Agabus didymus (Olivier, 1795)
   A lowland species occuring throughout England and Wales with the majority of records from the midlands and eastern England, only a few records from north west England and southern Scotland. Balfour-Browne considered it a running water species found among vegetation and that is certainly, although not exclusively, the case around Watford. It is common and sometimes very common indeed in the flowing drainage ditches and disused watercress beds around the nature reserve etc. in Cassiobury park but also occurs and often in large numbers in the weed choked and rather stagnant reserve pond. Whether in still or running water it always occurs among dense aquatic vegetation and then usually in company with other dytiscids e.g. A.bipustulatus and Ilybius ater. Large numbers were found during 2007 but this is not typical; the extraordinary rainfall has maintained spring water levels in ditches etc. and didymus has capitalized, many of these sites are usually wet soil by July and dry by mid August; where we were pitfalling and searching for carabids and Stenus in cracked mud during 2005/6 we were pond dipping in 2007. Although there are several local rivers with long stretches of luxuriant vegetation we have yet to find the species outside the Cassiobury park area. Adults first appear locally during May and by mid June are common, during late August 2007 they were abundant with single sweeps in drainage ditches producing a dozen or more, the species remained common until late October.
   7-8mm. Antennae clear yellow with last segment black at apex. Palpi red with apical half of terminal segment dark. Head black with 2 foveate punctures on clypeus inside front margins of eyes, sometimes with 2 orange ill-defined orange macula between posterior margin of eyes. Pronotum dark with lateral margins and sometimes base narrowly red, indistinctly punctate behind front margin and laterally before hind margin. Elytra each with 3 rows of weakly setose punctures, characteristically marked with transluscent cream 'N' shaped macula laterally behind middle and another, usually obscurely round or broadly 'U' shaped, subapically. Two other common species have distinct yellow subapical marks; A.bipustulatus and A.guttatus, both of which are at least 9mm and have differently shaped marks. Epipleurs dark, strongly narrowing posteriorly from level of first abdominal sternite. Upper surface dark, often with bronze lustre which can be very distinct in the pond net. Shining with dense granulate microsculpture throughout, indistinctly visible at X20. Upper surface not dimorphic, male basal protarsal segments dilated with suction hairs beneath. Front legs red, middle legs darker and hind legs pitchy red or black. Hind femur with small apical 'comb' clearly visible from side beneath tibiae in normal setting. Hind tibiae short, about twice as long as broad, longest terminal spur more than half its length. All claws more or less equal in length.
This description was taken from 11 Watford specimens taken between June and August 2007.

Description from 11 Watford specimens at X20