Males are easily separated by the form of the parameres; in both the internal margin is lined with fine hairs along the middle, in montanus the apical third or so lacks hairs, while in chalconatus the apical third is densely pubescent. Females can sometimes be identified by the antennal colour; those of montanus are generally more extensively darkened but this varies so that specimens with several darkened apical segments could belong to either species but those with only the apical segment darkened will belong to chalconatus.

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Ilybius chalconatus

This very widespread species occurs across the Palaearctic region from Europe to the far east of Siberia, it is present through the Near East and much of North Africa and occurs on many of the Mediterranean islands but is absent from the Atlantic islands, it is generally common throughout Europe from Portugal to Greece and the Caspian Sea although it seems to be absent from some regions of the Balkans, to the north it extends to the UK and central provinces of Finland and Sweden but is absent from Norway. In the UK it is generally common across central and eastern England, rather more local in the west and through Wales and very local and scarce in Scotland and Northern Ireland. Adults are present year-round; they overwinter in damp soil or under logs etc away from water and are active over a long season from early spring, peaking in abundance from May to July and again in September. Typical habitats are shallow pools with little or no vegetation although they also occur in marshes and among vegetation beside slow-moving rivers and drainage ditches, they frequently occur in temporary habitats such as flooded tyre ruts on heaths and in wooded areas and are common in cattle troughs and garden water butts, they usually occur in numbers and often along with other water beetles. Breeding probably occurs mainly in late summer as larvae have been found through the winter and into early spring, and the biology is typical of the group with predaceous aquatic larvae that breathe air at the surface and leave the water to pupate in a cell among marginal substrate. Adults are easily sampled by sweeping in likely habitats but they will need to be taken for critical examination, see below.

 

Ilybius montanus

Native to Europe and North Africa, this species has a rather patchy distribution, it is a mostly western and northern species occurring from Portugal to some of the southern Baltic countries, it is absent from much of central and southern Europe but is present on some Mediterranean islands, and the northern limit of the distribution appears to be the UK and Denmark. In general it is very local and rare and in many northern countries e.g. in Germany and Poland it is known from only a few localities, but the UK is exceptional as here it is common and often abundant throughout Great Britain and Ireland including all the islands north to Shetland. Adults are present year-round and peak in abundance during May and June and again in late summer, they generally occur in numbers and can be very abundant in small water bodies; we once found several thousand specimens in a single pool recently formed in a hollow created by a fallen tree on Bricket Wood Common in south Hertfordshire, there were about equal numbers of males and females present and no other species were found. More generally they occur in slow moving ditches and smaller heathland and woodland ponds, often on acidic substrates and they are common in peatland pools, in the spring they may be expected in any small water body such as garden ponds, flooded tyre ruts and cattle troughs and they may be common in small and heavily shaded woodland pools. The biology is not well known but is probably similar to the preceding species as adult phonologies are very similar; adults overwinter in soil or under debris away from water and become active very early in the year. Both adults and larvae are predaceous; the aquatic larvae surface to breath and when fully grown leave the water to pupate in a cell among marginal substrate. Sweeping small ponds and temporary water bodies, especially in shaded situations in the spring, is likely to produce the species but adults often occur with other medium sized dytiscids, including I. chalconatus, and so will need to be taken for critical examination.