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

https://doi.org/10.1016/j.asd.2019.03.001Get rights and content

Highlights

  • We analyse the anatomy and ultrastructure of Paussus favieri explosive defensive system.

  • We use histology, fluorescence and focused ion beam/scanning electron microscopy.

  • We compare the system of P. favieri with that of the Brachininae bombardier beetles.

  • We discuss whether the explosive system is a preadaptation for evolution of myrmecophily.

Abstract

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.

Introduction

“… I saw two rare beetles, and seized one in each hand; then I saw a third and new kind, which I could not bear to lose, so that I popped the one which I held in my right hand into my mouth. Alas! it ejected some intensely acrid fluid, which burnt my tongue so that I was forced to spit the beetle out, which was lost, as was the third one.” Charles Darwin

Bombardier beetles, the only animals capable of sustaining hot explosions inside their bodies (Aneshansley et al., 1969, Arndt et al., 2015), are commonly recognized as having one of the most sophisticated defensive mechanisms documented to date. Each blast is accompanied by an audible “pop”, heat, and a fine aerosolized spray resembling swirling smoke, all reminiscent of small cannons, which they aim at their enemies with astonishing accuracy (Eisner and Aneshansley, 1982, Eisner and Aneshansley, 1999, Dean, 1980a, Dean, 1980b, Eisner et al., 2006).

Since the beginning of the 19th century, this explosive defensive system has attracted the attention of generations of scientists and has been studied as a biomimetic model for several human applications (Beheshti and McIntosh, 2007, Lai, 2010, Booth et al., 2012, Schroeder et al., 2018). Scientists have most thoroughly investigated this system from chemical and functional points of view (Eisner, 1958, Schildknecht and Holoubek, 1961, Moore and Wallbank, 1968, Roach et al., 1979, Schildknecht et al., 1968, Schildknecht et al., 1970, Aneshansley et al., 1969, Aneshansley et al., 1983, Schildknecht, 1970, Eisner et al., 1977, Eisner et al., 1989, Eisner et al., 1991, Eisner et al., 2000, Eisner et al., 2001, Eisner and Aneshansley, 1982, Eisner and Aneshansley, 1999, Arndt et al., 2015). Most recently, the defensive systems of several genera within the subfamily Brachininae were investigated in detail (Di Giulio et al., 2015b) building upon previous work at a gross morphological level (Forsyth, 1972, Deuve, 1993).

Members of the subfamily Brachininae are the most famous bombardier beetles, but a similar explosive defensive system is also found in all members of the ground beetle subfamily Paussinae. Paussine beetles are commonly known as “flanged bombardier beetles” (Moore, 2006) due to the presence of a unique subapical fold on their elytra which they use to direct the spray toward the front of their bodies (Eisner and Aneshansley, 1982). There are approximately 800 species of Paussinae classified in 4 tribes. The tribe Paussini is the most speciose (with about 600 described species) and contains species that live obligately with ants and contain many highly derived and bizarre adaptations for this extreme lifestyle (Di Giulio et al., 2009, Di Giulio et al., 2012, Di Giulio et al., 2014, Di Giulio et al., 2015a, Moore and Robertson, 2014, Robertson and Moore, 2017).

The unique similarities between the defensive systems of Brachininae and Paussinae, including the gross structure of the defensive system (especially the unique presence of the reaction chamber in both lineages of bombardier beetles) and the ejected chemicals (mainly benzoquinones, hydrogen peroxide and various hydrocarbons such as alkanes, alkenes and alkadienes) have been stressed in the past as evidence that paussines and brachinines are sister groups (Eisner et al., 1977, Eisner et al., 1989, Eisner et al., 1991, Eisner et al., 2000). However, the phylogenetic relationship between Brachininae and Paussinae remains unclear (Maddison et al., 1999, Maddison et al., 2009, Eisner et al., 2000, Eisner et al., 2001), and some authors regard the similarities in the defensive systems of the bombardier beetles as products of convergent evolution (Forsyth, 1972, Ball and McCleve, 1990, Beutel, 1993, Arndt, 1998, Liebherr and Will, 1998).

In this paper we use optical, fluorescence and focused ion beam (FIB/SEM) microscopy to analyse the explosive defensive system of the flanged bombardier beetle Paussus favieri, a social parasite of Pheidole pallidula, and one of the most charismatic beetles in Europe (Fig. 1A). In order to facilitate comparisons between the defensive systems of paussines and brachinines, we followed the same methods and techniques used in our recently published work on the Brachininae genera Brachinus, Pheropsophus and Aptinus (Di Giulio et al., 2015b) and on Paussinae genera Metrius and Sinometrius (Muzzi et al., 2019).

Section snippets

Material examined

Twelve adults of P. favieri (6 females and 6 males) were collected in 2010 from nests of Ppallidula found under stones in a Mediterranean garigue on the High Atlas Mountains (Tizi-n-Test, about 3 km N to pass, 2063 m elevation, 30.87288° N– 8.36204° W).

