Bombardier Beetle Research Articles

Taxonomic and nomenclatorial revision of the Neotropical genus Phaeoxantha Chaudoir (Coleoptera: Cicindelidae).

Taxonomic and nomenclatorial revision of the Neotropical tiger beetle genus Phaeoxantha Chaudoir, 1850 (originally Megacephala Latreille, 1802) is presented. Ammosia Westwood, 1852 (with type species Megacephala bifasciata) is confirmed as a junior synonym of Phaeoxantha. The results by Naviaux (2008), who rectified commonly confused taxonomy and nomenclature of Phaeoxantha nocturna (Dejean, 1831) and P. laminata (Perty, 1830) versus P. limata, based on Megacephala limata Perty, 1833 which is an unjustified emendation by Perty (1833) in Perty (18301833), are confirmed the date of the unjustified emendation is newly rectified here. Megacephala laminata Perty, 1830 is confirmed as the type species of Phaeoxantha. Lectotype of Megacephala nocturna Dejean, 1831 is designated here, based on syntypes from the Dejean-Chaudoir collection in MNHN. The genus is subdivided here into two clearly differentiated subgenera: Phaeoxantha (Phaeoxantha) and Phaeoxantha (Euphaeoxantha) subgen. nov. The nominotypical subgenus is represented by Phaeoxantha laminata (Perty, 1830), P. (P.) nocturna (Dejean, 1831), P. (P.) nocturna crassipunctata ssp. nov., P. (P.) paranocturna sp. nov., P. (P.) epipleuralis W. Horn, 1923, P. (P.) tremolerasi (W. Horn, 1909), P. (P.) cruciata (Brull, 1837) and P. (P.) bifasciata (Brull, 1837). The latter, based on Megacephala bifasciata Brull, 1837, was published by Brull in order to rectify the concept of Megacephala aequinoctialis sensu auctorum, (primarily sensu Dejean 1825, 1833, 1836), which has been commonly yet incorrectly treated in literature as Phaeoxantha aequinoctialis aequinoctialis (Dejean, 1825). Although Dejean (1825) clearly characterized the same Megacephala species, his act cannot be interpreted as a valid description under Article 12 of the ICZN (1999), because he explicitly referred to the publications by Linnaeus (1763) and redescription by Fabricius (1775). Therefore, his act must be interpreted as a misidentification and subsequent usage of the name Cicindela aequinoctialis Linnaeus, 1763, which is in fact a bombardier beetle (Brachinini, Carabidae), presently known as Pheropsophus aequinoctialis (Linnaeus, 1763), misidentified by Dejean as Megacephala. In contrast, the name Megacephala bifasciata Brull, 1837 was validly published, supported by a rather appropriate illustration and preserved type specimen, being also in common use to the present day; therefore, the confused name aequinoctialis cannot be preserved and is considered an unavailable name being excluded here from Megacephalini and Cicindelidae. The new subgenus Phaeoxantha (Euphaeoxantha) subgen nov. (type species Megacephala testudinea Klug, 1834) comprises following five species: P. (E.) testudinea (Klug, 1834), P. (E.) klugii (Chaudoir, 1850), P. (E.) bucephala (W. Horn, 1909), P. (E.) wimmeri (Mandl, 1958) and P. (E.) lindemannae (Mandl, 1964). Keys to the two subgenera and to their species, descriptions or differential diagnoses and colour photographs of the habitus and of characters of available type specimens of all species of the genus, as well as the variability and characters of individual populations, are presented and their distribution discussed.

Bombardiers and assassins: mimetic interactions between unequally defended insects.

