BRACHININAE Bonelli, 1810

Bombardier Beetles

This large and diverse group of explosive beetles is under-represented in the UK. Brachinus crepitans is widespread but local, while B. sclopeta is an occasionally established vagrant.

ADEPHAGA Clairville, 1806

CARABIDAE Latreille, 1802

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4.5 - 9.5mm

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Introduction

This relatively small subfamily of ground beetles includes more than 500 species within 15 genera and two tribes, it has a worldwide distribution although Australasia is rather poor in species and the greatest diversity is in tropical regions. They are named after their highly-developed defence mechanism which involves a violent emission of hot toxic chemicals from the pygidium when the beetles are attacked, allowing them time to escape. They are small to medium sized beetles of a rather characteristic appearance with narrow forebody and broadly-oval elytra, many are bicoloured with contrasting forebody and elytra and may be distinguished from other ground beetles by the combinations of a single pair of supraorbital punctures, truncate elytra exposing various abdominal tergites, a setiferous puncture in the outer mandibular concavity and an extra ventral abdominal segment; 7 in the female and 8 in the male. The group is familiar to British coleopterists as the widespread Palaearctic species Brachinus crepitans (Linnaeus, 1758) occurs locally across the south of England.

Brachinus Weber, 1801 is the largest genus with about 300 species included in 9 subgenera, it occurs throughout the world and is well-represented in northern temperate regions; about 50 occur in the Nearctic region and 40 in Europe. Some subgenera are restricted to certain areas e.g. Metabrachinus Jeanell, 1949 occurs in South Africa and Madagascar, Bracinoaptinus Lastnik, 1926 is Mediterranean and Neobrachinus Erwin, 1970 occurs throughout the New World, Aptinomimus Alluaud, 1935 is endemic to Madagascar. They are generally medium sized beetles, 4-10mm, often bicoloured and of a typical appearance. Brachinini also includes the following genera: Brachinulus Basilewsky, 1958 includes the single species, B. viettei Basilewsky, 1958 from Isla Princile, off the west coast of Africa. Aptinoderus Hebenthal, 1919 includes about 5 species and restricted to the extreme south of Africa. Aptinus Bonelli, 1810 includes about 10 species and is native to Europe, Asia Minor and North Africa; the group is mostly southern in distribution but several e.g. the appropriately-named A. bombarda (Illiger, 1800) extend into central Europe. Mastax Fischer von  Waldheim, 1828  (which is the  term used for  the pharynx  of a

rotifer) includes more than 50 species and is widespread throughout the Old World, it does not extend into central Europe but M. thermarum Steven, 1806 occurs in the Caucasus and adjacent parts of Russia, and M. parrayssi Chaudoir, 1850 occurs in Mediterranean North Africa from Egypt and Algiers, members of this genus are characteristically small, from 2.5-4.0mm, and either entirely brown or have various while maculae to the elytra. Pheropsophus Solier, 1834 includes about 140 species in 3 subgenera that are sometimes regarded as distinct genera, it is most diverse in warmer southern-hemisphere regions, especially South America, Africa and Australia and is generally absent from the Nearctic and Palaearctic regions although many occur in the Oriental region and Japan, and several occur in Mediterranean North Africa and at least one, P. hispanicus (Latreille & Dejean, 1823) extends into Southern Spain. Styphlodromus Basilewsky, 1959 includes the single species, S. bicolour Basilewsky, 1959 which occurs in Central and Southern Africa. Styphlomerus Chaudoir, 1875) is divided into 2 subgenera, sometimes considered as distinct genera, and includes about 50 species; subgenus Styphlomerus s.str. occurs in Central and Southern Africa while subgenus Styphlomerinus Jeannel, 1949 is more widespread extending from Japan top India and much of the Oriental region, and including Madagascar but not the African mainland. Crepidogastrini Jeannel, 1940 includes 6 genera and is most diverse in Old-World tropical regions. Crepidogaster Boheman, 1848 includes more than 100 species from Africa south of the equator and Madagascar. Brachynillus Reitter, 1904 includes only 3 species from South Africa, at least one of which is a cave dweller. The monotypic Crepidogastrillus Basilewsky, 1959 and Crepidogastrinus Basilewsky, 1957 and the 5 species of Crepidonellus Basilewsky, 1959 occur in South Africa while the 2 species of Crepidolomus Basilewsky, 1959 occur near the southern tip of Madagascar.

