Predation avoidance mechanisms in a gregarious platynine carabid, Anchomenus dorsalis (Pontoppidan, 1763)

Anchomenus dorsalis (Pontoppidan, 1763) is a gregarious platynine carabid inhabiting muddy soils and fields across Europe. It is often found in association with species of Brachinus and, like Brachinus, it has a bright bicoloured coat body that contrasts with the background. These species usually aggregate under heavy stones in open lands with sparse vegetation, such as pasturelands, croplands or in humid, sun exposed soils (Bonacci et al. 2004b). Laboratory investigations showed that A. dorsalis use conspicuous colours and warning odours versus natural enemies (Bonacci et al. 2004a, 2006; 2008; 2011a,b). We supposed that the combination of visual and olfactory signals, common in many insects, can produce a multimodal warning display (Rowe and Guilford 1999) toward the predators. When disturbed, A. dorsalis releases a strong odour (Bonacci et al., 2011a), and quickly retreats (with dispersal movement) into deeper soil crevices. Based on these behavioural observations, chemical analysis (by all-glass aeration apparatus) were conducted to identify the volatile compounds produced by this carabid under undisturbed and disturbed conditions. GC-MS analysis of the extracts from individuals highlighted four major volatile compounds, undecane, heneicosane, Z-9 tricosene and tricosane, of which significantly more undecane was released by disturbed adults compared to undisturbed beetles. This suggests that this hydrocarbon could play a prominent role in the chemical defence of the species. The role of undecane as a defensive substance has been widely reported in other Insecta (Roger, 1862; Regnier and Wilson 1968; Löfqvist and Bergström, 1980; Morgan et al. 2005; Dettner and Schwinger 1982) but the occurrence of undecane in pygidial glands of ground beetles has been described in Pterostichini, Loxandrini, Morionini, Catapieseini, Perigonini and Odacanthini (Will et al. 2000). Nevertheless, in A. dorsalis, volatile compounds ejected after disturbance could have a double function: repellent function to predators and chemical signalling function for conspecifics. In fact, previous studies showed that the chemicals released by a number of disturbed A. dorsalis individuals is not only a repellent towards natural enemies, but also elicites dispersal behavior in conspecifics (Bonacci et al. 2004a,b, 2006, 2008, 2011b). Such an intra-specific dispersal function is supported by the gregarious nature of this species; gregariousness and high population density allowing rapid intra-specific communication is generally thought to be necessary for evolution of chemical signalling molecule (Nault and Phelan 1984). It is reasonable to believe that in A. dorsalis, undecane discharge by the pygidial glands causes dispersal movement of individuals inside the aggregation. Usually A. dorsalis occurs in dense aggregations of many individuals with other carabids belonging to the genus Brachinus. It can be expected that in species occurring in such masses an adequate defense mechanism has evolved towards potential predators, Brachinus spp producing several defensive chemicals (Zetto Brandmayr et al. 2006) and A. dorsalis producing a putative chemical signaling molecule, undecane, from the pygidial glands. Assemblages of mixed species that share common predators may experience benefits that are similar to or exceed those of monospecific groups. These benefits may be particularly pronounced if individuals of one species can recognize the alarm signals produced by individuals of other species in the assemblage (in Mathis and Smith 1993).