LIMNICHIDAE Erichson, 1846

Minute Marsh-Loving Beetles

Represented by a single species in the UK. Limnichus pygmaeus is a generally rare beetle of river margins and coastal estuaries.

Introduction

This is a small family of about 400 species in 37 genera and 4 subfamilies. They occur in all major biogeographical zones and are by far most diverse in tropical and subtropical areas. The European fauna includes 12 species while 28 have been recorded from the Nearctic. A single species, Limnichus pygmaeus (Sturm, 1807), occurs in the U.K. (see below). Thaumastodinae Champion, 1924 includes 4 genera and 12 species and is widely distributed; 3 species of Aconosceles Champion, 1924 occur in Japan and Southeast Asia, and the 4 species of Pseudeucinetus Heller, 1921 are from Southeast Asia. The two remaining genera are from the New World; 2 species of Mexico Spilman, 1972 from Mexico(!) and the Bahamas, and 3 species of Martinius Spilman, 1959 from Central America. The widespread Cephalobyrrhinae Champion, 1925 includes 4 genera. Throscinus LeConte, 1874 includes 7 Central American species, Parathroscinus Wooldridge, 1984 includes 5 Oriental and southeast Asian species, the monotypic Erichia Reitter, 1895 is from Turkey, and the 15 species of Cephalobyrrhus Pic, 1923 occur in China and Japan with a single species from India. Hyphalinae Britton, 1974 is monogeneric; Hyphalus Britton, 1971 includes 4 species from New Zealand, and single species endemic to Japan, Australia and the Seychelles. Limnichinae Erichson, 1846 is classified into 2 very unequal and widely distributed tribes. Bothriophorini Casey, 1912 includes 2 genera; the monotypic Bothriophorus Mulsant & Rey, 1852 occurs in Europe, while the 6 species of Physemus LeCont, 1854 are Neotropical. Limnichini Erichson, 1846 includes the majority of the species in about 28 genera although this may change as the group is dominated by several large genera which may be split on the future. Many genera are restricted to particular areas e.g. Babalimnichus Satô, 1994 includes 2 Oriental species, Afrolimnichus Deléve, 1968 (2 species) and Cyclolimnichus Deléve, 1968 (4 species) are African, and Cephalobyrrhinus Pic, 1923 (7 species), Corrinea Wooldridge, 1980 (13 species), Limnichoderus Casey, 1889 (24 species) and Phalacrichus Sharp, 1902 (11 species) are Neotropical. Byrrhinus Motschulsky, 1858 includes 90 species of mostly old-world tropical and sub-tropical species. Widespread old-world genera include Pelochares Mulsant & Rey, 1869 with 21 species, and Limnichus Latreille, 1829 with about 60 species, which is well-represented in Australia and New Zealand and includes many endemics.

POLYPHAGA Emery, 1886

BYRRHOIDEA Latreille, 1804

Limnichinae Erichson, 1846

Limnichini Erichson, 1846

Limnichus Dejean, 1821

L. pygmaeus (Sturm, 1807) 

1.5-1.8mm

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Description

Adult limnichids are small, up to 3mm, elongate-oval and convex dorsally, most are uniformly brown to black, usually with pale appendages, and clothed with fine golden to grey pubescence or scale-like setae. The head is weakly to strongly hypognathous and usually proportionally small, rarely larger as in Limnichites Casey, 1889, and usually deeply inserted into the prothorax so appearing transverse from above. Vertex flat to weakly impressed or convex, eyes round, small to large and usually rather flat, inserted on the lateral margin and so visible from above although sometimes e.g. in Eulimnichus Casey, 1889, hidden, and usually setose. Frontoclypeal suture faintly impressed or obliterated, clypeus distinct, labrum short and transverse. Antennae 11-segmented and short, most segments broad, and with a 1-7 segmented club, inserted between the mandibles and the anterior margin of the eyes. Mandibles small and usually hidden, maxillary palpi 4-segmented, labial palpi 2 or 3 segmented, all segments elongate and slender. Pronotum transverse to almost quadrate; broadest at distinct and usually sharp posterior angles and narrowed to anterior angles which are often produced forward, surface variously punctured and evenly convex or with distinct median or lateral basal impressions, the lateral margins bordered and the basal margin sinuate or bisinuate. Prosternum broad and flat to very convex in front of oval to transverse and posteriorly open coxal cavities, the process extending to at least the hind margin of the cavities, and often fitting into the anterior margin of the mesosternum, acute to bluntly pointed. Mesosternum short and transverse, the cavities round to oval, and widely separated. Metasternum broad and deep in front of transverse and narrowly separated or contiguous coxal cavities which generally extend to the elytral epipleura, sometimes with a longitudinal median impression. Abdomen with five visible sternites; all free and articulated. Pro-leg trocanters well-developed and exposed, meso- and meta-trocanters moderate and triangular. Femora and tibiae slender, both very variable in length. Tarsi 5-5-5, without bilobed segments, claws simple. In most species the underside is grooved for the reception of the legs, and the antennae generally reflex under the anterior pronotal angles. Scutellum small but distinct and usually triangular. Elytra convex, generally without prominent shoulders, and evenly curved to a continuously rounded apex, generally reflexed laterally and bordered outside well-developed epipleura. Surface randomly and usually quite finely punctured, sometimes with indistinct rows of punctures but without distinct striae.

