RIPIPHORIDAE Gemminger, 1870

Wedge Beetles

The one British species, Metoecus paradoxus, is widespread but rarely encountered. Larvae develop in wasp nests and the adults are generally found by chance. 

POLYPHAGA Emery, 1886

TENEBRIONOIDAE Latreille, 1802

Metoecus Dejean, 1834 

M. paradoxus  (Linnaeus, 1761) 

8-12mm

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Around the World

This small and cosmopolitan family includes more than 450 described species in about 50 genera and 6 subfamilies although these include many rare and poorly-defined genera where the morphology tends to overlap, some are described only from males, many specimens await description and some genera have yet to be assigned, it must also be said that the family includes some very unusual species and so the classification is likely to change. For the most part the family is badly in need of revision. Micholaeminae Viana, 1971 includes 3 genera; Ancholaemus Gerstaecker, 1855 (2 species) and the monotypic Micholaemus Viana, 1971 are Neotropical while Clinops Gerstaecker, 1855, more generally included in the Pelecotominae, includes 3 Palaearctic species. Hemirhipidiinae Heller, 1920 includes 5 genera and is most diverse in the Australasian region; the monotypic Hemirhipidius Heller, 1921 and 8 species of Sitarida White, 1846 are Australian, Nephrites Shuckard, 1838 includes 8 species; 4 are Australian and the others occur in China and Japan. Two species of Heteromeroxylon Pic, 1939 occur in Japan and South East Asia, and the Malaysian Ohananomia Toyama, 1986 is monotypic, another species having been transferred to Heteromeroxylon. Ptilophorinae Gerstaecker, 1855 includes 2 genera; the monotypic Nearctic Toposcopus LeConte, 1868 and Ptilophorus Dejean, 1834 with more than 20 species, 8 occur in Australia and 12 are Palaearctic with one occurring in Europe. Rhipiphorinae Gemminger & Harold, 1870 includes 3 genera in 2 tribes and is cosmopolitan in distribution. Ripiphorini Gemminger & Harold, 1870 includes the single genus Ripiphorus Bosc, 1792, 9 species are Nearctic and about 20 occur in the Palaearctic, a further 2 species are known from Ethiopia and 3 from South East Asia. Macrosiagonini Heyden, 1908 includes 2 genera and about 200 species, and one of each occurs in Europe. Metoecus Dejean, 1834 is sometimes considered as a subgenus of Macrosiagon and there is an extensive and confusing synonymy, the distribution is Palaearctic with a single widespread European species, M. paradoxus (Linnaeus, 1761) which extends to the U.K. Macrosiagon Hentz, 1830 is the largest genus of the family with about 150 species and is cosmopolitan although by far the greatest diversity is in the Palaearctic region with about 110 species, 20 are recorded from Australia , 7 each from the Nearctic and Neotropical regions

and 2 from Africa. M. bimaculata (Fabricius, 1787) is widespread across southern Europe and North Africa. Pelecotominae Seidlitz, 1875 includes about 12 genera and 50 species and is cosmopolitan; 14 are recorded from Australia and 1 from New Zealand, 10 are listed from the Palaearctic and 3 from the Nearctic region.  The subfamily is absent from Africa and only a single species occurs in each of Central and South America.  The European fauna includes a single species of the Holarctic genus Pelecotoma Fischer von Waldheim, 1809 (8 species) but does not extend to the U.K. Several endemic genera are included e.g. Madrasiindus Pic, 1911 with 3 species from India and the monotypic Allcinops Broun, 1921 from Polynesia. Micropelecotoides Pic, 1910 (7 species) is widespread in the eastern Palaearctic and South East Asia. Rhipidiinae Gertstaecker, 1855 is the most diverse subfamily in terms of genera with about 70 species in 18 genera and 2 tribes. Eorhipidiini Iablokoff-Khnzorian, 1986 is monogeneric with 5 species of the Asian genus Pterydrias Reitter, 1895. Ripidiini Gertstaecker, 1855 is a cosmopolitan group of 17 genera. Two species are recorded from Europe, Ripidius quadriceps Abeille de Perrin, 1872 extends from the south into central Europe while Rhizostylops inquirendus (Silvestri, 1906) is endemic to Italy. The genus Ripidius Thunberg, 1802 is Holarctic with species also recorded from Africa (Congo, Tanzania). Several genera are endemic to the New World; Neorrhipidius Viana, 1958 with 4 species and Quasipirhidius with 3 species are Central American while the monotypic Aporrhipis Pascoe, 1887 is from Brazil. At least 5 genera are endemic to Australia; Neonephrites Riek, 1995 with 2 species, Rhipidioides Riek, 1955 with 8 species, the monotypic Paranephrites Riek, 1955 and Neorhipidius Riek, 1955 and Riekella Selander, 1957 with 4 species. The quaintly named Blattivorus Chobaut, 1891 includes 3 Palaearctic species and a single Madagascan endemic, B. madagascariensis (Chobaut, 1906).

