Paederus

Paederus riparius, from here.

Belongs within: Paederini.

Paederus is a diverse genus of staphylinid beetles, generally coloured in contrasting patterns of red and black. Many species are found in damp habitats alongside rivers or in marshy fields. A number of species produce pederin and other haemolymph compounds that can cause severe dermatitis upon contacting human skin (Newton et al. 2001).

Irritating rove beetles
Published 13 October 2022

Despite being contenders to be recognised as the world’s most diverse family of animals, most representatives of the beetle family Staphylinidae garner relatively little attention. They are often small, nondescript and, though they may be important micro-predators, their direct impact on human lives tends to be minimal. However, the staphylinids of the genus Paederus represent notable exceptions to this trend. When they come into contact with humans, they burn.

Paederus littoralis, copyright Aleksandrs Balodis.

In the broad sense, Paederus is a cosmopolitan genus of over 600 known species (Frank 1988). Attempts to subdivide it have been made but with limited success, most being only regional in their application. Species are commonly found in damp habitats such as marshes or alongside streams, and they may be abundant in agricultural fields. They are distinguished from other staphylinid genera by the structure of the maxillary palps, in which the last segment is compressed, wider than long, apically truncate and densely hairy, and by their broadly dilated protarsi. Other notable features include the head narrowing behind the eyes to form a distinct ‘neck’, and a brick-like pattern on the intersegmental membranes of the abdomen (Newton et al. 2001). Most Paederus are strikingly coloured with the bright red or yellow pronotum contrasting with the black or metallic blue remainder of the body.

Paederus gratiosus, copyright Satish Nikam.

Their eye-catching coloration serves as a warning of their noxious nature. The haemolymph of many Paederus individuals contains a substance called pederin that causes the formation of burn-like blisters if it comes into contact with skin. Lesions often form a linear pattern as a result of the injured beetle’s movements as it escapes, leading to the condition being sometimes known as dermatitis linearis. Secondary conditions such as conjunctivitis may also result from the toxin being further spread by the fingers. The tendency of Paederus beetles to multiply in fields may increase the chance of contact, as does their attraction to lights (Bong et al. 2015). In some cases, outbreaks may be severe enough to have a significant impact on the communities where they occur. Here in Australia, a remote community in the Northern Territory had to be temporarily evacuated in 1995 due to swarms of Paederus australis (Todd et al. 1996).

Paederus dermatitis, copyright Jere Mammino.

Interestingly, production of pederin is not universal among Paederus. Where studied, pederin production only occurs in females. Its primary utility may not be to product the adult beetles themselves as much as the pederin-laden eggs they lay. However, not all females produce pederin, either. Pederin-negative females lay eggs lacking pederin but will start producing pederin themselves if fed eggs laid by a pederin-positive female (Kellner 1999). From this, it has been inferred that the pederin is not produced by the beetle itself but by an endosymbiotic bacterium. Kellner (2002) suggested that the symbiont was a species of Pseudomonas after identifying a molecular signature found in pederin-positive but not pederin-negative females. The power to produce pederin could then be passed on as an infection.

Systematics of Paederus

Characters (from Fagel 1961): Pronotum and prosternum not separate; weak prosternal extension; abdomen thick; pleurites well marked; sternite 1 with strong longitudinal carina; median lobe of aedeagus with ventral and dorsal lamella, median opening, parameres generally symmetrical, fused to median lobe over large part of their length and carrying numerous rows of bristles which do not reach the apex.

Paederus Fabricius 1775NT01 (see below for synonymy)
    |--P. adelaidae Blackb. 1887M96
    |--P. angusticepsF61
    |--P. australisB35
    |--P. caffer Boheman 1848F61
    |--P. cruenticollis [incl. P. cingulatus]M96
    |--P. fastuosusF61
    |--P. ferrugineusR26
    |--P. fuscipes Curtis 1823 (see below for synonymy)F61
    |--P. gestroiM86
    |--P. impressipennisF61
    |--P. littoralisLC20
    |--P. littorariusLC20
    |--P. longipennisG89
    |--P. meyricki Blackb. 1891M96
    |--P. nigripes Bernhauer 1932F61
    |--P. nigrolineatusF61
    |--P. obliteratusN90
    |--P. ochraceusR26
    |--P. orbiculatusR26
    |--P. riparius (Linnaeus 1758)B14
    |--P. rudebecki Fagel 1961F61
    |--P. simsoni Blackb. 1894M96
    |--P. termitophilus Wasmann 1911S57
    `--P. testaceusR26

Paederus Fabricius 1775NT01 [incl. Leucopaederus Casey 1905NT01, Neopaederus Blackwelder 1939F61, Paederillus Casey 1905NT01, Paederomorphus Gautier des Cottes 1862F61; Paederina]