In defensive mimicry, resemblance between unequally defended species can be parasitic; this phenomenon has been termed quasi-Batesian mimicry. Few studies have used real co-mimics and their predators to test whether the mimetic interactions were parasitic. Here, we investigated the mimetic interaction between two well-defended insect species, the bombardier beetle Pheropsophus occipitalis jessoensis (Coleoptera: Carabidae) and the assassin bug Sirthenea flavipes (Hemiptera: Reduviidae), using their potential predator, the pond frog Pelophylax nigromaculatus (Anura: Ranidae), which coexists with these insect species in the same habitat in Japan. We observed behavioural responses of this frog species (adults and juveniles) to adult Ph. occipitalis jessoensis and adult S. flavipes under laboratory conditions. Among the frogs, 100% and 75% rejected Ph. occipitalis jessoensis and S. flavipes, respectively, suggesting that, compared with the assassin bug S. flavipes, the bombardier beetle Ph. occipitalis jessoensis is more well-defended against frogs. An assassin bug or a bombardier beetle was provided to a frog that had encountered the other insect species. Frogs with a history of assassin bug encounter demonstrated a lower rate of attack toward bombardier beetles. Similarly, frogs with a history of bombardier beetle encounter demonstrated a lower rate of attack toward assassin bugs. Therefore, both the bombardier beetle Ph. occipitalis jessoensis and the assassin bug S. flavipes benefit from the mimetic interaction.

A Bio‐Inspired Temperature‐Arousing Battery with Giant Power for Fire Alarming

AbstractForest fire has been becoming a conscious theme as it causes huge loss of life and ghastly impact on the ecological environment. Temperature sensor appropriate for large‐area distribution in forest is rising as a subject of considerable speculation to build a fire alarming network. This strategy needs power supplies that can support fire alarming sensors in long term. Herein, a concept of temperature‐arousing battery (TAB) is conceived that has no energy consumption at normal temperaturewhile becoming activated in fire scenario. The TAB is made of a thermoelectric (TE) generator deposited with exothermic and endothermic materials on its hot sink and cold sink, respectively. Learning from bombardier beetles, the exothermic reaction and endothermic reaction can only happen when the thermal‐responsive switches to isolate reactants are triggered in the event of fire occurrence. Even in an isothermal atmosphere, a remarkable temperature difference is formed between two sinks of TE generator, accordingly producing a thermoelectric voltage higher than 5.0 V and a giant power over 1.0 W, which are high enough to power a wireless signal transmitter. In the practical tests, the integrated self‐powered alarming system can transmit an electrical signal in 79 s when the TAB touches a forest fire.

Molecular phylogeny and revision of species groups of Nearctic bombardier beetles (Carabidae, Brachininae, Brachinus ( Neobrachinus)).

Bombardier beetles of the genus Brachinus Weber are notorious for their explosive defensive chemistry. Despite ongoing research on their defense mechanism, life history, and ecology, the group lacks a robust molecular-based phylogeny. In this study, three loci from mitochondrial and nuclear genomes (COI, CAD, 28S) are used to reconstruct the phylogeny of the large subgenus Neobrachinus, and test species group boundaries hypothesized by Erwin (1970) based on morphological characters. Erwin's fumans species group is found to be polyphyletic, and is herein re-defined with eight new species groups erected to reflect clades based on molecular evidence: the cinctipennis, cyanipennis, galactoderus, gebhardis, mexicanus, phaeocerus, quadripennis, and tenuicollis species groups. Erwin's cordicollis group is also expanded to include Brachinus (Neobrachinus) medius and the americanus group.

Bombardier beetles repel invasive bullfrogs.

