Heterocerus fenestratus (Thunberg, 1784)
  Widely distributed and common throughout most of southern England and the midlands to south Yorkshire but with only scattered records from the west country and south west Wales. Found beside still and running fresh or brackish water where the margins are silt or mud. Often active on the surface during bright sunshine, they move slowly and with a little patience are easily observed. With careful observation the lines and casts of soil from the adults subsurface burrowing may be seen. During spring and early summer they are found on bare damp soil far from water. Pitfalling or sieving damp waterside soil may produce them in numbers. They come readily to MV light, often far from water e.g. Watford town centre gardens and Whippendell wood, often in hundreds. Searching suitable habitats throughout Watford will usually produce a specimen or two but occasionally a colony is found where they are very numerous. These may contain two or three species (Clarke) but not so far from the Watford area.
  Typical fenestratus may be recognised in the field by the characteristically marked elytra and yellow femora which contrast strongly against the black underside.
  3-4.7mm. Head black, mandibles and funiculus yellow. Mandibles and labrum produced. Pronotum transverse, with hind margin finely bordered throughout. Black with lateral margin distinctly and broadly yellow from front angles, this continues variously onto hind margin. Front margin very narrowly and sometimes only obscurely yellow (cf H.fusculus Kies ) Elytra bordered throughout, with typical pattern; the internal yellow marks usually discreet, there is variation in the extent of the markings but the dark template can usually be seen- illuminate from behind, this will avoid the pubescence obscuring the colour. Epipleurs yellow. Entire upper surface pubescent, two distinct layers appearing upon manipulating the specimen under strong light. Tibiae dark, sometimes lighter in apical half.

Front edge broadly yellow.

Description from 4 Watford specimens at X40