Haliplus ruficollis (De Geer, 1774)
This western Palaearctic species is generally common throughout Europe, extending east through Asia Minor to Uzbekistan and north to Scandinavia and the UK, it also occurs in Iran and Iraq and has recently (2005) been recorded from China. Here it is our commonest member of the genus, occurring throughout the UK including all the islands with the exception of Shetland. Adults occur year-round in a variety of wetland habitats, usually in permanent and well-vegetated or stagnant water bodies; ditches, ponds, marshes, lake margins, brackish pools and salt marshes, and usually along with other species, often H. sibiricus Motschulsky, 1860 or H. immaculatus Gerhardt, 1877, they will be found by sweeping among aquatic vegetation; usually leaving the sample within a minute or two and crawling across the net fabric, are common in extraction samples and year-round on the underside of floating marginal debris, logs etc. usually in numbers. Adults are active from early spring until late in the year, peaking in abundance in mid-summer, and dispersing nocturnally by flight in late spring when they may occur at light. Mating occurs in spring and early summer and oviposition begins in late spring and continues into the summer; eggs are laid singly inside the cells of submerged plants and larvae emerge within a week or two. Larval development proceeds through the summer and some will pupate in the autumn while most overwinter among marginal soil and roots out of the water and pupate in the spring. Pupation occurs in a cell beneath marginal debris or in the soil and adults eclose within two weeks, they generally remain in situ for a week or so before emerging and entering the water, they are thought to live for two years and so may reproduce twice. Both adults and larvae may be common among filamentous algae as larvae are phytophagous, sucking the contents from algal cells with their specialized mandibles while adults are omnivorous; feeding upon algae as well as small crustaceans, hydrozoans, oligochaet worms and eggs etc.
The UK species of the subgenus Halpilus s.str. are recognized by the short grooves at the base of the pronotum (c.f. lineatocollis) and the very fine dorsal punctation (X50), but specific identification is difficult and specimens will often need to be dissected. The ruficollis group of species is recognized by the metaventral process being either smooth or impressed in the centre, and in ruficollis it is distinctly impressed. The prosternal process is longitudinally impressed, at least in the anterior half, but this may be shallow and should be looked for very carefully with low-angle lighting. The outline of ruficollis becomes distinctive with experience, it is widest about one-quarter of the elytral length from the base while in the common and widespread H. sibiricus Motschulsky, 1860 it is widest further back, nearer the middle. In male ruficollis the pro-tarsal claws are unequal, the outer claw is longer and less curved, and the basal mesotarsomere is not deeply indented and angles as seen in H. lineolatus Mannerheim, 1844. Female ruficollis have at least the basal third of the elytra smooth and unpunctured, and at least the sub-apical area finely punctured (X50). H. ruficollis is a rather broad species with the forebody and appendages pale and the elytra variable; pale to medium brown with black puncture rows and darker various darker marks across the middle and towards the apex. Males of species within the ruficollis-group should be dissected as their genitalia are very distinct and make identification straightforward; in ruficollis the aedeagus (fig. 1) is narrowed from the inner angle to the tip while the paramere (fig. 2) is very broad across the base and continuously pubescent along the inner margin to the outer apical angle.
Fig. 2 H. ruficollis paramere
Fig. 1 H. ruficollis aedeagus