Anacaena Thomson, C.G., 1859
POLYPHAGA Emery, 1886
CUCUJOIDEA Latreille, 1802
HYDROPHILINAE Latreille, 1802
A. bipustulata (Marsham, 1802)
A. globulus (Paykull, 1798)
A. limbata (Fabricius, 1792)
A. lutescens (Stephens, 1829)
This is a large genus of more than 100 species represented in all the major biogeographical regions; it is essentially an Old-World genus with the greatest diversity in eastern Palaearctic and Oriental regions; only 4 species are recorded from the Nearctic region and this includes 2 adventive Palaearctic species, and 13 from the Neotropical, but despite several recent revisions the fauna of many regions is only poorly understood and many more species are likely to be described e.g. only 14 are known from the Afrotropical region, 5 from the Indian Subcontinent and 8 from Australia. The European fauna includes 11 species although only 4 extend north to the UK.
All species are small, 1.5-3.5mm and most are oval to broadly-oval, continuous in outline and convex dorsally although there is great variation across the genus; our UK species are rather typical while some exotic species are comparatively narrow and flattened. All are glabrous and rather shiny and most are drab coloured, non-metallic black to pale brown, usually with paler margins and sometimes well-defined pale maculae to the head, pronotum or elytra. The underside is flat and covered with dense hydrofuge pubescence. The head is transverse and smoothly convex with various fine microsculpture and punctures; eyes moderately large and weakly convex, continuous or nearly so with the outline of the head, only weakly emarginate anteriorly and widely separated. Maxillary palpi shorter than half the head width and about as long as the antennae; second segment thicker than those following, terminal segment asymmetrical with the outer margin more curved. Antennae usually 9-segmented, more rarely (exotic species) 7- or 8-segmented; scape long and wide, pedicel narrow and shorter than the scape, club elongate with all segments distinct. Pronotum transverse, broadest across the base and narrowed to rounded anterior angles, posterior angles not projecting; distinct
to widely rounded, surface finely punctured and microsculptured, without distinct fovea or other depressions. Prosternum flat or convex but not carinate in front of contiguous and weakly transverse coxae, mesosternum steeply inclined anterior to closely approximated and strongly transverse coxae, without cavities for the reception of the pro-coxae. Metasternum not projecting between mesocoxae, a little more convex centrally and with a small glabrous postero-median area, sometimes narrowly produced between closely approximated and widely transverse metacoxae. Abdomen with five distinct and unfused visible sternites, the first without lines or impressions and about as long as the second. Elytra evenly rounded from distinct shoulders to a continually rounded apex, randomly punctured and lacking striae but for a variously-developed sutural stria in the posterior half, epipleura declined, sometimes strongly so, sharply delimited by a narrow border, broad at the shoulders and narrowed to the basal third or middle then narrow to the apex. Legs short and robust, trocanters small and obliquely joined to the femora which have tibial grooves along the inner margin, tibiae long and flattened with longitudinal series of stout projecting setae. Tarsi 5-segmented, the basal metatarsomere much shorter than the second, male pro-tarsi weakly dilated.
So far as is known all species are associated with wetland habitats and have an aquatic lifestyle, adults occur year-round and are generally abundant through the warmer months, overwintering in marginal situations and appearing early in the spring. Breeding occurs in the spring when batches of eggs are enclosed in a small cocoon made of silk and plant debris and attached to marginal vegetation and larvae develop through the summer to produce a summer/autumn generation that will overwinter. With the exception of A. bipustulata (Marsham, 1802), which is a local though rather common species mostly of southern and eastern England, our species are widespread and abundant and should soon occur when working any type of wetland habitat.
Our species may be identified from the following key. Of particular importance is the extent of the pubescence on the ventral surface of the hind femora and this should always be examined after specimens have been assigned using the mesosternal character.
Strongly convex above, form broadly-oval and colouration dark, almost black with contrasting pale pronotal and elytral margins. Distinguished from our other species by the form of the mesosternum; the sharply-declined part in front of the coxae is at most slightly raised medially but lacks any transverse or longitudinal keels. 2.5-3.5mm.
Less strongly convex and usually less broadly-oval. Colouration lighter, often distinctly and extensively so. Mesosternum with a distinct median longitudinal keel which is joined at the base to a short transverse carina.
Pubescence below hind femora extending along the hind margin towards the apex where it is narrowly and more or less transversely delimited, figure 1.. 2.2-2.8mm.