Description

Adults are very variable but somehow soon become familiar; most are only sparsely pubescent or glabrous, many are brightly coloured and/or patterned and many mimic various diptera or hymenoptera, sometimes superficially very closely, in overall form as well as detail e.g. the large head, pronotum and exposed abdomen, short or modified antennae and elytra and long legs etc. Many are laterally flattened and strongly ‘hump-backed’, much like Mordellids but having distinctive antennae and lacking the abdominal style and metatibial ridges. Many species are widely sexually dimorphic both in overall form and in detail. In most species the head is large, convex and strongly deflexed, and in some elongate anteriorly, with short temples and neck which are variously retracted into the prothorax. Eyes large, convex and oval or weakly emarginate anteriorly, sometimes reniform, and often almost contiguous dorsally. Antennae 10- or 11-segmented, in males often strongly developed; uni- or bi-pectinate or flabellate, in females serrate to uni-pectinate, generally relatively short to moderately long and inserted in front of the eyes. Clypeus and labrum distinct, often with a visible membrane between, sometimes either may be reduced; labrum sometimes hinged under the anterior margin of the frons, the apical margin truncate to deeply bilobed. Mandibles well-developed; robust and triangular with internal setae or teeth. Maxillary palps 4-segmented, the terminal segment fusiform and acute to truncate apically, never expanded. Labial palps 3-segmented with the terminal segment fusiform. Pronotum large and prominent, campanulate or broadest at the base and straight or curved to a narrow rounded or straight anterior margin, posterior angles generally acute and often produced backward, mostly without distinct lateral borders or surface sculpture although often with basal depressions, generally convex but in some also broadly explanate. Prosternum usually small and with a distinct intercoxal process (absent in Ripiphorus), coxal cavities large and open behind. Mesosternum generally short and convex, with or without a distinct anterior keel, coxal cavities oval and contiguous to narrowly separated. Metasternum large and usually very convex both transversely and longitudinally anterior to the widely transverse coxal cavities, with or without a longitudinal median furrow or broad depression, often strongly sinuate posteriorly and produced around the coxae. Scutellum moderate to short, sometimes diminutive or covered by the median production of the pronotum and often with a distinct basal keel. Elytra very variable; long and separately acuminate apically although generally not reaching the abdominal apex to short or abbreviated and truncate or rounded, generally rather flat but in short forms often strongly convex, almost like large scales (in diptera and hymenoptera mimicking species), without longitudinal striae, rows of punctures or costae and usually smooth or finely punctured. Epipleura very variable, sometimes missing entirely, and in almost all cases much of the abdomen and the hind wings are exposed. Hind wings large and functional, in species with long elytra either entirely or partially folded, in some with many transverse pleats under the elytra, sometimes e.g. in Ripiphorus, large and exposed and without folding mechanisms, as in diptera. Where the wings are large and exposed the venation is usually reduced. Abdomen with 5 or e.g. in Pirhidius, 7 visible sternites which may be modified; sometimes difficult to see in Macrosiagon, or greatly enlarged in Ripiphorus, apically often down curved or acuminate but not produced as in Mordellids, form very variable; well-sclerotized and rigid to lightly sclerotized, flexible or ‘telescopic’, in Pirhidius lightly sclerotized and cylindrical. In most species at least the last few tergites are visible from above. Legs long and slender although often with well-developed femora, trocantins absent. Procoxae large and projecting, generally contiguous although separate in Ripiphora, mesocoxae oval and oblique to transverse, projecting in most species and closely to moderately separated, metacoxae rounded to widely transverse and contiguous or nearly so, sometimes lamellate or with a well-developed apophasis. Trochanters small and triangular or occasionally e.g. in Pirhidius, elongate, femora generally elongate and slender to weakly claviform. Tibiae long and slender, straight or curved, sometimes with an apical ring of spurs or stiff setae and with various long apical spurs, from 2-2-2 to absent on all tibiae, equal or nearly so. Tarsi always 5-5-4, the segments generally simply expanded apically, in some Ripiphorus with enlarged and truncate basal segments. Males sometimes with some dilated pro-tarsal segments. Claws simple, bifid or pectinate.

