|Agabus sturmii (Gyllenhal in Schonherr, 1808)|
|Common and usually abundant throughout the British Isles including Anglesea,
Man, the Western Isles and Orkneys (but not Shetland) although records are more scattered through the southwest,
Wales and northern Scotland (NBN). This species occurs in a variety of wetland habitats, typically preferring
small stagnant pools rich in vegetation but in general any fresh or peaty still water bodies are likely habitats.
Not usually found in running water but Balfour-Browne
records them from 'slow-flowing drains on the marshes where there is plenty of vegetation', and this is our
experience of the drainage channels around the watercress beds below Cassiobury park where the species is abundant.
Balfour-Browne also records the species at high altitudes and, on a few occasions, on salt marshes. By far the most
common Agabus (2006-8), or indeed dytiscid, ocurring in Watford, they are usually found in numbers and often in company
with other species eg A.bipustulatus, A.didymus, Ilybius ater and I.fuliginosus etc.
Adults overwinter although we have yet to find them before May, typically they become common during June and through July,
August and September they are abundant and likely to be found locally in any vegetated stagant water body.
Jackson (1958) observed oviposition in spring and early summer (in Fife); eggs were found under leaves of Juncus articulatus and J.bulbosus, often in rows under primarily withered leaves, also on rootlets of Juncus and on withered leaves of Carex and Potamogeton. In captivity also in Sphagnum leaflets. Balfour-Browne records full grown larvae in July and freshly emerged adults from July to September although most common during July and August. Most specimens have normally developed flight muscles and the species has been observed flying (Jackson, 1952).
In the field they may be confused with Ilybius fuliginosus but the contrasting pronotal/elytral colouration of A.sturmii is diagnostic. Under the microscope the posterior claws are equal in Agabus, in Ilybius the anterior claw is distinctly shorter than the posterior.
7.5-9mm. Body broad oval, elytra weakly sinuate subapically in male. otherwise only weakly constricted at pronotum-elytra junction. Head black with anterior margin and two spots at base rufous, cuticle distinctly metallic. Eyes emarginate behind antennal insertions. Antennae testaceous with apical part of distal segments darkened. Palps testaceous, terminal segment dark apically. Pronotum strongly transverse, front angle produced and lateral margin finely bordered, with a row of punctures along base close to margin, generally obsolete near middle. Black and distinctly metallic, as head, with broadly testaceous lateral margin, anterior and basal margin narrowly testaceous but this varies. Head and pronotum with coarse reticulation; triangular, quadrate or variously polygonal but generally somewhat elongate, with sparse and very fine puncturation among this, clearly visible at X20. Cuticle of head smooth and shiny, on pronotum very finely microreticulate, just visible at X20. Elytra distinctly lighter than centre of pronotum; dark testaceous with obscure darker areas, lateral margin broadly testaceous to before apex, basal margin narrowly but often obscurely testaceous. Elytral reticulation dual, as pronotum but much more uniformly quadrate. Without any discreet paler marks. Scutellum finely microsculptured but without the larger meshes. Longitudinal series of punctures on elytra weak, often random and obscure. Lateral edge weak, epipleurum testaceous and narrowed at level of first abdominal sternite. Legs rufous. Metafemoral comb tiny, spines short. Basal segments of protarsus dilated in male, apical segment elongate in both sexes, claws subequal in length in both sexes, elongate, longer and more robust in male. Basal mesotarsal segments dilated in male, terminal segment elongate and curved in both sexes, much longer in male, claws elongate, subequal. Prosternal process almost flat, lanceolate with well defined but fine lateral edges. Penis distinctive; bifid, with subapical spine.
A melanic, northern Scottish, form carbo was described by Foster (1981); entirely black but for first antennal segment pitchy. Also mentioned in this article is ab. fallax (Munster) in which the pale pronotal border is absent. As pointed out by Foster, the melanic form may be confused with A.melanarius but for the very different shape.
Description from five Watford specimens at X20
Foster, G.N. 1980. An Abberation of Agabus sturmii. Ent.Mon.Mag 116:213-214
Jackson, D.J. 1952 Observations on the capacity for flight of water beetles. Proc R.Ent.Soc.Lond.(A) 27(7-9)
Jackson, D.J. 1958 Egg laying and egg hatching in Agabus bipustulatus with notes on oviposition in other species of Agabus. Trans.R.Ent.Soc.Lond 110 Part 3 70-71.