TENEBRIONIDAE | uk beetles

TENEBRIONIDAE Latreille, 1802

Darkling Beetles

This very diverse group includes many common species found in a variety of habitats. Some species are stored product pests or otherwise synanthropic.

POLYPHAGA Emery, 1886

TENEBRIONOIDEA Latreille, 1802

Approx. 50

1.5-30mm

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Introduction

The following brief discussion is intended to give an impression of the family as a whole, morphological and ecological diversity is huge and so can be mentioned in only the broadest terms but a more thorough account of the European, and especially the U.K. fauna, is given below. This huge family is the most highly evolved of the Tenebrionoidea series and is estimated to include more than 20000 species in some 2300 genera, 96 tribes and 9 or 10 subfamilies. The alternative common name of Nocturnal Ground Beetles is a reflection of the state of knowledge at the time the family was named, there are many nocturnal and photophobic species but this is not characteristic of the family as a whole. Most species are general omnivores, feeding and developing on both living and decaying vegetation, wood in all stages of decay, fungi, organic detritus including dead insects etc. and carrion. The family includes several notorious pests of stored products e.g. species of Tenebrio Linnaeus, 1758, Tribolium MacLeay, 1825 or Alphitobius Stephens, 1829, and several are bred on a commercial scale to provide live food for exotic pets e.g. Zophobas morio Fabricius, 1776 or dried food in bulk for bird feeders and, nowadays, human consumption e.g. Tenebrio. Ecologically the family can be divided into 2 very broad groups, those associated with wooded ecosystems and those associated with terrestrial ecosystems. In wooded areas many are associated with fungi, either among fruiting bodies or mycelia on damaged but otherwise healthy trees and shrubs, or as grazers on the fungal cultures in the galleries of other wood-boring insects. Some develop in stems or consume leaves or flowers etc. and some graze lichens, mosses or algae on wood or stones etc. Adults of some of these saproxylic species will also visit sap to feed. Many species are terrestrial, living on or under the soil surface both as adults and larvae; occurring under debris or among leaf-litter and tussocks etc. many of these occur in sandy areas, especially coastal and of these some live at the edge of the tidal zone, consuming organic matter and developing as larvae under the sand. Some are adapted to life in very arid conditions, having sub-elytral cavities that reduce respiratory water loss, and some species in the Namib desert are adapted to collect water droplets from the morning fogs that roll in from the sea. A few species are adapted to live in caves and many are associated with the nests of mammals, birds and a range of social insects,

in a few tropical species the larvae are free-living and predate other insects among palm fronds. A few species have become agricultural pests e.g. Opatrum sabulosum (Linnaeus, 1760). The group is cosmopolitan and most areas have a diverse fauna, the tropics are the most diverse but not by the margin seen in many other families; 1200 species of 190 genera occur in the United States while 7800 species occur across the Palaearctic, of which more than 130 occur in Europe.

Overall the adults are difficult to characterize, let alone recognize, as a family as there are exceptions to all the defining characteristics of the group, but in general it is rather better defined than most of the Tenebrionoidea series by using a combination of adult and larval features. Adults are, very generally, well-sclerotized with robust and hard bodies, they have closed pro-coxal cavities, connate basal abdominal ventrites and eyes that are variously emarginate or reniform. The two characters by which they are generally recognized are the heteromerid tarsal arrangement, 5-5-4, and the 11-segmented antennae which are inserted beneath laterally-expanded genae. Larvae are cylindrical or nearly so, with a distinct frontoclypeal suture, they are generally glabrous (except in Lagriinae), have the pro-legs longer than the others, annular or annular multiferous spiracles, and the ninth segment either rounded apically or with several spines, paired urogomphi or a single median projection. They are commonly called mealworms or false-wireworms but are distinct from those of the Elateridae in having a more convex head, articulated labrum, mandibular mola and by lacking lateral stomata.

Adult habitus very variable but most are convex and rather elongate, some are flattened and sometimes unusually so, probably best illustrated by the ‘Pie Dish Beetles’ (Helea Latreille, 1804) of Australia. Size varies from about 1mm to more than 80mm, the largest is said to be the Frigate Island Great Tenebrionid, Polposipus herculeanus Solier, 1848, which now survives on a single island in the Seychelles, having been wiped out by introduced rats on the others. The adults of many species are fully-winged and fly well but many are apterous and where these are terrestrial they tend to have the elytra fused. The vast majority have eyes which are well-developed and eyeless forms are rare, the eyes are rarely round, usually emarginate to some extent or reniform, often very narrow and transversely wrapped around the antennal insertions, and often partially or even completely divided horizontally by a canthus. The antennae are generally rather short and robust (there are many exceptions), moniliform to filiform, often with transverse segments and sometimes otherwise modified e.g. apically thickened or loosely clubbed etc. they are generally 11-segmented, rarely 9- or 10-segmented, and the insertions are almost always hidden beneath variously expanded genae. Labrum transverse to elongate but usually quadrate or nearly so, mandibles variously developed but often visible and sometimes prominent, bi- or tridentate apically and with a mola with or without transverse ridges. Pronotum very variable in shape and sculpture, often toothed or explanate laterally, procoxal cavities closed posteriorly and open or closed internally. Mesocoxal cavities closed laterally. Some Pie Dish Beetles have the Pronotum recurved anteriorly around the head, forming an aperture. Visible abdominal sternites 1-3 connate, 4 and 5 articulated, intersternal membrane usually exposed.

