but this failed to refind the species at Moyola Waterfoot, Shane’s Castle or what is believed to be the Toome Spit referred to by Halbert (1910)
- Four subsequent visits have been made to the Moyola Waterfoot area but have also failed to refind this species. The species is now presumed extinct on Lough Neagh
- Lough Neagh is designated as an ASSI
- Implementation of the Northern Ireland habitat action plan for Eutrophic Standing Waters.
Proposed objectives/actions
- It will be necessary to monitor suitable shorelines for a few more years but the presumption is that it has gone
- Ensure that suitable fine sand habitats along the north-west shores are maintained.
What you can do
Report any sightings of Stenus which have silvery hairs on the elytra forming ill-defined pale areas or spots, on sandy shores of Lough Neagh, or indeed, on sandy banks of streams running into the Lough. Please note the locality using an Ordnance Survey map and report the details to CEDaR at the National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, cedar.info [at] nmni.com or to roy.anderson [at] ntlworld.com.
Further information
Links
http://www.coleopterist.org.uk/
http://www.ukbap.org.uk/UKPlans.aspx?ID=586
Northern Ireland Habitat Action Plans
Literature
Anderson, R. (1979). The Coleoptera of a Lough Neagh sandy shoreline with recent records of Stenus palposus Zetterstedt (Staphylinidae) and Dyschirius obscurus Gyllenhal (Carabidae). Ir. Nat. J. 19: 297-302.
Anderson, R. (1997). Species inventory of Northern Ireland: rove beetles (Coleoptera: Staphylinidae). Environment and Heritage Service Research Series, Belfast.
Anderson, R., Nash, R. and O'Connor, J.P. (1997). A revised and annotated list of Irish Coleoptera. Irish Naturalists’ Journal Special Entomological Suppllement 1997: 1-81.
Halbert, J.N. (1910). Cryptophagus bimaculatus, Panz., and other Coleoptera at Lough Neagh. Entomologist's Mon. Mag. 46: 62-66.
Janson, O.E. (1924). Coleoptera at Lough Neagh (Co. Antrim) and Newcastle (Co. Down). Ir, Nat. 33: 69-72.
Johnson, W.F. & Halbert, J. N. (1902). A list of the beetles of Ireland. Proc. R. Ir. Acad. 6(3): 535-827.
Text written by:
Dr Roy Anderson