Heterocerus fenestratus (Thunberg, 1784)
This Holarctic species occurs throughout Mediterranean North Africa, Europe north to southern Scandinavia and the UK, and from Portugal east through Asia Minor to China and the far east of Russia, Japan, Thailand and Vietnam etc. and is widespread in Canada and the United States. Throughout much of Europe, and certainly in the UK, it is by far the most common member of the family and here it is one of only three species to occur inland, the others being the local and rare southern species H. obsoletus Curtis, 1828 and the widespread and frequent H. marginatus (Fabricius, 1787). The typical habitat is among fine silty sediments beside still or slow-moving fresh or brackish water; adults tend to remain among the sediment but in warm weather are active on the surface and may swarm; during spring and again from mid-summer they disperse by flight, sometimes occurring far from suitable habitats and sometimes in numbers at light. Adults are active from the first warm days of early spring and remain so through the spring and summer, oviposition occurs in spring and early summer when eggs are deposited in small chambers about 25mm below the surface of the sediment. Larvae emerge after about 2 weeks, they remain in the egg chamber or roam the adult galleries for a while before boring through the sediment to feed; both adults and larvae are thought to consume organic debris and algae among the sediment, development is rapid and they are fully grown within 6 weeks, they generally feed near the surface and careful examination may reveal the winding tracks of galleries on the sediment. Pupation occurs in a cell within the sediment and adults eclose after a few days, those emerging through the summer are active until the autumn when they will overwinter among the sediment, but late eclosing adults may remain in the pupal cell through the winter and become active in early spring. Like the larvae, adults may be detected by the presence of raised tracks on the surface and sieving likely sediments may produce them in numbers, but simply splashing water over the sediment will usually force them to emerge, they are generally slow-moving but on warm days will quickly take flight when flooded out of the sediment. In general they will occur in numbers and very large colonies often occur, these may include
several species and so it is worthwhile becoming familiar with the present species where the extensively patterned elytra soon becomes familiar; the name fenestratus refers to the pale elytral maculae or ‘windows’. Typical specimens may be recognized in the field by the characteristically patterned elytra and yellow femora which contrast strongly against the black ventral surface.
3.0-4.7mm. Dorsal surface with double pubescence; dense, recumbent and fine, and much less dense, longer and semi-erect setae. Head black with pale mandibles and funiculus (in obsoletus Curtis, 1888 it is dark), labrum and mandibles produced forward and usually obvious from above. Pronotum transverse and broadest in front of weakly-defined posterior angles, hind margin finely bordered throughout; black with red or yellow lateral margins, this may continue onto the basal margin and, only narrowly, across the anterior margin (in fusculus Kiesenwetter, 1843 it is broadly pale). Elytra bordered throughout, with wide yellow epipleura which narrow from the basal third, surface finely and densely punctured and characteristically patterned; the internal pale marks are usually discreet, otherwise they are variable from extensively pale to dark but the basic pattern can usually be seen, the basal third is usually at least partly pale (in marginatus (Fabricius, 1787) it is dark). Abdominal ventrites abruptly pale laterally. Femora pale, tibiae and tarsi often extensively dark. H. marginatus is distinct in having a small oblique ridge on the epipleura behind the shoulder and this will always distinguish it from fenestratus but specimens will need to be examined under the microscope to be sure of this.