True water bugs (Hemiptera: Heteroptera: Nepomorpha) are among the most common insects of freshwater ecosystems, comprising approximately 2300 extant species spread over five recognized superfamilies, including Corixoidea, Nepoidea, Ochteroidea, Notonectoidea and Naucoroidea. They exhibit striking morphological and behavioural adaptations to various freshwater environments, including oar‐like swimming legs, breathing siphons or plastron respiration. The phylogeny of Nepomorpha remains contentious, particularly for the early‐diverging lineage, which has hindered the understanding of the evolution of morphology and respiratory behaviour within the clade. In the present study, we assembled a large‐scale phylogenomic dataset, including 2018 single‐copy, protein‐coding gene sequences from 85 representative species of heteropterans (44 nepomorphans) to investigate the phylogeny of Nepomorpha and the corresponding implications for character evolution. Our inferences suggest that Corixoidea is the sister group of the remaining Nepomorpha, then Nepoidea and Ochteroidea; these clades successively branched in the Triassic, following the end‐Permian extinction event about 251 million years ago. The five superfamilies radiated in the Jurassic, when geological reconfigurations and drastic climate changes occurred. An ancestral state reconstruction demonstrated that the ancestral respiration type in true water bugs is likely a simple air‐bubble type, which was widely utilized in true water bugs. Subsequently, different clades have evolved variously specialized adaptations to improve its efficiency. We propose that the crawling legs of Nepidae are secondary or symplesiomorphic characters, which cannot serve as the evidence for the sister group role of Nepoidea to the remaining nepomorphans.

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Among hundreds of insect families, Hermatobatidae (commonly known as coral treaders) is one of the most unique. They are small, wingless predaceous bugs in the suborder Heteroptera. Adults are almost black in colour, measuring about 5 mm in body length and 3 mm in width. Thirteen species are known from tropical coral reefs or rocky shores, but their origin and evolutionary adaptation to their unusual marine habitat were unexplored. We report here the genome and metagenome of Hermatobates lingyangjiaoensis , hitherto known only from its type locality in the South China Sea. We further reconstructed the evolutionary history and origin of these marine bugs in the broader context of Arthropoda. The dated phylogeny indicates that Hexapoda diverged from their marine sister groups approximately 498 Ma and that Hermatobatidae originated 192 Ma, indicating that they returned to an oceanic life some 300 Myr after their ancestors became terrestrial. Their origin is consistent with the recovery of tropical reef ecosystems after the end-Triassic mass extinction, which might have provided new and open niches for them to occupy and thrive. Our analyses also revealed that both the genome changes and the symbiotic bacteria might have contributed to adaptations necessary for life in the sea.

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Symbiotic bacteria and fungi generally colonize insects and provide various benefits for hosts. Although numerous studies have investigated symbionts in terrestrial plant-feeding insects, explorations of symbiotic bacterial and fungal communities in aquatic and semiaquatic insects are rare.