Northern Ireland's Priority Species

Hydraena minutissima – diminutive moss beetle


Hydraena minutissima Stephens, 1829
Family: Hydraenidae

Hydraena flavipes (formerly H. minutissima) belongs to a group of tiny water beetles in the family Hydraenidae. These are mainly inhabitants of river and stream margins. Hydraena are often described as water beetles but they are partly amphibious and crawl on vegetation and bottom mud both in and out of the water, though more fully aquatic than Ochthebius. Like the members of Ochthebius, there is a strong resemblance between species. All are tiny (1-2 mm), and relatively narrow and flattened compared to Ochthebius, with long, conspicuous labial palps and a pronotum lacking the strong sculpturing or transparent margins of Ochthebius.
Hydraena flavipes lives along the margins of fast streams, often choosing moss on sills or marginal gravel and silt to live in. It was formerly widely if thinly distributed but is now rare in Ireland with a single recent record, for Wicklow (Tierney et al., 2002).

In brief

  • This species is rated as Critically Endangered (CE) in Ireland (Foster et al., 2009) and is a Northern Ireland Priority species
  • Formerly recorded from the River Faughan in Londonderry and from near Armagh
  • Its favoured habitat is submerged weed, moss in riffles or sills and gravel at the margins of fast flowing water

Species description
Species of Hydraena, of which there are 9 in Ireland, are tiny, relatively narrow and flattened beetles with very long palps, short antennae, and a pronotum which comes to obtuse angles at the sides forming a characteristic hexagonal shape.

Life cycle
Adults are present mainly in spring.

Similar species
Hydraena flavipes is the smallest species at 1 mm long. It may be separated from its near relative H. pulchella (1.2-1.5 mm) by having elytra which are squared off at the apex rather than smoothly rounded, and, in the male, by having a small nodule or tooth on the last segment of the labial palps. Hydraena pulchella has a dark band across the middle of the pronotum, whereas in H. flavipes the pronotum is uniformly dark.

How to see this species
Hydraena flavipes lives in vegetation or at the margins of rapidly flowing fresh water. They are usually seen in spring. Their very small size will cause problems both in retaining them within a net and in detecting them. Kick sampling is probably the best approach using a large, fine-meshed net.

Current status
A very rare species which may be extinct in Northern Ireland.

Why is this species a priority in Northern Ireland?
Its designation rest on its rarity and association with unpolluted fast streams.

Threats/Causes of decline
Eutrophication and river and farm drainage schemes are the main threats.

Conservation of this species

Current action
Designation as a Priority Species.

Proposed objectives/actions
Survey of suitable streams on the Antrim or Londonderry basalt is suggested.

What you can do
Information is required on the status of this species in N. Ireland. It should be looked in fast streams in areas of basic rocks. If you encounter something which suggests this species please note the locality from an OS map and report the details, with a specimen or specimens to CEDaR at the Ulster Museum (Record Centre Manager, CEDaR, National Museums Northern Ireland, Cultra, Holywood, Co. Down, BT18 0EU;

Further information


Balfour-Browne, F. (1958). British water beetles. Volume III. The Ray Society, London. pp. 1-210.
Foster, G.N., Nelson, B. & O’Connor, Á. (2009). A regional red list for water beetles in Ireland. Report to National Parks and Wildlife. Dublin.
Johnson, W.F. & Halbert, J.N. (1902). A list of the beetles of Ireland. Proceedings of the Royal Irish Academy 6(3): 535-827.
Tierney, D., Bradley, C., Kelly-Quinn, M. and Murray, D. (2002). Modern records of Hydraena minutissima Stephens, 1829 and of Agabus biguttatus Olivier, 1795 (Coleoptera) from Ireland Bulletin of the Irish Biogeographical Society 26:220-223.

Text written by:
dr Roy Anderson