Paederus fuscipes Curtis 1823 [incl. P. abyssinicus Cam. 1950, P. aestuans Erichson 1839-1840, P. alfierii Koch 1934, P. angolensis Er. 1843, P. erichsoni Wollaston 1867, P. mayumbeanus Cameron 1939]F61

*Type species of generic name indicated

References

[B35] Boisduval, J. B. 1835. Voyage de Découvertes de l’Astrolabe. Exécuté par ordre du Roi, pendant les années 1826–1827–1828–1829, sous le commandement de M. J. Dumont d’Urville. Faune entomologique de l’océan Pacifique, avec l’illustration des insectes nouveaux recueillis pendant le voyage vol. 2. Coléoptères et autres ordres. J. Tastu: Paris.

Bong, L.-J., K.-B. Neoh, Z. Jaal & C.-Y. Lee. 2015. Paederus outbreaks in human settings: a review of current knowledge. Journal of Medical Entomology 52 (4): 517–526.

[B14] Bouchard, P. (ed.) 2014. The Book of Beetles: A lifesize guide to six hundred of nature’s gems. Ivy Press: Lewes (United Kingdom).

[F61] Fagel, G. 1961. Coleoptera (Staphylinidae): Paederinae. In: Hanström, B., P. Brinck & G. Rudebeck (eds) South African Animal Life: Results of the Lund University Expedition in 1950–1951 vol. 8 pp. 259–295. Almqvist & Wiksell: Uppsala.

Frank, J. H. 1988. Paederus, sensu lato (Coleoptera: Staphylinidae): an index and review of the taxa. Insecta Mundi 2 (2): 97–159.

[G89] Gestro, R. 1889. Viaggio ab Assab nel Mar Rosso dei signori G. Doria ed O. Beccari con il R. Avviso “Esploratore” dal 16 Novembre 1879 al 26 Febbraio 1880.—IV. Coleotteri. Annali del Museo Civico di Storia Naturale di Genova, Serie 2a, 7: 5–72.

Kellner, R. L. L. 1999. What is the basis of pederin polymorphism in Paederus riparius rove beetles? The endosymbiotic hypothesis. Entomologia Experimentalis et Applicata 93: 41–49.

Kellner, R. L. L. 2002. Molecular identification of an endosymbiotic bacterium associated with pederin biosynthesis in Paederus sabaeus (Coleoptera: Staphylinidae). Insect Biochemistry and Molecular Biology 32: 389–395.

[LC20] Lü, L., C.-Y. Cai, X. Zhang, A. F. Newton, M. K. Thayer & H.-Z. Zhou. 2020. Linking evolutionary mode to palaeoclimate change reveals rapid radiations of staphylinoid beetles in low-energy conditions. Current Zoology 66 (4): 435–444.

[M86] Macleay, W. 1886. The insects of the Fly River, New Guinea, “Coleoptera”. Proceedings of the Linnean Society of New South Wales, series 2, 1 (1): 136–157.

[M96] Masters, G. 1896. Catalogue of the described Coleoptera of Australia. Supplement, part II. Dytiscidae, Gyrinidae, Hydrophyllidae, Staphylinidae, Pselaphidae, Paussidae, Silphidae, Scaphididae, Histeridae, Phalacridae, Nitidulidae, Trogositidae, Colydiidae, Cucujidae, Cryptophagidae, Lathridiidae, Mycetophagidae, Dermestidae, Byrrhidae, Parnidae, Heteroceridae. Proceedings of the Linnean Society of New South Wales 21 (Suppl.): 695–754.

[N90] Newton, A. F., Jr. 1990. Insecta: Coleoptera Staphylinidae adults and larvae. In: Dindal, D. L. (ed.) Soil Biology Guide pp. 1137–1174. John Wiley & Sones: New York.

[NT01] Newton, A. F., M. K. Thayer, J. S. Ashe & D. S. Chandler. 2001. Staphylinidae Latreille, 1802. In: Arnett, R. H., Jr & M. C. Thomas (eds) American Beetles vol. 1. Archostemata, Myxophaga, Adephaga, Polyphaga: Staphyliniformia pp. 272–418. CRC Press: Boca Raton.

[R26] Risso, A. 1826. Histoire naturelle des principales productions de l’Europe méridionale et particulièrement de celles des environs de Nice et des Alpes maritimes vol. 5. F.-G. Levrault: Paris.

[S57] Seevers, C. H. 1957. A monograph on the termitophilous Staphylinidae (Coleoptera). Fieldiana Zoology 40: 1–334.

Todd, R. E., S. L. Guthridge & B. L. Montgomery. 1996. Evacuation of an Aboriginal community in response to an outbreak of blistering dermatitis induced by a beetle (Paederus australis). Medical Journal of Australia 164: 238–240.

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