Invasive non-native predators negatively affect native species; however, some native species can survive the predation pressures of invasive species by using pre-existing antipredator strategies or evolving defenses against invasive predators. The American bullfrog Lithobates catesbeianus (Anura: Ranidae) has been intentionally introduced to many countries and regions, and has impacted native animals through direct predation. Bombardier beetles (Coleoptera: Carabidae: Brachininae: Brachinini) discharge chemicals at a temperature of approximately 100 °C from the tip of the abdomen when they are attacked by predators. This “bombing” can successfully repel predators. However, adults of a native bombardier beetle Pheropsophus (Stenaptinus) occipitalis jessoensis have been reportedly found in the gut contents of the introduced bullfrog L. catesbeianus in Japan. These records suggest that the invasive bullfrog L. catesbeianus attacks the native bombardier beetle P. occipitalis jessoensis under field conditions in Japan; however, the effectiveness of the bombing defense against invasive bullfrogs is unclear. To test the effectiveness of the bombing defense against bullfrogs, we investigated the behavioral responses of L. catesbeianus juveniles to P. occipitalis jessoensis adults under laboratory conditions. Contrary to previous gut content results, almost all the bullfrogs (96.3%) rejected bombardier beetles before swallowing them; 88.9% rejected the beetles after being bombed, and 7.4% stopped attacking the beetles before being bombed. Only 3.7% successfully swallowed and digested the beetle. All of the beetles collected from non-bullfrog-invaded sites could deter bullfrogs, suggesting that the pre-existing defenses of bombardier beetles played an essential role in repelling bullfrogs. When treated beetles that were unable to discharge hot chemicals were provided, 77.8% of bullfrogs successfully swallowed and digested the treated beetles. These results indicate that bombing is important for the successful defense of P. occipitalis jessoensis against invasive bullfrogs. Although invasive bullfrogs have reportedly impacted native insect species, P. occipitalis jessoensis has an existing defense mechanism strong enough to repel the invasive predators.

First Report on Karyotypic, Morphometric and Meiotic Analysis of a Predatory Bombardier Beetle Pherosophus catoirai (Coleoptera: Carabidae) from Jammu region of Outer Himalayas, India

Chromosomal studies and manual karyotyping are the aged techniques for determining the identity of a species on evolutionary scale; however, these techniques are simple, reliable and inexpensive to authenticate the existence of a particular species. In the present work, the chromosome complement and meiotic processes of a predatory bombardier beetle Pherosophus catoirai were investigated. This species presented 2n=35 as diploid chromosome number and the chromosomal formula was found to be 12m+8sm+12st+X0. Sex mechanism was X0 type with metacentric X chromosome. Y chromosome was absent in this species. Karyotype revealed small chromosomes except X chromosome which is found to be largest in the spermatogonial metaphase stage. Meiotic stages were pachytene, diplotene, diakinesis and metaphase-I. Present study may find importance to analyse evolution of chromosomes in order Coleoptera particularly in family Carabidae.

Beetle bombing always deters praying mantises.

Some animals have evolved chemical weapons to deter predators. Bombardier beetles (Coleoptera: Carabidae: Brachininae: Brachinini) can eject toxic chemicals at temperatures of 100 °C from the tips of their abdomens, ‘bombing’ the attackers. Although some bombardier beetles can reportedly deter predators, few studies have tested whether bombing is essential for successful defence. Praying mantises (Mantodea) are ambush predators that attack various arthropods. However, it is unclear whether bombardier beetles deter mantises. To test the defensive function of bombing against praying mantises, I observed three mantis species, Tenodera sinensis, Tenodera angustipennis, and Hierodula patellifera (Mantidae), attacking the bombardier beetle Pheropsophus jessoensis (Carabidae: Brachininae: Brachinini) under laboratory conditions. All mantises easily caught the beetles using their raptorial forelegs, but released them immediately after being bombed. All of the counterattacked mantises were observed to groom the body parts sprayed with hot chemicals after releasing the beetles. When treated P. jessoensis that were unable to eject hot chemicals were provided, all mantises successfully caught and devoured the treated beetles. Therefore, bombing is essential for the successful defence of P. jessoensis against praying mantises. Consequently, P. jessoensis can always deter mantises.

A Bombardier Beetle Pheropsophus aequinoctialis (L.) (Insecta: Coleoptera: Carabidae)

Pheropsophus aequinoctialis (L.) is a ground beetle in the tribe, Brachinini, native to parts of South and Central America. Ground beetles of this tribe are commonly referred to as bombardier beetles due to their ability to produce a powerful and hot defensive chemical spray directed at would-be predators. This spray is capable of harming humans, resulting in discomfort, physical burns (due to the spray temperature), and possibly contact dermatitis. The adults of this species are nocturnal and believed to be generalist predators and scavengers. However, larval stages appear to depend on an exclusive diet of mole cricket eggs. As such, Pheropsophus aequinoctialis may have potential use as a biological control agent against certain invasive mole cricket pests in North America. This article describes this insects biology, development, and behavior.
 Also published on the Featured Creatures website at http://entnemdept.ufl.edu/Creatures/BeneFICIAL/beetles/Pheropsophus_aequinoctialis.html

