Rhantus suturalis (MacLeay, 1825)
This is among the most widespread of the world’s water beetles; originally from New Guinea it has spread south to Australia and New Zealand, and north to Europe and the entire southern Palaearctic region, including Mediterranean North Africa, from Asia Minor to China. It is generally common throughout Europe, extending north to Fennoscandia, where it is locally common and mostly coastal in the south but with scattered records north to the Arctic Circle, and the UK. Here it is generally common across England and Wales, though more so in eastern regions, and more local and mostly coastal in southern Scotland although it seems to be increasing in range and there are recent isolated records from the far north. Adult beetles are present year round, they probably overwinter in the water and they become active very early in the year, from January or February depending on the season, and during mild winters may remain active throughout, and they usually remain active into December. Typical habitats include small water bodies with little or no vegetation, including garden ponds and water butts, flooded tyre ruts and temporary pools in almost any situation; they are tolerant of pollutants and also occur in brackish water ponds. Adults are strong flyers, they fly at night and may disperse over long distances to find suitable habitats and sometimes appear in light traps. Breeding occurs in the spring and early summer and development from egg to adult is rapid, eggs are deposited among submerged debris and larvae emerge within a few days. Both adults and larvae predate mosquito larvae, these are common in the beetle’s preferred habitats whereas other predators are rare or absent. Larvae are aggressive and will attack any smaller animals, the smaller ones being capable of killing and feeding from larvae larger than themselves, they often occur in numbers and remain together and it has been shown that groups of larvae are more successful in hunting than solitary specimens although in the absence of prey they are also known to become cannibalistic. They are voracious feeders and may consume up to 30 larvae each day, their legs are partially fringed with hairs and they are good swimmers but they will also crawl among debris or vegetation looking for prey. Larvae pass through three instars and are fully grown, measuring about 15mm, within three weeks, at which time they become
inactive and remain submerged for a while, not surfacing to replenish their air, before leaving the water to construct a cell in the marginal substrate in which to pupate. The pupal period is very brief and the adult may eclose after only a few days but it remains within the cell for a while to become pigmented and harden, the entire process from digging the cell to the adult emerging from the soil takes less than two weeks. In temperate zones new generation adults occur from August and at this time there may be a peak in abundance as they add to the previous generation. This rapid life-cycle coupled with strong dispersal ability and specialized prey is ideally adapted to the type of habitat preferred by the beetle. A further adaptation to this lifestyle ( although probably widespread in the family) is the way females produce pheromones to attract males, in this way males arriving by flight at new sites will be able to begin breeding very quickly. For an interesting discussion of the life-cycle click HERE.
At 10.5-13mm this is our largest member of the genus, it may be identified by the lack of a ‘comb’ of setae near the apex on the ventral margin of the metafemora (present in all members of Agabinae) and the distinctive colouration; the pronotum is pale with a variable central dark marking and the abdominal sternites are completely black. Head black with a pale macula between the eyes which is usually confluent with the pale anterior margin, antennae and palps pale. Pronotum pale with a variable and usually well-defined central dark mark, there may be other, less distinct and poorly-defined darker markings but these are sites of muscle attachment rather than pigmentation. Elytra pale brown, usually with dense dark mottling so that they appear darker than the pronotum, with paler margins except for sometimes laterally towards the apex. Each elytron with two longitudinal series of fine punctures, and microsculpture consisting of finely impressed and irregular meshes overlying a much finer reticulation which may be missing towards the basal margin in the male and is generally absent in the female. Legs brown, often with the meta-tibiae and tarsi darker. Male with weakly dilated basal pro- and mesotarsomeres and unequal protarsal claws that are much shorter than the terminal tarsomere.