What is going on between aposematic carabid beetles? The case of Anchomenus dorsalis (Pontoppidan 1763) and Brachinus sclopeta (Fabricius 1792) (Coleoptera Carabidae)

Interspecific aggregations among insects are quite common, but not among carabid beetles. Moreover, aggregations of aposematic species, and morphologically similar beetles such as Anchomenus dorsalis and Brachinus sclopeta are even rarer. Most interestingly, in these aggregations, we observed a peculiar rubbing behaviour of A. dorsalis towards B. sclopeta. In this study we aimed (i) to verify if A. dorsalis displays the rubbing behaviour towards non-aposematic species, (ii) to describe in detail and to quantify this behaviour between A. dorsalis and B. sclopeta, and (iii) to verify if previous experience of inter-specific gregariousness with the same species may exert some effects on the frequency of the rubbing display. We have investigated the qualitative and quantitative characteristics of rubbing behaviour towards aposematic and non-aposematic carabide beetle species, and then we used A. dorsalis and B. sclopeta as model species. We tested the rubbing of A. dorsalis towards B. sclopeta with either solitary A. dorsalis or in A. dorsalis coming from monospecific (A. dorsalis) or from heterospecific (A. dorsalis and B. sclopeta) aggregations. We finally tested the effectiveness of the B. sclopeta cuticular odour in eliciting rubbing from A. dorsalis by presenting olfactory dummies (special paper) either soaked with water or after it had previously been placed in a monospecific group of B. sclopeta for 15 days. First of all, we found important differences in the occurrence of rubbing behaviour towards aposematic and non-aposematic species, as A. dorsalis displayed the rubbing only towards the former. Secondly, rubbing frequency depended on the pre-test conditions (isolated, mono- or heterospecific aggregations) of the tested A. dorsalis individuals, as no rubbing occurred in individuals coming from heterospecific aggregations. The paper presentation tests demonstrated that the odour of B. sclopeta was sufficient to provoke rubbing in A. dorsalis.We conclude that aggregation in these species of carabid beetles may influence their anti-predatory defences possibly increasing the effectiveness of the aposematic (warning) signals. More work is needed to delineate in detail the evolutionary significance of this behaviour, focusing on the possibility that interspecific aggregations may not be so rare in ground beetles, and that this positive interaction could involve a larger number of species and genera, at least in wet, sun-exposed and open land habitat types.