A new bombardier beetle species of the genus Brachinus (Coleoptera: Carabidae: Brachininae) from the Western Ghats in south west India with a new synonymy

ABSTRACT Description of a new bombardier beetle species, Brachinus paikadai sp. nov., first report of B. peltastes Andrewes, 1931 from south India and outside the type locality in Himalaya and synonymisation of B. cinctellus Chaudoir, 1876 with B. limbellus Chaudoir, 1876 are provided. http://www.zoobank.org/urn:lsid:zoobank.org:pub:93B7754A-8088-4631-AACD-AA55D48DFED9

Defense of bombardier beetles against avian predators.

Bombardier beetles (Coleoptera, Carabidae, Brachininae) possess a remarkable defense mechanism where a hot chemical spray is released from the tip of their abdomen, with an audible explosive sound. To date, the repellent properties of these chemicals have been tested against a limited number of taxa, such as amphibians and insects. To investigate the impact of bombardier beetle defenses on avian predators, feeding trials were conducted using the bombardier beetle (Pheropsophus jessoensis) and the Japanese quail (Coturnix japonica), a sympatric and generalist predator. All naïve, hand-reared quail attacked live beetles, indicating the absence of an innate aversion to them. However, most of the quail rejected consuming the beetles whether or not the beetles sprayed them with chemicals. Naïve quail also rejected dead P. jessoensis individuals. These results support the recent hypothesis that it is not essential for P. jessoensis to spray noxious chemicals to deter predators. We also found that some of the quail exposed to live P. jessoensis remembered to avoid them for up to 5weeks. Our results provide the first evidence of the repelling effects of bombardier beetle defense mechanisms on avian predators.

Biosynthetic origin of benzoquinones in the explosive discharge of the bombardier beetle Brachinus elongatulus.

Bombardier beetles are well-known for their remarkable defensive mechanism. Their defensive apparatus consists of two compartments known as the reservoir and the reaction chamber. When challenged, muscles surrounding the reservoir contract sending chemical precursors into the reaction chamber where they mix with enzymes resulting in an explosive discharge of a hot noxious chemical spray containing two major quinones: 1,4-benzoquinone and 2-methyl-1,4-benzoquinone (toluquinone). Previously, it has been speculated that the biosynthesis of all benzoquinones originates from one core precursor, 1,4-hydroquinone. Careful ligation of the base of the reservoir chamber enabled us to prevent the explosive reaction and sample untransformed reservoir fluid, which showed that it accumulates significant quantities of 1,4-hydroquinone and 2-methyl-1,4-hydroquinone. We investigated the biosynthetic mechanisms leading to quinone formation by injecting or feeding Brachinus elongatulus beetles with stable-isotope-labeled precursors. Chemical analysis of defensive secretion samples obtained from 1,4-hydroquinone-d6-administered beetles demonstrated that it underwent conversion specifically to 1,4-benzoquinone. Analogously, results from m-cresol-d8 injected or fed beetles confirmed that m-cresol is metabolized to 2-methyl-1,4-hydroquinone, which is then oxidized to 2-methyl-1,4-benzoquinone in the hot spray. Our results refute the previous claim that 1,4-hydroquinone is the precursor of all substituted benzoquinones in bombardier beetles and reveal that they are biosynthetic products of two independent pathways. Most likely, the aforementioned biosynthetic channel of hydroxylation of appropriate phenolic precursors and subsequent oxidation is not restricted to bombardier beetles; it could well be a general pathway that leads to the formation of all congeners of benzoquinones, one of the most widely distributed groups of defensive compounds in arthropods. Graphical abstract.

