MONOTOMIDAE Laporte, 1840

Root-Eating Beetles

Includes several common species occurring under bark (Rhizophagus) or among decaying vegetation (Monotoma).

POLYPHAGA Emery, 1886 

CUCUJOIDEA Latreille, 1802

2

3

23

1.5-4.7mm

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Around the World

A small family of about 260 species included in 35 genera-depending upon how the limits of some generic and sub-generic groups are defined-and 4 subfamilies. The group includes many important economic and ecological species; saproxylic predators, mycetophagous species and forest pollinators. Several are myrmecophilous and at least one genus lives among hymenopterous nests. Many species within the Monotominae are mould feeders. The family occurs worldwide and there have been many introductions so that various genera now occur in regions where no native fauna exists. The greatest diversity is in tropical regions and this decreases with increasing latitude; there are 55 species in 11 genera in the United States and 9 species in 2 genera in Canada. The U.K. hosts 2 (or 3) genera and 23 species.

 

Ecology

Notwithstanding the common name many of the species are predatory rather than root feeding, most species occur in moist conditions among decaying organic material; compost, grass cuttings, bird and mammal nests, dung and under bark or in wood are typical habitats. Some species are subterranean for at least part of their life cycle; Rhizophagus paralellocollis Gyllenhal, 1827 frequently occurs in coffins and is thought to feed on dipterous larvae. Several Rhizophagus species occur in the tunnels of other saproxylic species and some have been used as biological control agents e.g. R. grandis Gyllenhal, 1827 has been introduced to the U.K. to help control the scolytid Dendroctonus micans (Kugelnan, 1794) in spruce plantations. Two U.K. species are myrmecophilous, Monotoma conicicollis Guérin-Méneville, 1837 and M. angusticollis (Gyllenhal, 1827).

Description

The majority of species are elongate, rather parallel-sided and slender with the upper surface either glabrous, finely pubescent or with broad setae which resemble narrow scales. In general the Rhizophaginae are convex, or at the very least weakly so, while many of the Monotominae are much more flattened. The North American species Pycnotomina cavicolle (Horn, 1879) is atypically broadly oval in outline while species of Thione Sharp, 1899 are very elongate with the head, and especially the temples, abnormally long. 1.5-6.0mm. The head is often large in proportion to the pronotum and usually produced forward in front of the eyes. The mandibles and palps, while well-developed, are generally not visible from above. Antennae 10-segmented with a 2-segmented club; the 10th and 11th segments are fused and so the club often appears to be a single segment, in many Monotominae the antennae are gradually thickened towards the apex. This 10-segmented arrangement will separate the family from superficially similar cucujoid groups e.g. Nitidulids. The basal segment is broad, large and visible from above but the insertions, on the side of the head in front of the eyes, are sometimes hidden. Eyes weakly to very strongly convex and protruding e.g. in Monotoma species. Pronotum elongate to quadrate, the surface smooth and punctured or, more usually, sculptured with depressions or carinae. Margins bordered, sometimes appearing serrate and/or with distinct tubercles at the angles. Scutellum large and obvious e.g. Rhizophagus or small and cryptic e.g. Monotoma. Elytra elongate and as long as, or longer than, the head and pronotum but there are exceptions. Generally with punctured striae or interstices, sometimes pubescent and this may be arranged randomly or in neat rows. Apices truncate, flatly rounded or separately rounded, generally leaving a part of the abdomen exposed, in many genera the last tergite is exposed in the female while the last two are exposed in the male. Abdomen with 5 visible articulated sternites, the first as long as the next 2 combined. The legs are generally short and robust with the tibiae often expanded apically and toothed externally. The tarsal formula varies from 5-5-5 to 4-4-4; in many the males are heteromerous, 5-5-4 while the females are 5-5-5. Many species show some degree of sexual dimorphism in the relative proportions of the head and pronotum while the middle and hind tibiae of males are often curved when compared to the females. In many species the shape of the terminal abdominal sternite differs between the sexes.

