HETEROCERIDAE MacLeay, 1825
Generally abundant in fine-grained riparian substrates.
Heterocerus fenestratus is by far the most abundant species, and the vast majority of specimens will belong to this species.
POLYPHAGA Emery, 1886
BYRRHOIDEA Latreille, 1804
Around the World
This small family includes about 500 species in 15 genera and 2 subfamilies although the generic limits are only poorly understood and various other classifications have been proposed based on a number of tribes. The number of genera will be found to vary widely in the literature as several have been split or recombined and the number of species is likely to increase as many have been added over recent decades and continue to be described, many are morphologically very similar and very variable and so historically there have been many misidentifications and re-descriptions leading to a large and complicated synonymy. In many cases reliable identifications depend on dissections and there are now good figures of the aedagophores for most species although females generally rely on morphology or association. The Elythomerinae Pacheco, 1964 includes the monogeneric Elythomerus Waterhouse, 1874, known only from a few specimens collected around Rockhampton in Queensland, Australia; it resembles other members of the family but is more elongate and dilated towards the apex. The Heterocerinae MacLeay, 1825 is cosmopolitan with by far the greatest diversity in tropical regions, and temperate faunas tend to be small; 20 species of 3 genera occur in central Europe, 16 species of 2 genera in Spain, and the U.K. fauna includes 9 species of 2 genera, of the 90 or so recorded from the New World 40 are Nearctic, and the Australian fauna includes only 7 species of Heterocerus Fabricius, 1792. The subfamily is sometimes divided into 4 tribes although not all the genera are so conveniently placed, the monogeneric New World Tropicini Pacheco, 1964, Augyliini Pacheco, 1964 with 4 genera, Heterocerini Pacheco, 1964 with 6 genera and the monotypic Palaearctic Micilini Pacheco, 1964, but with such a morphologically similar group the need for supra-generic taxa is questionable.
Adults and larvae live among fine substrates in marginal wetland situations, lakes, ponds and streams as well as coastal mudflats, dunes, shores and salt marshes although they do not inhabit sandy areas without mud or a high organic content and the ideal substrate contains between 30 and 60% water. They occur from sea-level to about 3000m; the adults are good fliers and quick to colonize suitable environments, are generally nocturnal and during the spring and summer may be attracted to light when dispersing from their usual habitat. Adults form galleries as they push through sediments consuming algae, zooplankton and organic debris, they generally work through fresh substrate but will also follow cracks in drying substrate or the burrows of other beetles etc. they usually move with the head held down against the close-fitting anterior margin of the pronotum so that the forebody is rigid and the mouthparts are protected from wear as they go, the femora are broad and flattened and act to scrape away sediment which is then pushed behind them by the tibiae which are lined externally with long rake-like setae and spurs for the purpose, and while tunnelling the tarsi are held reflexed against the upper surface to protect them. The mandibles are curved upwards with broad basal plates and robust multi-pointed teeth adapted to scraping away substrate in front of the beetle and in old specimens these will be worn down, along with the tibiae, from constant erosion. All species have dense hydrophobic hairs which prevent them from getting wet and also act as a plastron, the legs are similarly clothed in very fine and dense pubescence to prevent them from becoming clogged with sediment during tunnelling. The burrows are usually obvious from the rings of sediment piled around the entrance and often by raised lines as the beetles work their way just below the surface. During heavy rain or wave action the substrate may become flooded and then the beetles quickly leave the burrows and take flight, under such circumstances they may fly several kilometres to escape the flooding and find a suitable habitat; a good way to sample them is to flood small areas and tube them as they emerge, many will fly immediately they emerge but some will float on the surface, protected by the hydrofuge pubescence, and are easily taken. In temperate regions most species are thought to be univoltine. Eggs are laid in the spring and early summer and may continue over a long period. Females ready to oviposit may spend some time seeking out suitably boggy, almost waterlogged areas, before they lay small, spherical and pale eggs in small groups inside specially constructed incubation chambers among the sediment, in the few species studied these are located about 25mm below the surface, and in one Nearctic species the female displays egg-guarding behaviour. Eggs hatch after a week or two and the small larvae remain in the adult burrows for a while, sometimes emerging onto the surface on warm days, before they begin to start their own horizontal burrows just below the surface, like the adults they are densely pubescent and are thought to respire, at least partly, by a plastron. They develop rapidly and pupate from mid-summer in a cell within the substrate, adults eclose after a few days and emerge from the substrate from mid-summer but later eclosing adults will remain in the cell through the winter, those early-emerging new- generation adults will return to the substrate in the autumn and construct a cell in which they will pass the winter. So far as is known neither larvae nor pupae overwinter. Adults become active early in the year, late January or February if conditions permit, and mating occurs through the spring after a period of feeding. In some areas heterocerids are a significant prey items for waders and amphibians and also play a role in seed burial and dispersal in riparian habitats. Adults often occur in very large numbers along localized marginal areas and these aggregations may contain more than one species, they leave their burrows in the evening and often disperse en masse, and at this time they may be attracted to light in numbers. The open and moist habitats frequented by Heterocerids will often host many other kinds of beetles e.g. Dryopidae, Carabidae, Staphylinidae, Chrysomelidae and Curculionidae, and so when found colonies should be observed carefully before being disturbed. Our U.K. species inhabit a wide variety of habitats; H. fossor Kiesenwetter, 1843, H. flexuosus Stephens, 1828 and Augyles maritimus (Guérin-Méneville, 1844) in salt-marshes and coastal mud-flats, H. obsoletus Curtis, 1828 and H. fusculus Kiesenwetter, 1843 in marginal mixohaline situations, and the others in various fresh water habitats.