Eustra yinggelingensis sp. nov., a new flanged bombardier beetle from Yinggeling Nature Reserve, Hainan, China with contribution to the knowledge of Chinese Eustra species (Coleoptera, Carabidae, Paussinae).

A new species of flanged bombardier beetle form Hainan Island, China, Eustra yinggelingensis sp. nov. (Coleoptera, Carabidae, Paussinae, Ozaenini) is described. The new species was collected in rotten wood from Yinggeling Nature Reserve. Habitus and diagnostic features of the new species are illustrated. List, key and distribution map of all known Chinese Eustra species are given.

New Species of Bombardier Beetles of the Genus Styphlomerus Chaudoir, 1875 (Coleoptera: Carabidae) from Southern India, with a Key to the Indomalayan and Palearctic Species

Two new species of Styphlomerus Chaudoir, 1875 (Styphlomerus devagiriensis Akhil and Thomas and Styphlomerus striatus Akhil and Thomas) are described from southern India. A key to the Indomalayan and Palearctic species of Styphlomerus is provided.

The ant nest “bomber”: Explosive defensive system of the flanged bombardier beetle Paussus favieri (Coleoptera, Carabidae)

Bombardier beetles are famous for their unique ability to explosively discharge hot quinones from their pygidial glands when threatened. Here we provide the first detailed description of the ultrastructure of the defensive gland system of the genus Paussus, the most speciose genus in the ground beetle subfamily Paussinae. Paussine beetles are commonly known as “flanged bombardier beetles” due to the presence of a flange on their elytra that assists in directing their defensive chemicals toward the front of their bodies. In this paper, we use optical, fluorescence and focused ion beam (FIB/SEM) microscopy to analyse and illustrate anatomy and ultrastructure of the explosive defensive system of Paussus favieri, a charismatic myrmecophilous species. The defensive system of this species consists of two independent, symmetrical glands each composed of secretory lobes, a long collecting duct, a bilobed reservoir chamber, a cuticular valve, a sclerotized reaction chamber, and an accessory chamber, associated with the reaction chamber, that is surrounded by several isolated glandular cells. Differences between the pygidial defensive systems of Paussus favieri and those of Brachininae are discussed.

Measuring thermal conductivity of ultra-small materials exampled by the reaction chambers of bombardier beetles

Bombardier beetles, as their name implies, defend themselves against the potential predators by spraying boiling-hot and corrosive chemicals which are products of an explosive biochemical reaction taking place in the paired organs called reaction chambers (RCs). Why these beetles can withstand the explosive reaction that takes place in their bodies? Knowledge of thermal properties of the RCs will help to shed light on this puzzle. However, the submillimeter dimension and irregular shape of the RCs discourage the traditional methods for thermal characterization. To overcome this challenge, in this paper a method is developed particularly for measuring the thermal conductivity of a material with dimension as small as submillimeter. The measurement results by this method are demonstrated reliable especially for polymeric or organic materials including the RCs of bombardier beetles. The results in this paper not only help to understand the superior thermal insulation of the RC wall but also offer a facile approach to measuring the thermal conductivity of materials in submillimeter scale.

NOx-Conversion Comparison of a SCR-Catalyst Using a Novel Biomimetic Effervescent Injector on a Heavy-Duty Engine