Monotomid larvae are recognized by the presence of a pair of ventral epicranial ridges which run more or less parallel to the lateral margin of the head. , its presence separates the family from other cucujoid groups. They are elongate and cylindrical or rather flattened, parallel-sided and weakly sclerotized, generally pale yellow to white with the segment margins and the head darker. The head is round and sharply constricted to a narrow neck, and the mandibles are usually prominent. The legs are short. The 9th abdominal segment has setiferous tubercles and a pair of cerci or elongate processes. They occur in a wide range of situations; many are saproxylic or develop in decaying vegetation.

Four subfamilies are recognized, of which 2 occur in the U.K.

Lenacinae Crowson, 1952 includes a single genus and species, Lenax mirandus Sharp, 1877 from New Zealand.

Thioninae Crowson, 1952 includes 3 genera and about 14 species of mostly tropical beetles. Included is the very distinctive genus Thione Sharp, 1899 with 5 species from the Australasian region and South America. These have the head and pronotum much longer than the elytra. They are large species (for monotomids), around 5mm that inhabit the galleries of wood-boring beetles.

Monotominae Laporte, 1840 Includes 28 genera and the majority of the species included within the family. The group has a cosmopolitan distribution and is particularly rich in the New World tropics. Monotoma Herbst, 1793 is by far the largest genus with about 45 species included in 3 subgenera. It is the only genus to occur in the U.K. The subgenus Monotomina Nikitsky, 1986 includes 2 European species of which one, M. quadrifoveolata Motschulsky, 1837 occurs in the U.K. and has become cosmopolitan through human transport. Gyrocecis Thomson, C.G., 1863 includes 2 European and Nearctic species, all are myrmecophilous and associated with Formica species. The remaining 40 or so species are classified in Monotoma s.str. The species of Monotoma are moderately pubescent and distinctly and coarsely punctured. The anterior pronotal angles are tuberculate and the eyes strongly protruding. Most are found among decaying vegetation and they are thought to be mould feeders.  Although the genera differ in detailed morphology they should, for the most part, be recognizable once a familiarity with the U.K. species is gained. Perhaps the most atypical genus is Crowsonius Pakaluk & Slipinski, 1993 with 2 Brazilian species and one from Costa Rica. They live in the nests of Hymenoptera and are thought to disperse by phoresy. The pronotum and elytra have prominent longitudinal carinae, they are wingless and the tiny eyes consist of a single facet.

Rhizophaginae Redtenbacher, 1845 includes only the Holarctic genus Rhizophagus Herbst, 1793 with 55 species included in 4 (or 5) subgenera. The type species, R. bipustulatus Fabricius, 1792 occurs in the U.K. The subgenus Cyanostolus Ganglbauer, 1899 is sometimes considered a full genus, it contains the single European species R. aeneus Richter, 1820. The species occurs at sap and beneath the bark of wet timber; large numbers have been found under the bark of floating logs, and it is rarely found away from water. The subgenus Anomophagus Reitter, 1907 contains 2 Palaearctic species, including the British R. cribratus (Gyllenhal, 1827), and 3 Nearctic species. The subgenus Eurhizophagus Méquignon, 1909 includes a single Nearctic species, R. grouvellei Méquignon, 1913 and 3 Palaearctic species including the widespread and common U.K. species R. depressus (Fabricius, 1792). The remaining species, well represented in the U.K, are included in Rhizophagus s.str. The genus is Holarctic and well represented in all regions, and in the New World it extends south to Mexico at high altitudes. Adults and larvae of many, or even most, are saproxylic, feeding on fungi and scolytid larvae. Only one species is known to be an obligate predator; R. grandis Gyllenhal, 1820 preys on Dendroctonus micans, The Spruce Bark Beetle. The species are generally glabrous and shiny. Distinct from the monotominae genera by the transverse front coxae with exposed trocantins, and the presence of antennal grooves or cavities beneath the lateral margins of the head. Many can be surveyed by searching under bark or among fungal fruiting bodies. Some are strictly seasonal with some species occurring only in the winter. Most are generalists but a few are restricted to either deciduous or coniferous wood.

UK Species

R. fenestralis

R. parallelocollis

R. perforatus

Cyanostolus aeneus

Monotoma angusticollis

R. picipes

M. conicollis

M. quadricollis

R. ferrugineus

R. nitidulus

R. oblongicollis

R. cribratus

R. depressus

M. quadrifoveolata 

M. bicolor

M. brevicollis

M. spinicollis

M. testacea

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