Adults are very characteristic and will not be mistaken for any other group, they vary from between 1 to 10mm in length; the smallest European species is Micilus murinus (Kiesenwetter, 1843) at 1.5mm while some Heterocerus reach 8mm, are elongate, parallel-sided and flattened. The entire insect has a covering of fine and dense hydrofuge pubescence and there may be other, sparser layers of longer erect or semi-erect setae on the dorsal surface. The elytra may have a pattern of lighter spots or streaks on a dark ground but most species vary from entirely pale to entirely dark, immature specimens tend to be entirely pale and many species have entirely pale elytra that do not vary. The head is proportionally large, prognathous and retracted into the thorax and so the temples are not usually visible from above, the vertex is convex and variously punctured, often finely and densely so, and generally without impressions or sculpture. The eyes are circular and convex but usually relatively small, a frontoclypeal suture is distinct and well-impressed and the labrum is large with a dense fringe of setae along the anterior margin. Mandibles large and prominent, sometimes more so in the males, with a large internal lobe, serrate internally and setose externally; some species produce males with greatly enlarged and projecting ‘hypermandibles’. Antennae 9-11 segmented and inserted in front of the eyes at the top of a transverse furrow in which they are withdrawn when at rest, they are highly modified; short with an elongate basal segment and transverse distal segments which form a loose and serrate club, and a round terminal segment. Pronotum strongly transverse, convex and rounded laterally, with or without basal and (often indistinct) lateral borders and with rounded or indistinct angles. The surface lacks sculpture and is variously punctured, generally finely and sparsely to densely, and with single or double pubescence, the lateral margins are usually lined with longer and sparser setae. Prosternum well-developed in front of broadly rounded coxal cavities, the anterior margin produced and covering the gular area. Prosternal process narrow between the coxae then apically expanded and rounded further back. Mesosternum very short, fitting the prosternal process anteriorly and the metasternal process posteriorly, with or without a transverse groove, mesocoxal cavities round, or nearly so, and widely separated. Metasternum large; flat and usually with a median longitudinal impression from between the coxae, and with or without a narrow anterior border. Metacoxae narrowly separated and transverse, reaching to the elytral margin or nearly so. Scutellum distinct, usually triangular, and punctured and pubescent as the pronotum. Elytra entire, completely covering the abdomen and continuously rounded apically, the surface variously punctured, often with both larger and finer punctures. The elytra are always finely and densely pubescent and there may be one or more layers of longer and more erect setae, the possession of which may be an adaptation to the habitat in which they live. Epipleura well-developed at least towards the base and sometimes with a short oblique ridge below the humeri. Abdomen with 5 visible sternites, the second and third fused, the eighth with functional spiracles, in most species the apical segments have long posteriorly-directed setae. Stridulatory ridges are present on either side of the first sternite; they run from the outer basal angle obliquely towards the apical margin and sometimes recurved towards the metacoxae. The legs are fossorial, especially the anterior pair, with broadened tibiae lined externally with long and stiff setae and spines, and with strong, often proportionally very long, apical spurs which may become worn in older specimens. Tarsi 4-segmented; all segments elongate and cylindrical, the apical segment bearing equally long, curved and simply pointed claws. In general males are shorter and more compact than females, there is always a good deal of overlap but in a series this is a good indication of which specimens are worth dissecting.