<div class="section abstract"><div class="htmlview paragraph">NOx pollution from diesel engines has been stated as causing over 10 000 pre-mature deaths annually and predictions are showing that this level will increase [<span class="xref">1</span>]. In order to decrease this growing global problem, exhaust after-treatment systems for diesel engines have to be improved, this is especially so for vehicles carrying freight as their use of diesel engines is expected to carry on into the future [<span class="xref">2</span>].</div><div class="htmlview paragraph">The most common way to reduce diesel engine NOx out emissions is to use SCR. SCR operates by injecting aqueous Urea solution, 32.5% by volume (AUS-32), that evaporates prior the catalytic surface of the SCR-catalyst. Due to a catalytic reaction within the catalyst, NOx is converted nominally into Nitrogen and Water.</div><div class="htmlview paragraph">Currently, the evaporative process is enhanced by aggressive mixer plates and long flow paths. The mixer plates create extra exhaust back pressure and cool the exhaust gases, which decreases engine and catalyst efficiency, resulting in overall poor NOx conversion (<40%) and higher CO<sub>2</sub> production under real life drive cycle conditions.</div><div class="htmlview paragraph">To achieve future emission legislation targets, SCR efficiency has to be improved, especially under low catalyst temperature conditions. It should also be noted that Ammonia slip has to be avoided as it is now legislated against [<span class="xref">3</span>].</div><div class="htmlview paragraph">In this study a novel biomimetic effervescent aqueous urea injector, its design inspired by the natural spray phenomenon of the Bombardier Beetle, is used to compare against a market-leading aqueous urea doser in comparable exhaust conditions with and without aggressive mixer plates being installed.</div><div class="htmlview paragraph">The novel biomimetic effervescent injector operates by increasing the temperature of the Urea solution inside a constant volume chamber to its saturated vapour pressure. At the required time of injection an electromagnetically controlled outlet valve opens exposing the now superheated fluid to lower pressure conditions, this results in a hot, effervescent spray being ejected which rapidly breaks up into very fine droplets (<20um) projected at high velocity (60m/s) into the exhaust stream targeted at the face of the SCR catalyst.</div><div class="htmlview paragraph">The novel biomimetic effervescent injector shows great potential with a significant higher NOx conversion rate and lower ammonia slip compared to the market-leading doser on a heavy-duty engine.</div></div>

Morpho-functional analysis of the explosive defensive system of basal bombardier beetles (Carabidae: Paussinae: Metriini)

Bombardier beetles, belonging to the carabid subfamilies Paussinae and Brachininae, are famous for their unique ability to explosively discharge a hot spray of quinones from their pygidial glands when threatened. The paussine tribe Metriini is broadly acknowledged as the most basal group of bombardiers. In order to complement the available information on the chemical substances and the primitive discharging mechanism of Metriini, we provide a detailed morpho-functional analysis of the explosive defensive system of Metrius contractus Eschscholtz, 1829 and Sinometrius turnai Wrase and J. Schmidt, 2006, representatives of the two genera in this tribe. We use dissections, histology, scanning electron microscopy (SEM), and focused ion beam microscopy (FIB/SEM) to describe and illustrate various levels of anatomical complexity. FIB/SEM microscopy is used to analyse ultrastructural features of the cellular regions, replacing the classical transmission electron microscopy (TEM). Compared to other Paussinae tribes, Metriini lacks the typical flange of Coanda, the elytral fold used to direct the defensive secretions forward, but has a similar arrangement of internal components. We find that the internal components of the explosive defensive system, including the secretory lobes, collecting duct, reservoir chamber, valve, reaction chamber, accessory chamber and accessory glands, are only slightly different between Metrius Eschscholtz, 1829 and Sinometrius Wrase and J. Schmidt, 2006. The accessory chamber to the reaction chamber is a unique, derived character state common to all Paussinae examined and therefore represents a clear apomorphy of the Paussinae. We use the same microscopy techniques as used in a recent publication on the Brachininae, to compare the defensive systems of Metriini and Brachininae. We find a lack of morphological similarity at the ultrastructural level, suggesting that the bombarding mechanism may have evolved independently in the Paussinae and the Brachininae, perhaps in response to different ecological pressures.

Anti-predator defences of a bombardier beetle: is bombing essential for successful escape from frogs?