Ecology

So far as is known ripiphorid larvae are hypermetamorphic endoparasites of the early stages of other insects and the adults are free living and independent of their larval habitats. In general most species are localized in distribution, adults tend to be short-lived and have a short season, are very active, agile and fast moving. Many are diurnal, frequenting flowers and foliage and all are thought to be univoltine. Some species occur at light and among these some Ripidiinae and Trogonodera have large eye facets which would suggest a nocturnal lifestyle. Many, including our own Metoecus, may become locally abundant for a season or two as they develop in large numbers in host nests. With the exception of the Ripidiinae the larvae of all species completely consume their hosts and all larvae leave their hosts before pupation. Hosts are mostly species of Hymenoptera but a range of other insect associations have developed; all species of Ripidiinae parasitize cockroaches while Allocinops, Nephrites and Rhipistena attack cerambycid larvae and Plecotoma parasitize saproxylic larvae of Ptinidae. Species of Ripiphorinae parasitize ground-nesting hymenoptera; social nesting wasps host Metoecus while bees are attacked by Ripiphorus. In many cases eggs deposited on flowers produce triungulin stage larvae which attach to bees etc., Metoecus is unusual in this respect as they oviposit in crevices on decaying dry trunks and fence panels etc. in late summer or autumn where they will overwinter and produce triungulins that will attach to wasps in the spring as they arrive to shave-off wood for nest construction. In many species newly emerged triungulins display searching behaviour, they usually occur in numbers as multiple eggs are laid on a single flower head and when visiting insects arrive they adopt an upright position, standing on the abdominal apex, and cling to the first insect they contact. Once transferred to the nest the larva enters an unoccupied cell and waits for an egg to be laid among the pollen or food mass, or in some species will enter an occupied cell, and waits for the larva to feed and grow before penetrating its tissue and living endo-parasitically while its host develops, here it will overwinter inside the larva and then begin a further period of feeding in the spring before its host pupates. The larvae of some species leave the pupa and feed upon it, moulting several times and consuming it entirely before pupating in the host cell and emerging as an adult. Some larvae remain inside as the host develops and emerges as an adult, here the first-instar larva will then emerge from the host mesothorax and moult to produce a grub-like and lightly-sclerotized second instar which attaches itself to the adult bee and consumes it while passing through several more instars over a period of about two weeks and pupating while still attached to the dead host. Adults eclose after about two weeks, emerge from the host cell and leave the nest to disperse, in many cases living for only a few days. Where adult hymenoptera overwinter and build nests in the spring the ripiphorid triungulins may remain on the host through the winter and enter egg chambers in the spring when they contain an egg or larva. Many variations of this general life-cycle occur but many species remain to be studied in detail e.g. some Macrosiagon oviposit on leaves or in the soil as well as on flowers and it is not certain how hosts are located from these multiple sites, but it seems that most larvae wait until the host has undergone a period of feeding before entering, and that all known larvae undergo multiple ecdyses, generally passing through six stages.