Pubescence below hind femora obliquely and widely delimited to the front margin, figure 2, the apical glabrous area much more extensive.
Pubescence on hind femora not, or only very narrowly at the base, reaching the hind margin. Head with a distinct and usually bright-yellow macula anterior to each eye, pronotum extensively pale, usually with several obscurely delimited dark marks which may be united on the disc. Terminal maxillary palpomere bicolored; pale basally and rather abruptly darkened in the apical half or third. 2.3-3.2mm.
Pubescence extending to at least half way along the hind margin. Head entirely black or only obscurely pale anteriorly. Pronotum extensively dark brown with paler margins. Terminal maxillary palpomere entirely dark. 2.4-3.1mm.
Anacaena bipustulata (Marsham, 1802)
The UK distribution of this species is mostly southern and eastern; it is widespread and locally common north to Yorkshire but absent from much of the West Country and the west midlands, further west there are a few scattered records from the coast around north and south Wales, Merseyside and Lancashire. With the exception of the Isle of Wight it is absent from the islands. The European distribution is mainly southern and western; it is common in Portugal, southern France and the channel coastal regions from France to Holland but absent or very rare in most central and northern areas, beyond this there are a few coastal records from Algeria and Morocco. European habitats are therefore mostly coastal and fluviatile, the species showing an association with estuaries, deltas and river catchments, which is not the case in the UK although throughout it is a lowland species recorded to 800m in river valleys in Europe. Habitats include all types of standing or slow-moving water with plenty of marginal vegetation and detritus, adults occur year round, they overwinter among marginal debris and are active over a long season, from the first mild spells until late in the autumn. Locally we recorded adults from several south Herts. sites that had been regularly worked for about 10 years without success, they were common and remained so for 2 seasons and then vanished and have remained absent since 2012, a specimen then turned up a few miles further north in 2017, and that is our only experience with the species; common but ephemeral and a demonstration that sites should be worked over long periods in order to understand a local fauna. Recorded habitats are weed choked margins of the River Colne in Watford, ditto river Ver at How Wood, St Albans and several specimens, along with our other 3 Anacaena species, from a cattle trough at Bricket Wood Common, Herts. All our records have been from June and July.
This species is usually obvious in the field due to the pale overall colour and patches of yellow in front of the eyes. 1.8-3.0mm. Elongate-oval and only moderately convex, dorsal aspect dull yellow-brown to bright yellow with the head and labrum dark brown to black, clypeus anterior to and along the inner margin of each eye pale yellow. Head densely punctured with punctures varying in size, antennae pale with the club darkened, maxillary palps pale but for the bicoloured apical segment. Pronotum extensively pale, with one or three dark marks on the disc; a central mark and usually another each side although they may be confluent, punctation fine and sparse but denser laterally. Scutellum variable, from pale yellow to black, punctured as the pronotum. Elytra pale or with various diffuse darker markings, often with series of tiny dark spots below the punctures, punctures random and quite dense, without distinct striae on the disc but with a well-impressed sutural stria in the posterior half and there may be longitudinal series of larger punctures towards the lateral margins. Mesosternum with a sharp transverse carina and a variable longitudinal ridge. Pubescence on ventral surface of hind femora not, or only narrowly at the base, reaching the posterior margin. Male pro-tarsi not dilated compared with the female.
Anacaena globulus (Paykull, 1798)
This species, the type of the genus, is generally abundant throughout the UK including all the islands from lowland to all but the highest mountain altitudes, in Europe it is common from Portugal north to above the Arctic Circle in Fennoscandia and east to western Russia, the Black Sea, Turkey, Israel, Syria and various regions of Mediterranean North Africa, extending to above the tree-line to 2000m in mountain areas. Adults occur year-round, they overwinter among marginal litter etc. and are active from early spring until late in the year, they show no particular preference for water type or habitat and have been recorded from peat cuttings and sphagnum pools, all still-water habitats, coastal pools and occasionally from seashore drift. Through the summer they are common in temporary habitats which lack vegetation such as woodland pools and tyre-ruts and we have sampled them in large numbers from cattle troughs in a range of situations, in our experience they may be more common among marginal vegetation by slow moving water and they are common locally among reed litter where the water-level fluctuates widely through the summer. Adults are readily sampled by sweeping among marginal vegetation, especially where the water is shallow, or sieving marginal litter, they are also common on floating logs and debris and, during the summer they occasionally come to light. Through the winter they often occur in numbers among extraction samples from suitable marginal habitats.