Tenebrionidae in Europe

The U.K. fauna includes 49 species in 34 genera, 21 tribes and 4 subfamilies, all of which are part of a larger central European fauna of about 130 species. Many of these are saproxylic and, in line with many woodland species of other families across the region, have suffered a decline in recent decades and at least 5 central European species have not been recorded for at least 50 years. The following two European subfamilies do not occur in the U.K. Stenochinae Kirby, 1837 is a very diverse and worldwide group of 3 tribes, and more than 50 species of all 3 tribes are Nearctic while about 300 species of 64 genera and 2 tribes are Palaearctic. The European fauna includes 2 genera of the Cnodalonini Oken, 1848. Upis Fabricius, 1792 is monotypic and includes the Holarctic U. ceramboides (Linnaeus, 1758). Menephilus Mulsant, 1854 is a Palaearctic genus of 10 species, of which M. cylindricus (Herbst, 1784) occurs throughout Europe. The subfamily includes medium to large and robust species, 12-45mm (12-19mm in Europe), which are mostly elongate and flattened. Both European species have short, apically broadened antennae, the bases of which are not completely concealed beneath the genae and weakly emarginate eyes. The penultimate tarsomere is simple and the claws are smooth. Both have an exposed membrane between ventrites 3-5. Pimelinae Latreille, 1802 is a large and very diverse group including up to 40 tribes, depending on how it is classified, of which 17 occur in the Palaearctic and 12 in the Nearctic. Many of the species are diurnal and terrestrial; they occur throughout drier regions of the world but are rare in the Oriental region and almost absent from Australia and New Zealand. Most arid regions of the world have their own diverse and endemic fauna and diversity is greatest in tropical regions. Of the 3200 Palaearctic species only 2 occur in Central Europe. The Asidini Fleming, 1821 includes about 1000 species in 30 genera; 300 species of 8 genera occur in the Nearctic and more than 460 in the Palaearctic. A single species, Asida sabulosa (Fuessly, 1775) occurs on sandy soils in Central Europe. Cnemeplatiini Jacquelin du Val, 1861 includes 15 species in 5 genera in the Palaearctic region and is absent from the Nearctic. The single species Cnemeplatia atropos Costa, A., 1847 occurs throughout Europe, North Africa and Southeast Asia; it is primarily a Mediterranean species developing in decaying vegetation and active nocturnally. Members of this subfamily vary widely in size, 2-35mm, and morphology (e.g. see the South African Cryptochile echinata Fabricius, 1781) but our European species are small, around 3mm (Cnemeplatia) to 15mm (Asida), with the head and pronotum widely transverse and the elytra elongate, sub-parallel and continuously rounded, and with various striae, longitudinal carinae or other sculpture. They have small eyes and short antennae, the insertions of which are concealed. The tarsi lack lobed segments, the claws are smooth and there are no membranes visible between the basal abdominal ventrites.

Species such as Zophobas morio are used as food in the exotic pet trade.

Tenebrionidae in the UK

The U.K. fauna is ecologically fairly representative of much of the world fauna; Tenebrioninae and Diaperinae include synanthropic as well as wild species; saproxylic and terrestrial species and some e.g. Tenebrio, that are more eurytopic. Of the terrestrial species ours are typical in inhabiting exposed and often sparsely vegetated sandy, often maritime, conditions. Many of our species are nocturnally active. The morphologically distinct Lagriinae and Alleculinae are mostly diurnal species found on flowers and foliage. In terms of morphology however the U.K. fauna, as well as that of much of Europe, hardly begins to represent the wider world fauna.

