The classic East Asia–Eastern North America (EA‐ENA) disjunction, well‐documented in temperate plants, is typically attributed to the fragmentation of once‐continuous high‐latitude ranges by climatic cooling and regional extinction. Whether this model extends to insects, particularly aquatic groups, remains largely untested with genomic data. Here, we integrate whole‐genome sequencing, ddRAD‐seq and morphology with phylogeneti...