A single major transition followed by multiple successful colonisations characterises the evolution of the semi-aquatic bugs (Gerromorpha), which now live as pond skaters, water crickets, or sea skaters at the interface between air and water. This mosaic-like evolutionary trajectory is described by Chinese entomologists, including Wageningen master’s student Zhaoqi Leng, following phylogenetic and morphological analyses of 215 species of such bugs (PNAS, 9 March). The ancestors of these insects lived some 247 million years ago in moist terrestrial habitats and underwent a single major transition to life on the water surface, accompanied by a substantial shift from ‘walking’ to ‘rowing’ during the Middle Jurassic (193–179 million years ago).
These semi-aquatic insects subsequently colonised a broad range of different habitats at the water surface successfully, from the Middle Jurassic to the Late Cretaceous (160–79 million years ago): from fast-flowing to standing freshwater, and from intertidal zones to the open ocean surface. In marginal habitats, multiple reversals from ‘rowing’ to ‘walking’ took place, whilst species in open surface habitats retained the same mode of locomotion throughout their evolutionary history. The research group, led by entomologists at Nankai University, argues that their study ‘is a typical example of macroevolutionary dynamics in species adapted to specialised habitats’. In summary, they describe the history of habitat occupation in these species as both ‘highly complex and ordered’, owing to the major role of ‘environmental context and functional constraints’ at the air–water interface.