The convex form and rather abruptly yellow pronotal margins will, with a little experience, allow identification in the field.
2.7-3.5mm. Broadly-oval and convex, more so than in our other members of the genus. Outline continuous, dorsal surface shining and not, or only very slightly, metallic, finely and rather densely punctured. Head entirely black or with diffuse pale areas before the eyes, more densely punctured than the pronotum. Eyes slightly transverse, with the anterior and posterior margins incised, weakly convex and continuous with the lateral margin. Antennae 9-segmented with a pubescent 3-segmented club; testaceous with the club dark and distinctly pubescent. Maxillary palps as long as antennae, the terminal segment a little longer than the penultimate; testaceous with the terminal segment darker. Pronotum black with rather abrupt yellow margins; transverse, broadest near the base and very finely bordered. Anterior and posterior angles rounded. Scutellum equilateral and finely punctured. Elytra broadest behind the shoulders or near middle, finely and randomly punctured and without striae but for an impressed sutural stria in the apical half; black with pale margins; the apex often extensively pale and usually with rows of black spots. Legs dark red or brown. All tibiae with several rows of strong spines and long apical spurs but without swimming hairs. First tarsal segment small so that the tarsi appear four segmented. Protarsal segments 2-4 equal, the fifth longer. Meso- and meta-tarsomeres 2 and 5 longer than 3 and 4, Claws smooth and toothed at the base. Distinct from our other species in lacking a median mesosternal ridge.
Anacaena limbata (Fabricius, 1792) / lutescens (Stephens, 1829)
These species are dealt with together because both are abundant in all freshwater habitats and they often occur together where the range overlaps, they are very similar and will usually need to be examined under the microscope for certain identification. A. lutescens is the more widespread, being common to the far north of Scotland including all the islands except for the most northern of the Western Isles, Orkney and Shetland; A. limbata by comparison is generally absent from Scotland and is sparser in the most northern parts of England but otherwise equally abundant. Both species occur throughout Ireland. A. lutescens is abundant throughout Europe to the north of Scandinavia and is present in North Africa, A. limbata is similarly abundant across Europe, extending north beyond the Arctic Circle, its range includes most of the Palaearctic region although it may be absent from North Africa. Both species have become established and are widespread in North America. Parthenogenetic populations of A. lutescens, consisting entirely of females, are known from many areas of Europe including northern England and Scotland, and in some eastern regions bisexual populations are known only from mountain regions. Adults of both species occur year-round and are active over a long period from early spring; A. limbata peaks in the spring and again during the summer while A. lutescens peaks in the spring. Habitats include most still and slow-moving water environments; generally well-vegetated margins of ponds and rivers but also fens, marshes and sphagnum bogs, temporary environments such as puddles and tyre-ruts as well as small garden ponds and cattle troughs etc. A. lutescens is the more common species in acid water situations. We have found them together in a wide range of habitats across the south of England and they are generally abundant; sweeping a net through marginal vegetation will usually produce them, especially where the water is shallow and rich in vegetation and detritus, and through the winter they are common among samples of marginal litter from any water body or reed bed. They are likely to be the commonest species in wetland situations and among the first to be recorded.
The colour of A. lutescens varies according to distribution with some northern populations being very dark but in general the species are very similar in appearance and so the hind femora will need to be examined for certain identification. 2.2-3.1mm. Head dark or narrowly and diffusely pale in front of the eyes, antennae pale with the club darker, maxillary palps pale with the terminal segment entirely dark. Pronotum variously brown or reddish-brown, often with the disc darker and sometimes extensively so, or with several darker spots which may be confluent, punctures fine and uniform on the disc becoming denser laterally. Elytra pale to dark brown with the suture narrowly darker and the margins variously lighter, randomly and quite densely punctured and with a well-impressed sutural stria in the apical half. The pubescence on the ventral surface of the hind femora is diagnostic; in limbata it extends along both margins and ends rather transversely behind the apex, in lutescens it extends much further along the anterior margin, ending very obliquely and some distance from the apex on the posterior margin, see below. From A. bipustulata they are distinct in the generally darker colouration, especially the entirely dark terminal palpomere, and from A. globulus by the less convex form and carinate mesosternum, see here.