Metoecus paradoxus (Linnaeus, 1761)

This is the only member of the family to occur in the U.K., it is a locally common though rather sporadic beetle occurring throughout Europe north to Scandinavia and east to the Caucasus and Russia and it also occurs in Japan. In the U.K. it is generally distributed through England and Wales and there are a few records from south east Scotland. The species is a parasite of various social nesting wasps, typically Vespula vulgaris (Linnaeus, 1758) and V. germanica (Fabricius, 1793) but also other species. The typical habitat is wherever wasp nests might occur, wooded borders and dry parkland etc. and adults are active on flowers or among foliage from July until September or October, wasp nests in all situations may be parasitized as first instar Metoecus triungulins are transported along with wasps that are in the process of constructing nests and they have been recorded from nests in trees, sheds, greenhouses and roof-spaces. From adults occurring indoors, having emerged from a bungalow roof-space, it has been estimated that at least 500 Metoecus must have emerged from a single nest. Adults are short-lived; females have been observed to live between 6 and 12 days, and are thought not to feed. Females oviposit on exposed dry wood in late summer and autumn; using a long and slender telescopic ovipositor they insert small to large batches, 10 to 50, of tiny white mucus-covered eggs in crevices etc. and each may produce several hundred during its brief lifetime. Eggs overwinter and small triungulin larvae emerge in the spring, they have long legs with areas of adhesive setae that cling effectively to wasps as they shave-off wood for nest building, larvae display searching behaviour and cling to the first wasp they encounter, often in numbers, and are transported back to the nest. Once inside they leave the wasp and enter the brood cells, they enter the developing wasp larvae but then develop very slowly over the next few months, living as a largely inactive endoparasite, and only when the host has moulted 4 or 5 times do they bore out through the tissue and moult, sealing the exit hole with the shed skin, and attach themselves to the ventral surface and begin to feed as an ectoparasite, sucking the host fluids through modified mouthparts. From this time development is rapid; the larva passes through a further 4 instars and consumes the entire host before it pupates inside the cell. Development from emerging triungulin to adult may be completed within 24 days. The final larval stage is the longest, up to several days, and here growth is rapid before the pupal stage which lasts about 10 days. Occasionally a second triungulin may enter a brood cell but here only one will survive and begin developing. Adults generally leave the cells during late morning, they bite around the cap and then exit the nest rapidly, at this stage they secrete defensive chemicals that were obtained from the wasp larvae and which protect them from attack by any wasps present in the nest. In the U.K. Metoecus is univoltine while further south, on the continent, there may be two generations in a year.

8-12mm, absolutely distinctive among the U.K. species; the long divergent elytra are unique. The colour varies widely and at least 8 colour varieties have been named but in general males have pale elytra with darkened apices while females have dark elytra with pale apices. The head is proportionally very small and when the beetle is at rest generally concealed from above, typically dark with convex and entire eyes that occupy the lateral margin. Antennae 11-segmented and sexually dimorphic; bipectinate in males, unipecinate and much less developed in females, basal segment elongate and unmodified in both sexes. Thorax and elytra flattened laterally, pronotum broadest at acute and produced posterior angles and narrowed to a rounded anterior margin, basal margin produced backwards medially. Pronotum very finely punctured and pubescent, extensively black with the lateral margins broadly pale in the posterior half. Elytra convex at the shoulders and separately narrowed from near the base to sharply acuminate apices leaving much of the abdomen and hind wings exposed, the surface very finely punctured and pubescent. Apical abdominal segments freely articulated. Appendages entirely black but for red tibial spurs and claws. Femora long and weakly curved, tibiae relatively short; meso- and meta-tibiae with 2 long spurs at the inner apical angle, pro-tibiae without spurs. Tarsi 5-5-4, without dilated segments, the basal segment on all tarsi very long, remaining segments elongate. Claws smooth, without a basal tooth and bifid apically.

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