Biology

Very little has been written regarding the life histories of bombardier beetles but larvae are known to be ectoparasitoids on the pupae of other beetles and various other insect hosts have been recorded e.g. some Pheropsophus larvae have been observed feeding on mole cricket eggs. Larvae of Brachinus have been recorded attacking water beetles pupae and scavenging dead insects and in Europe they are known to feed on the pupae of a wide range of beetle families including chrysomelidae, Staphylinidae, Carabidae and Hydrophilidae. Adults occur in a wide range of habitats and it is likely that, at least in the case of Brachinus, species have specific habitat requirements; many are associated with wetland margins, sphagnum bogs and estuarine environments while many also occur in dry habitats such as coastal dunes or calcareous grasslands. In tropical regions many genera are associated with rainforests or open arid regions. Adults are terrestrial and generally nocturnal, they may be found running in open situations or hiding by day under debris or bark, and many species will be found in numbers. Some Nearctic species have been observed laying single eggs in mud cells on rocks or plant stems but otherwise little is known of the oviposition habits, larvae are atypical for carabids in having reduced legs, they are known to feed externally and to pupate inside the host pupa. Adults of many Brachinus have been found year-round and in some species teneral adults occur towards the end of summer and are known to overwinter.

Species of this subfamily vary from 2.0mm (some Mastax) to 30mm in some Pherosophus while the Brachinina vary from 3.0-18.2mm, and there is often considerable variation in size within a species, especially in Brachinus and Pheropsophus. Crepidogastrini are the more primitive group, they have short elytra exposing 2 abdominal tergites, and moniliform antennae. The tribes are otherwise very distinct and differ as follows:

-Mesepimera broad and clearly visible. Male pro-tarsi with two rows of setae beneath the basal segments. Terminal palpomere cylindrical to wedge-shaped. Gular sutures divergent towards the base.

Brachinini

-Mesepimera absent or almost absent. Male pro-tarsi soft and padded ventrally. Terminal palpomere expanded and often securiform. Gular sutures convergent towards the base.

Crepidogastrini

The following description applies generally to the tribe but there are exceptions. The habitus is typified by Brachinus, colour varies widely e.g. many are unicoloured, black to variously brown, or with spots or strips of contrasting, usually paler, colour but many species follow a bicoloured pattern of pale to dark forebody and appendages with coloured, sometimes metallic, elytra. Most are finely pubescent, at least on the elytra. Head quadrate or elongate and smoothly convex or nearly so, antennae long and inserted laterally in front of well-developed and convex eyes. Brachynillus is eyeless and some Crepidogaster have very reduced eyes. All have one pair of supraorbital setae except Mastax where a second, reduced, pair is present. Pronotum quadrate to elongate, usually broadest before the middle and sinuate before distinct posterior angles, surface weakly convex and variously explanate, often with distinct sub-basal fovea and a fine median impression and usually punctured and isodiametrically microsculptured. Elytra usually with well-developed shoulders and much broader than the forebody, rounded laterally and often widest behind the middle, apical margin truncate leaving at least the terminal tergite exposed. Surface finely punctured and finely to coarsely microsculptured, with striae consisting of ridges or weakly-impressed and punctured striae that fade towards the apex, interstices usually weakly convex in the basal half and flattening apically. Wings vary enormously and there are apterous or brachypterous members in all groups but the majority are fully-winged and capable of flight.  Legs long and slender; femora slender and unarmed, tibiae slender with tiny apical spurs, the anterior tibia with a well-developed antennal-cleaning notch internally with a longer spur at the base and apex. Tarsi 5-segmented with all segments long and slender, basal pro-tarsomeres dilated in the male.

