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Published June 20, 2018 | Version v1
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A Fatal Agonistic Interaction between Ant and Darkling Beetle (Coleoptera: Tenebrionidae: Pimeliinae: Adesmiini) in the Northern Namib Desert

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Lamb, Trip (2018): A Fatal Agonistic Interaction between Ant and Darkling Beetle (Coleoptera: Tenebrionidae: Pimeliinae: Adesmiini) in the Northern Namib Desert. The Coleopterists Bulletin 72 (2): 314-316, DOI: 10.1649/0010-065X-72.2.314, URL: http://dx.doi.org/10.1649/0010-065x-72.2.314

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urn:lsid:plazi.org:pub:FFC0FF88FFE1FFA098564368FF90FFE3

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Figure: 10.5281/zenodo.5382533 (DOI)

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  • Africa' s Namib Desert harbors a diverse array of darkling beetles (Tenebrionidae), in which species richness greatly exceeds that of desert biomes elsewhere on the continent (Matthews et al. 2010). Endemism of darking beetles in this desert is concomitantly high, particularly for the psammophilic species inhabiting the Namib' s expansive dunefields, or sand seas (Simmons et al. 1998). In these dunefields, many darkling beetles assume an important role as detritivores, cycling nutrients and energy from the windblown detritus that they consume along dune slip faces (Louw 1983; Seely 1991). The beetles also exhibit several adaptations associated with additional environmental challenges posed by Namib sand seas, including substrate, thermal, and water relations (Henschel and Seely 2008; Lease et al. 2014). Perhaps the most striking adaptation is fog basking (Hamilton and Seely 1976), a water gathering behavior whereby beetles crest dune ridges and tilt head down in advection fog to drink condensate that forms on their elytra. Fog basking is practiced by just two species of adesmiine tenebrionids: Onymacris unguicularis (Haag) and Onymacris bicolor (Haag) (Hamilton et al. 2003; Henschel and Seely 2008). Here, I report for O. bicolor another unusual account-an incident involving interspecific agonistic interaction.
  • Onymacris bicolor, a species defined in part by possessing entirely white elytra, is a member of the distinctive 'white' Onymacris Allard clade, whose elytral color ranges from white to tan-striped. Although certain beetle species assume a whitish cast through production of waxy pruinescence, white coloration outright is rare and is, in all cases, expressed structurally through non-ordered scattering of incidental light (Seago et al. 2009). Onymacris contains a 'black' clade as well, although the genus was recently determined to be paraphyletic with respect to Physadesmia Penrith (Lamb and Bond 2013). Namely, Physadesmia and the 'white' Onymacris are sister taxa, constituting a clade that, in turn, is sister to the 'black' Onymacris. Members of the 'white' clade occur only in the northern Namib (Angola and Namibia), where they are patchily distributed (Penrith 1975). Onymacris bicolor is restricted to dunefields, with populations inhabiting both Cunene and Skeleton Coast sand seas.
  • I collected a series of five O. bicolor at the base of a dune slip face near the Cunene River in northern Namibia. The locality data are: Namibia: Cunene Sand Sea, Kunene District, ~5 km E of Skeleton Coast Park boundary and 1.6 km S of the Angolan border; -17.1892°, 12.1556°; 21.v.2015. Beetles were captured by hand, held alive in empty plastic containers, and later killed by ethanol injection. While processing the specimens, I noticed a nodulelike structure encircling one beetle' s right antenna; closer inspection revealed it to be the head of an ant, fastened tightly by clinched mandibles (Fig. 1). Head size (width = 2.9 mm, length = 3.1 mm) and other morphological features distinguish the ant as a major worker of a large species of Camponotus Mayr (Hymenoptera: Formicidae: Formicinae).
  • Camponotus is a large (1,022 species), cosmopolitan genus of stingless ants, with several subgenera occurring in southern Africa (AntWeb, www.antweb.org). Among the more conspicuous species, in terms of size (6-16 mm), are members of the subgenus Myrmopiromis Wheeler, which includes the ubiquitous, diurnal Camponotus fulvopilosus species-group (Marsh 1986; Robertson and Zachariades 1997). Although species identification in this group centers on pronotal and gastral characters, the attached head-as a function of aforementioned size, eye length (0.94 mm), mandibular tooth count (eight), and coloration-is almost certainly the savanna form of Camponotus fulvopilosus (De Geer). Skeleton Coast populations of C. fulvopilosus are characterized by reddish brown heads, as opposed to brownish black head color observed elsewhere across the range. Moreover, C. fulvopilosus is the only species-group member documented from Namibia' s Kunene District, for which records exist in both Cunene and Skeleton Coast sand seas (Robertson and Zachariades 1997), including one locality (17.11° S, 12.17° E; 24.iv.1991; E. Marais) quite close to the collection site of O. bicolor.
  • The particular circumstances leading to initial contact between ant and beetle are unknown. Both species appear to be abundant in the northern Namib. Moreover, comparable patterns of diurnal activity routinely place them in close proximity, but encounters are probably random, unlikely to elicit substantive behavioral interaction, and thus of little circumstance. Attempted predation by O. bicolor can probably be dismissed. As noted, most Namib tenebrionids are detritivores, and that appears to be the case for Onymacris. Hanrahan and Seely' s (1990) detailed dietary study on Onymacris rugatipennis (Haag) identified both plant and animal detrital material in dissected beetle crops. They concluded that O. rugatipennis forages preferentially, selecting flower material and arthropod fragments, both of which are underrepresented in available detritus. Although small insects were found occasionally in dissected crops, the authors considered such consumption to be inadvertent and more likely a function of the consumed insects' close association with flower material. During a two-year mark-recapture study on O. bicolor, Hanrahan and Yeaton (1987) regularly observed beetles consuming succulent leaves and slip face detritus. Thus, there is little evidence that Onymacris-O. bicolor, in particular-engages in predation.
  • The diet of C. fulvopilosus is predominantly honeydew secured from homopterans, but worker ants maintained in laboratory colonies will feed on caterpillars, roaches, and other soft-bodied insects (Robertson and Zachariades 1997). The closely related Camponotus detritus Emery, a Namib Sand Sea endemic, supplements its honeydew diet with "pollen, nectar, dead animal material, and possibly bird and lizard faeces" (Curtis 1985). In nature, workers of both species tend to forage individually, typically in transit to or from honeydew sources, and thus seem unlikely to attempt to prey on darkling beetles larger than themselves.
  • What other behavioral interactions might account for the outcome experienced between C. fulvopilosus and O. bicolor? Workers of C. fulvopilosus exhibit aggressive behavior towards intruders that attempt nest damage and will attack them by biting, spraying formic acid, or both (Robertson and Zachariades 1997). Such aggression towards Onymacris has not been reported, nor are there any records of Onymacris damaging ant nests. Given the stereotypic defensive behavior of Camponotus, however, there is a feasible scenario by which this encounter could have unfolded-a beetle' s incidental presence near a recently damaged nest. Colony workers would be assuming defensive posture and, though not actually an intruder, the beetle, by timing and placement, could trigger their aggressive behavior. Thus, a worker bites the beetle, latching on to its antenna; the beetle bites defensively in response, severing the ant' s head. Of course, this explanation (or any other) can never be confirmed, but whatever behavioral sequence transpired, the stark evidence of agonistic interaction remains…firmly attached.