A project undertaken at the Western Australian Museum, and supervised by Dr Fernando Soley.
It remains unclear why tool use, despite its high ecological advantage, is rare amongst animals. Contrary to the cognitively-driven tool use of vertebrates, in arthropods, tool use is innate. How innate tool use originates remains a mystery. Previous research suggested that, amongst insects, tool use seems to be particularly common in assassin bugs. These insects collect plant resins to cover their legs or bodies, but the function of such behaviour remains underexplored. Taxonomists coined the term ‘resin bugs’ to refer collectively to this paraphyletic group of assassin bugs. We studied tool-using behaviour in several species of resin bugs in the remote Kimberley region of Western Australia.
Through detailed recordings of behaviour under natural conditions, we are describing the complex motor routines performed by these animals to efficiently extract the resin from different plants. Contrary to other instances of innate tool use, where pre-existing motor routines seem to have been directly co-opted for tool use (i.e., a ‘single evolutionary step), here, tool use seems to have evolved in a more gradual fashion, which is consistent with morphological evolution. Experiments in two species from different genera revealed that resin-equipped individuals were more likely to capture ants or flies in arenas. Hence, resin provides a clear function in prey capture, and video analyses indicate that during assassin bug attacks, resin creates extra adhesion that hinders prey escape. However, eggs in all of these species are also covered in resin. Through careful observations and micro-CT imaging, we discovered that females engage in complex maternal care by absorbing resin through their genitalia to later apply to their eggs as they are laid. Experiments using ants as predators revealed that resin protects the eggs from being taken into ant nests. Hence, resin serves a dual purpose in this group.
We discovered that resin is extracted from several plant species, including different grasses, but also non-herbaceous plants. This and additional natural history information that we collected (including diet and plants used) for at least six species of resin bugs will be very useful when deciphering the road map for tool evolution in this group.
Based on molecular characters, we revised a global phylogeny of resin bugs to include a large sample of Australian taxa for the first time. The new phylogeny, in combination with accurate descriptions of morphology and tool-using behaviour in these species, will provide insights into the origin of tool use in this particular group of animals, as well as explanations for animal tool use more generally. Based on our phylogeny and examination of specimens, we identified several undescribed species. We described a new species, Gorareduvius gajarrangarnang, and also uncovered at least two additional undescribed resin bug species near the genera Poecilosphodrus, and Poecillobdallus, as well as yet another putatively new species of Gorareduvius. Taxonomic work on these taxa is now progressing. Our phylogenetic analysis reveals that in addition to occurring multiply overseas, resin use is also present in two separate Australian clades. This provides further evidence that resin use is prolific in the assassin bug subfamily Harpactorinae, revealing assassin bugs as the prime tool users among insects. Whether this has either evolved multiple times or instead shares a common origin and has since been repeatedly lost, remains to be confirmed. While several resin-gathering behaviours show similarities across clades, suggesting a common origin, we have identified specialized setae and other leg structures used to collect and apply resin, and these structures differ between the Australian clades, providing evidence of repeated evolution of tool-use.