Defence mechanism

Bombardier beetles are best known for their chemical defence mechanism which produces a hot corrosive spray from the abdomen but this is also present in another group of carabids, the Paussinae Latreille, 1807, or ant-nest beetles. This is used when the beetle is threatened; it curves the abdomen under the body and directs the spray forward between its front legs, moving the abdomen to aim as necessary and the resulting liquid is generally lethal to other insects. The spray is the result of several chemical reactions which occur in 2 robust cavities at the abdominal apex; each is lined with secretory cells which produce catalase and peroxidise enzymes, these produce oxygen from hydrogen peroxide and oxidize hydroquinones into p-quinone, both of which are secreted by the beetles when threatened, and when these are combined a very exothermic reaction occurs and the resulting increase in pressure is used to direct the products at high speed towards a potential threat. It is released in a series of short bursts as the pressure relaxes and increases as the reaction proceeds; the beetle stores enough hydroquinone and peroxide for about 20 emissions, each consisting of about 70 rapid pulses, the resulting spray is a foul-smelling and highly-corrosive mixture of water and (mostly) 1,4-Benzoquinone at about 100 Celsius which is extremely irritating to the respiratory and visual system of other insects and quickly fatal. The entire process takes only a fraction of a second, the pulses occurring at about 500 per second, and if necessary gives the beetle time to escape either by running or by unfolding its wings and taking flight. For more information click HERE.

Brachinus crepitans (Linnaeus, 1758)

Native to the western Palaearctic region this is the only member of the subfamily established in the UK, it has a very wide continental distribution and is generally common from lowland to low mountain altitudes from North Africa through Europe to central Fennoscandia and east to Asia Minor and Russia to Lake Baikal. Here it is locally common across South Wales and southern and central England though generally absent from the West Country and most of East Anglia, there has been a general decline in recent decades and many modern records are from coastal areas, including the Isle of Wight, and it has disappeared from many sites where it was once common. Adults have been recorded from February until October and it is likely they overwinter, they often occur in small colonies and typical habitats include open and dry grassland on calcareous or loamy soils exposed to the sun where they spend the day under hedgerows or debris on headlands, and roam among tussocks or shrubby vegetation at night. In more southern parts of Europe they occur in damp situations or even on wetland margins and may be abundant in vineyards on calcareous soils. Larvae are associated with pupae of various carabids, staphylinids and hydrophilids, and on the continent they are often found parasitizing pupae of Anchomenus dorsalis (Pontoppidan, 1763), the adults of which are often found in colonies of Brachinus and are thought to benefit from aposematic mimicry.

 

Immediately recognized by the extra abdominal segment but in any case very distinct among our fauna due to the size and bicoloured body; head and pronotum pale brown or orange and the elytra metallic dark blue or green. 6.0-9.5mm, elongate with the abdomen much broader than the forebody. Head finely punctured and pubescent, vertex convex, frons flat, eyes large, convex and prominent, terminal maxillary palpomere elongate and narrow, antennae long and filiform with only the second segment reduced. Pronotum transverse, broadest before the middle and strongly sinuate before sharp posterior angles, surface finely punctured and pubescent, transversely rugose across the disc, especially towards the base, lateral margins strongly bordered, basal margin less strongly so. Scutellum pale, as the pronotum, large and triangular. Elytra broadest behind the middle, with rounded shoulders and weakly-rounded or truncate apical margins which bear a fringe of fine setae, striae weakly impressed and without punctures and entire surface finely punctured and pubescent. Wings fully developed. Legs entirely pale. Male with eight visible sternites, female with seven.

 

Brachinus sclopeta (Fabricius, 1792)

This has always been a very rare insect in the UK and it is likely that all records are of specimens from the continent; it was recorded from several sites on the southeast coast of England up to 1928 but then vanished until 2006 when it was discovered on an abandoned ruderal site in east London near the Thames Barrier, and since that time there have been a few further records from various similar sites in east London. Adults occur year-round, they seem to prefer open sites on grassland with patches of vegetation and plenty of debris and have sometimes been found in small numbers sheltering in crevices among brick and concrete debris or under debris tightly embedded in the ground. Little is known of the life-cycle but adults have been found nocturnally active throughout the year, they breed in the summer and then overwinter, presumably alongside the larvae which will produce a new generation in the spring or early summer. The larvae are thought to be ectoparasites of other carabids such as Amara, Harpalus and Ophonus which occur in similar habitats, attaching themselves to the pupae upon which they will feed and pupate within the empty shell.  The continental distribution is more southern than that of B. crepitans, it is often common around the Mediterranean but generally scarce and sporadic further north, they often occur in aggregations alongside Anchomenus dorsalis which are thought to gain protection by mimicking  the aposematic colouration; in tests various predators displayed a preference for non-aposematic ground beetles, see HERE.

Distinguished among our fauna by the size, 4.5-7.5mm, and unique colouration; body entirely pale, elytra metallic blue with the base of the suture broadly pale. Sexes differ in the structure of the abdomen, as above.

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