Zophobus morio Fabricius, 1776 deserves a special mention as it must be among the most common of our species; the larvae are sold worldwide as food for exotic pets. Zophobus Blanchard, 1845 is a New World genus of the Tenebrionini, in Cuba they are known as blind mealworms and they occur among bat guano in caves, but more generally they are detritivores much like Tenebrio. Z. morio is bred and sold in huge numbers in the U.K. but it will not survive our temperate climate and has not been recorded in any situation other than being bred artificially, in tropical climates they occur in mills and among grain storage, under bark or among decaying vegetation and have been reared on a very wide range of vegetable products.

The European fauna is rather less varied than the wider world fauna and more easily described in general terms. Species vary widely in size, the smallest are probably Myrmechixenus at less than 1.5mm and the largest is Blaps gigas (Linnaeus, 1767) which can reach 40mm. The shape varies from elongate and parallel-sided e.g. Palorus Mulsant, 1854 or Tribolium MacLeay, 1825, to strongly rounded and discontinuous e.g. Blaps Fabricius, 1775 or Gnaptor Brullé, 1832, or oval and continuously rounded as in Scaphidema Redtenbacher, 1849, and from virtually flat to strongly convex. In general they are well-sclerotized and robust although Lagriinae and Alleculinae are relatively soft-bodied. Most are drab, black to dark brown or testaceous, a few have red or yellow markings and a few are brightly coloured; Alleculinae and Lagriinae are commonly bicoloured with pale elytra and dark forebody. Only a few are metallic e.g. Helops Fabricius, 1775 or Platydema Laporte & Brullé, 1831. The dorsal surface may be glabrous or with various setae, and in softer bodies species finely pubescent, smooth or variously microsculptured or rugose, and the head and pronotum sometimes have larger sculpture e.g. tubercles or horns. The head is usually large and visible from above, often produced forward beyond the eyes and with temples that are variously retracted into the prothorax. A frontoclypeal suture is usually visible and may have a partially exposed membrane which is visible from in front, the clypeus varies from straight to emarginate anteriorly, the labrum transverse to quadrate and often concealed, and the maxillary palps are 4-segmented, the penultimate is the smallest and the terminal segment is very variable, from cylindrical to securiform. The eyes vary from small and widely separated to large and closely approximated, round to widely transverse, and generally emarginate anteriorly; in many partly or completely divided horizontally by backwardly produced genae. Antennae 11-segmented and very variable, narrow and filiform in many Alleculinae and Lagriinae, short and gradually expanded towards the apex, serrate or with a distinct club. The antennal insertions are generally hidden, or partially hidden, beneath laterally expanded genae. Pronotum generally transverse to quadrate, with distinct posterior angles and rounded or distinct anterior angles, in many Alleculinae continuously rounded anteriorly, rounded or straight to sinuate laterally and generally sinuate across the base. Surface varies from almost flat to strongly convex, often with impressions, punctures, microsculpture or rugosity. Scutellum generally well-developed; triangular to rounded or pentagonal. Elytra shiny and smooth to distinctly striate or strongly sculptured e.g. Opatrum Fabricius, 1775, generally covering the abdomen and continuously rounded apically, produced or ‘mucronate’ in Blaps. In some flightless species the elytra are fused along the suture. Epipleura usually well-developed and complete to the apex. Abdomen with 5-7 ventrites, in some species there is an extra one in the male, in most (all U.K. species) with a transverse membrane visible between 3 and 4 or 4 and 5. Legs generally long and robust although in Alleculinae long and slender; In terrestrial burrowing species the front and middle tibiae are usually dilated widely towards the apex, probably most developed in Melanimon tibialis (Fabricius, 1781), and have variously developed external teeth or spines, and in some cases this is sexually dimorphic. Tarsi 5-5-4, 4-4-4 only in Myrmechixenus, generally without modified segments although often sexually dimorphic with males having some basal pro- and mesotarsomeres dilated.

Larvae

Larvae are typically elongate and well-sclerotized, more strongly so in Tenebrioninae and Diaperinae, with the dorsal surface smooth or with scattered setae, or pubescent in Lagriini. The head is generally darker and more sclerotized, with distinctly toothed mandibles, 3-segmented maxillary palps and 2-segmented labial palps, 1 or 2 small ocelli on the vertex and 2- or 3-segmented antennae. The thoracic segments are not wider than the abdomen, in Lagriini the prothorax is atypically longer than the meso or metathorax. Abdomen with 10 segments although the terminal segment is diminutive and not always visible, ninth segment acuminate or produced into a tooth or with spines or paired urogomphi. In most the prolegs are longer than the others, Lagriinae being an exception. Both adults and larvae of all European species feed on decaying plant or fungal matter, some occasionally consume animal detritus but none are known to be predatory. Tenebrio larvae may become cannibalistic when bred in large numbers and larvae of Corticeus Piller & Mitterpacher, 1783 may predate other larvae in scolytid galleries but more research is needed here.