Acilius sulcatus (Linnaeus, 1758)
Lesser Diving Beetle
This is one of the most widespread and common Palaearctic members of the genus; it occurs in North Africa and throughout Europe to the far north of Scandinavia and extends east through Asia Minor and Siberia to the eastern shores of Russia. It is generally distributed throughout the UK, including all the islands north to Shetland, and is locally common although in our experience adults are rarely found in numbers. Adults are present year-round; they are active from early in the year until October or November depending on the season and peak in late spring or early summer, they inhabit a wide variety of still and slow-moving water; typically small ponds and lake margins but also reed beds, moorland pools and canals etc. and we have sampled them from a cattle-trough on a local common, they are tolerant of a wide range of conditions from densely-vegetated ponds in wooded areas to lake margins devoid of vegetation. Adults occur year-round and mating pairs have been recorded in both spring and autumn although breeding typically occurs in the spring; females lay eggs above the water among vegetation or debris and larvae emerge after a week or so. The larvae are predaceous and actively hunt a range of small prey; under artificial conditions they have been shown to be very effective predators of mosquito larvae and have been considered for use as bio-control agents, they digest prey by injecting them with enzymes and then sucking out the contents whereas adults consume prey directly. They develop rapidly and are fully-grown after about a month at which time they leave the water to pupate in a cell among marginal substrate or debris. Adults eclose between two weeks and a month following pupation and return to the water for the summer but they are good fliers and move readily between water bodies, occasionally appearing at light traps or in temporary ponds etc, they remain active until late in the year when they move to permanent aquatic habitats in which to overwinter. They are active and hunt both by day and by night, during the day they evade predation by swimming but by night they use poisonous and very effective defence fluids to deter predators. Sampling is usually straightforward using the basic methods of sweeping along the margins of ponds or leaving out baited bottle-trapping but adults can swim very quickly and are adept at avoiding threats and so vigorous sweeping might be needed.
Among our UK fauna this species is distinguished by the size, 15.7-18.0mm, broadly oval shape and colouration. Only A. canaliculatus (Nicolai, 1822), our other member of the genus, might be confused with the present species but here the hind femora are entirely pale and the abdominal sternites are predominantly pale, in sulcatus the hind femora are bicoloured and the sternites are extensively dark. Broadly rounded and continuous in outline, mid-legs small and slender compared with the large hind legs. Female fore-legs slender in the female, dilated in the male, especially the very broad basal tarsomeres. Head pale with various dark markings or sometimes extensively darkened, often with a pale V-shaped macula behind the clypeus and a transverse pale macula on the frons. Pronotum pale with two transverse dark markings, one behind the anterior margin and one in front of the basal margin, which are united laterally by a narrower dark line. Elytra dull yellow mottled with dense small darker spots, in set specimens they appear drab brown with paler margins but in life, submerged, they are much lighter and the margins more distinct. Dimorphic; smooth in the male but with four broad longitudinal and densely-pubescent grooves delimited by raised ridges in the female.
The function of the female elytral sculpture has long been thought to be of assistance to the male during mating but a more recent theory sees it as deterrence to males trying to mate as this will help reduce the vulnerability of females to predators during mating.
Generally smaller (14-16mm)
Elytra smooth in both sexes.
Underside mostly pale brown,