Larvae are small, less than 5mm when fully grown, elongate and sub-cylindrical, weakly curved and usually pubescent. Pale with the head generally darker, prognathous and partly retracted into the thorax. Antennae 3-segmented and short, the terminal segment with an apical spine. Clypeus and labrum transverse. Mandibles bi- or tri-dentate and lacking a molar or basal tooth. Maxillae with 4-segmented palps, labium transverse with 2-segmented palps. Thorax generally a little wider than the head, with short 5-segmented legs bearing a single moveable claw. Abdomen parallel-sided and decurved towards the apex; segments 1-7 with broad membranous sterna, sternum of segment 8 narrower, sternum 9 operculiform and lacking gills. Posterior margin of all thoracic segments, and abdominal segments 1-8 with longitudinally-striated borders, terminal abdominal segment without cerci. The larvae are semi-aquatic, living among decaying plant material at the water margin, among moss or in tunnels in sandy or silty and waterlogged substrates which are covered with algae and mosses. They are thought to consume decaying plant material among the substrate or to graze the surface mosses from below. Pupation occurs in the same habitats.

Ecology

Adults occur on damp waterside substrates or emergent vegetation and shoreline debris and are often common among flood or splash refuse. They are thought to feed on decaying plant material among the substrate or on stems etc. When submerged they can move through the water by conventional alternate leg movements although they have not been observes to enter the water intentionally. Most can be sampled by sweeping likely vegetation etc. or by pootering individual insects from plants but when disturbed they will drop to the soil and remain still for some time, or drop to the water, remain still and inconspicuous, or take flight. Worldwide species are known from coral reefs, coastal lagoons and mangrove swamps and in many warmer regions they occur at light in numbers. In temperate regions most species are thought to be univoltine.

Limnichus pygmaeus (Sturm, 1807)

This is a widespread central and northern European species extending north into southern Scandinavia and the U.K., it is generally rare and sporadic and in some countries e.g. the Czech Republic, classed as threatened, it has not been recorded from Russia. It occurs along river margins on sandy and loamy soils and is occasionally attracted to light. Both adults and larvae are thought to feed on organic debris, algae and mosses. Here it occurs sporadically in southern and eastern England, South Wales, Anglesey and the Isle of Wight; the majority of records are coastal and it has been recorded from most counties from Devon to Yorkshire. Adults occur from April or May until August on damp substrates and among moss etc. beside running water in coastal wetlands, rivers and fens, and the species is thought to be univoltine. Although very local this tiny beetle can hardly be the easiest species to find and so may be more widespread than supposed.

1.5-1.8mm. Elongate-oval, convex and continuous in outline, entirely light to dark brown or obscurely reddish towards the lateral and apical elytral margins. Upper surface rather dull, finely and densely punctured or sometimes with larger punctures adjacent to the elytral suture, and with pale, short and recumbent pubescence which forms patterns due to changes in its orientation. Head short and transverse, eyes and antennal insertions concealed from above, palps short; about 2/3 the antennal length. Antennae 11-segmented and at least as long as the head and pronotum combined, the 2 basal segments large, 3-6 elongate, 7 and 8 triangular, 9 and 10 quadrate to transverse, and 11 forming a single segmented club. Pronotum broadest at the base and narrowed to distinct anterior angles, the surface evenly convex and without sculpture. Elytra entirely covering the abdomen and continuously rounded apically, without any trace of striae. Appendages pale.

Very similar to some species of the Byrrhidae Latreille, 1804 although smaller than most. In the UK only Chaetophora spinosa (Rossi, 1794) is similar in size but here the antennae have an abrupt 3-segmented club and the pubescence is straight, erect and scale-like.

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