Some animals, such as the bombardier beetles (Coleoptera: Carabidae: Brachinini), have evolved chemical defences against predators. When attacked, bombardier beetles can discharge noxious chemicals at temperatures of approximately 100 °C from the tip of their abdomens, “bombing” their attackers. Although many studies to date have investigated how bombardier beetles discharge defensive chemicals against predators, relatively little research has examined how predators modify their attacks on bombardier beetles to avoid being bombed. In this study, I observed the black-spotted pond frog Pelophylax nigromaculatus (Anura: Ranidae) attacking the bombardier beetle Pheropsophus jessoensis under laboratory conditions. In Japan, Pe. nigromaculatus is a generalist predator in grasslands where the bombardier beetle frequently occurs. Almost all the frogs (92.9%) observed rejected live bombardier beetles; 67.9% stopped their attacks once their tongues touched the beetles, and 25.0% spat out the beetles immediately after taking the beetles into their mouths. No beetle bombed a frog before being taken into a frog’s mouth. All beetles taken into mouths bombed the frogs. Only 7.1% of the frogs swallowed live bombardier beetles after being bombed in the mouth. When dead beetles were provided instead, 85.7% of the frogs rejected the dead beetles, 71.4% stopped their attacks after their tongues touched the beetles, and 14.3% spat out the beetles. Only 14.3% of the frogs swallowed the dead beetles. The results suggest that the frogs tended to stop their predatory attack before receiving a bombing response from the beetles. Therefore, bombing was not essential for the beetles to successfully defend against the frogs. Using its tongue, Pe. nigromaculatus may be able to rapidly detect a deterrent chemical or physical characteristics of its potential prey Ph. jessoensis and thus avoid injury by stopping its predatory attack before the beetle bombs it.

Molecular phylogeny, ecology and multispecies aggregation behaviour of bombardier beetles in Arizona.

Aggregations of conspecific animals are common and have been documented in most phyla. Multispecies aggregations are less common and less well studied. Eight species of Brachinus beetles —famous for their unique, highly effective, chemical defense—regularly settle together form large diurnal multispecies aggregations in dark, moist areas in riparian habitats in the Sonoran Desert Region. Here, we document these multispecies aggregations and investigate the incidence and dynamics of aggregation behavior. Analysis of species composition of 59 field-collected aggregations revealed that 71% contained more than one species, eight species regularly co-occurred in aggregations, and no two species showed a preference to aggregate with one another. We provide the first phylogenetic analyses of participants in multispecies aggregations, and find that Brachinus species found together in aggregations are not each other’s closest relatives but rather are dispersed throughout the phylogeny of the genus. Further, we find no tendency for species to aggregate with close relatives more frequently than distant relatives. Laboratory experiments on B. elongatulus showed that it chose to settle in occupied shelters over empty shelters. Experiments with B. hirsutus and B. elongatulus showed that B. hirsutus prefers to settle under shelters housing heterospecifics over conspecifics. Our findings suggest that these multispecies aggregations do not form by chance, but rather are initiated by a genus-wide aggregation cue associated with the presence of individuals already in a shelter, which is likely to be chemical and potentially tactile in nature.

Bacterial Associates of a Gregarious Riparian Beetle With Explosive Defensive Chemistry.

Bombardier beetles (Carabidae: Brachininae) are well known for their unique explosive defensive chemistry. These beetles are found in riparian corridors throughout the American Southwest, where they commonly form large diurnal multispecies aggregations in moist areas under rocks, in crevices, and in leaf litter. Using high throughput 16S amplicon sequencing, we provide the first microbiome survey of a bombardier beetle, Brachinus elongatulus, collected from two sites in Arizona. Two bacterial taxa were present in all individuals sampled: Enterococcus and Dysgonomonas. Enterococcus has been implicated in the production of fecal aggregation pheromone components, which have been shown to regulate aggregation in the German cockroach; it is possible that Enterococcus plays a similar role in Brachinus. Dysgonomonas was found in all the secretory cells of the defensive system and gut samples. Additional studies are needed to determine if these microbes play a role in these beetles’ unique chemical defense. Results also show that the majority of B. elongatulus individuals collected from both sites were infected with Spiroplasma. Many Spiroplasma are intracellular, vertically transmitted insect symbionts that may manipulate host reproduction (e.g., cause male-killing) or provide resistance to nematodes and/or parasitoid wasps. Defensive protection could be especially beneficial to B. elongatulus, which are frequently parasitized by horsehair worms (Nematomorpha). In sum, findings suggest several testable hypotheses on the effects bacteria may have on bombardier beetle behavior and physiology.