Histology

Two specimens of Pfavieri were anesthetized with CO2, fixed with Bouin's solution, dehydrated in a graded ethanol series, and embedded in paraffin. Serial sections were cut at 5–7 μm with a rotary microtome CUT 6062 (SLEE, Mainz, Germany) and

General structure of the defensive system

As in all adephagan beetles, the defensive system of Pfavieri (Fig. 1B,C) comprises two symmetrical glandular systems located in the posterior part of the abdomen, between the sixth and ninth abdominal segments, dorsal to the reproductive organs and adjacent to the hindgut. Each glandular system is physically and functionally independent of the other. Secretory lobes discharge chemical precursors into a collecting duct, which empties into a voluminous reservoir chamber. A cuticular valve

Discussion

Pfavieri is a West-mediterranean species emerging as a model for the entire tribe Paussini, thanks to the recent studies dealing with larval development (Di Giulio et al., 2011), behaviour (Le Masne, 1961, Maurizi et al., 2012), parasitic strategies (Cammaerts et al., 1990, Di Giulio et al., 2014, Di Giulio et al., 2015a), sensorial organs (Di Giulio et al., 2012) and glandular anatomy (Di Giulio et al., 2009). Below we compare the fine morphology and anatomy of the different components of

Acknowledgements

We are grateful to Prof. Sandra Moreno (University Roma Tre, Rome, Italy) for the help in preparing the biological samples, and Eng. Daniele De Felicis (University Roma Tre, Rome, Italy) for the kind availability and technical assistance in the LIME lab. We thank Prof. Ahmed El Hassani (Institut Scientifique, Université Mohammed V Agdal, Rabat, Morocco) for granting us the permit to collect Paussus favieri specimens. We thank three anonymous reviewers for the helpful suggestions. The research

References (62)

  • F. Alvarez-Padilla et al.

    A protocol for digesting internal soft tissues and mounting spiders for scanning electron microscopy

    J. Arachnol.

    (2007)
  • D.J. Aneshansley et al.

    Biochemistry at 100°C: explosive secretory discharge of bombardier beetles (Brachinus)

    Science

    (1969)
  • D.J. Aneshansley et al.

    Thermal concomitants and biochemistry of the explosive discharge mechanism of some little known bombardier beetles

    Experientia

    (1983)
  • E. Arndt

    Phylogenetic investigation of Carabidae (Coleoptera) using larval characters. Phylogeny and classification of Caraboidea (Coleoptera: Adephaga)

  • E.M. Arndt et al.

    Mechanistic origins of bombardier beetle (Brachinini) explosion-induced defensive spray pulsation

    Science

    (2015)
  • G.E. Ball et al.

    The middle American genera of the tribe Ozaenini, with notes about the species in southwestern United States and selected species from Mexico

    Quaest. Entomol.

    (1990)
  • N. Beheshti et al.

    A biomimetic study of the explosive discharge of the bombardier beetle

    Int. J. Des. Nat. Ecodyn.

    (2007)
  • R.G. Beutel

    Phylogenetic analysis of Adephaga (Coleoptera) based on characters of the larval head

    Syst. Entomol.

    (1993)
  • A. Booth et al.

    Spray technologies inspired by bombardier beetle

  • R. Cammaerts et al.

    Host trail following by the myrmecophilous beetle Edaphopaussus favieri (Fairmaire)(Carabidae Paussinae)

    Insectes Soc.

    (1990)
  • A. Casale et al.

    Coleoptera, Carabidae. I. Introduzione, Paussinae, Carabinae

  • R.A. Crowson

    The Biology of the Coleoptera

    (1981)
  • J. Dean

    Effect of thermal and chemical components of bombardier beetle chemical defense: glossopharyngeal response in two species of toads (Bufo americanus, B. marinus)

    J. Comp. Physiol.

    (1980)
  • J. Dean

    Encounters between bombardier beetles and two species of toads (Bufo americanus, B. marinus): speed of prey-capture does not determine success

    J. Comp. Physiol.

    (1980)
  • J. Dean et al.

    Defensive spray of the bombardier beetle: a biological pulse jet

    Science

    (1990)
  • T. Deuve

    L'abdomen et les genitalia des femelles de Coléoptères Adephaga. Mémoires du Muséum national d'histoire naturelle

    (1993)
  • A. Di Giulio et al.

    Review of competing hypotheses of phylogenetic relationships of Paussinae (Coleoptera: Carabidae) based on larval characters

    Syst. Entomol.

    (2003)
  • A. Di Giulio et al.

    The long-awaited first instar larva of Paussus favieri (Coleoptera: Carabidae: Paussini)

    Eur. J. Entomol.

    (2011)
  • A. Di Giulio et al.

    Form, function and evolutionary significance of stridulatory organs in ant nest beetles (Coleoptera: Carabidae: Paussini)

    Eur. J. Entomol.

    (2014)
  • A. Di Giulio et al.

    The pied piper: a parasitic beetle's melodies modulate ant behaviours

    PLoS One

    (2015)
  • T. Eisner et al.

    Spray aiming in bombardier beetles: jet deflection by Coanda effect

    Science

    (1982)
  • Cited by (4)

    • Anatomical specializations of the gizzard in soil-feeding termites (Termitidae, Apicotermitinae): Taxonomical and functional implications

      2020, Arthropod Structure and Development
      Citation Excerpt :

      The musculature around the gizzard in termites provides an optimal mechanical pressure to control the movement of the particles through the digestive tract (Noirot and Noirot-Timothée, 1969). As established in other arthropods (Michels et al., 2016; Muzzi and Di Giulio, 2019; Rork et al., 2019), the weakly sclerotized tissues throughout the gizzard wall seem to reveal the presence of a resilin-like protein. In Apicotermitinae, with a regressed gizzard, the presence of this elastic protein could be important to reduce mechanical stress on the pulvillar belt and increase the extensibility (Andersen and Weis-Fogh, 1964; Michels et al., 2016) of the whole gizzard during particle flow and peristalsis.

    View full text