Ilybius fenestratus (Fabricius, 1781)

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ADEPHAGA Clairville, 1806

DYTISCIDAE Leach, 1815

AGABINAE Thomson, C.G., 1867

Ilybius Erichson, 1832

A locally common species distributed throughout the western Palaearctic from Portugal east to Mongolia and north to high Scandinavian latitudes, reaching the Arctic sea in the north of Norway, in the U.K. it is widespread and generally common through central and eastern England, very local and sporadic in the west and Wales and in Scotland known only from lochs in the southwest. It usually inhabits the margins of larger water bodies, reservoirs, and canals etc. where the adults may be found among vegetation. Adults occur year-round, they are nocturnally active and thought to be flightless, they overwinter, probably in the water, and become active in the spring, feeding for a while before mating and ovipositing in the stems of marginal vegetation from May or June and during this time they may be active during the day. Larvae appear from July and develop in marginal situations, these will overwinter and produce adults the following summer and so the life-cycle is semivoltine, alternating between larvae and adults overwintering. In Scandinavia the larvae have been recorded active in ice-covered lakes in Scandinavia. Freshly eclosed and teneral adults are attracted to thick aggregations of weeds in marginal situations, and the species is known to occur in lakes stocked with large fish for angling purposes, adults have a characteristic and not very pleasant smell and the defence chemicals are well-documented, as well as producing steroids they secrete toxins that have been shown to produce clonic spasms in amphibians.

With experience adults become obvious in the field from the characteristic dark colour and broad-oval shape which is widest behind the middle; more specifically it is recognized among the U.K. fauna by the combination of the pre-apical ventral series of setae on the metatibiae, emarginate eyes, shape and size, 10-12mm, and the form of the metasternum. The dorsal surface is black with a metallic-bronze reflection which may be faint; head with two spots on the vertex and the clypeus and labrum red, at least in mature specimens. Lateral pronotal margin very narrowly red, the elytra with two apico-lateral spots and the lateral margin narrowly red (I.f. fuliginosus).  Antennae and palps

pale red, legs dark red to brown. Pronotum weakly sinuate behind weakly produced anterior margins, the lateral margins become almost parallel towards the posterior angles and the basal margin is widely sinuate. Ventral surface dark red, the lateral metasternal extensions long and narrow compared to other U.K. species.  Elytral of distinctive shape; laterally almost straight in the basal half, broadest and curved behind the middle and strongly narrowed to the apex, sculpture of random and irregular meshes, which tend to be much more deeply impressed in the female, and microsculpture which is often only visible towards the apex, several longitudinal series of setiferous punctures are usually present but may be weak or absent altogether. Metatarsal claws unequal; the outer claw distinctly shorter than the inner. Male with basal pro- and meso-tarsal segments dilated and the inner pro-tarsal claw weakly expanded towards the apex.

Similar Species

Ilybius fenestratus can be distinguished from all other Ilybius species by the elongated metasternal wings.

Other Ilybius species

Ilybius fenestratus

Agabus bipustulatus
  • Hind claws equal in length.
  • More evenly rounded.
  • Microsculpture consists of elongate cells.
  • Two pale spots on head (often difficult to see.)

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