Through integrating molecular, morphological and natural history evidence, nominal Hemaris diffinis (Boisduval) of eastern North America is shown to include a second, cryptic species, Hemaris aethra (Strecker) stat. rev. Despite highly divergent mtDNA sequences and differing larval phenotypes, genitalic morphology, habitat and larval host plants, adults of H. aethra and sympatric H. diffinis are externally so similar that H. aethra has remained unrecognized for over a century. With a more northerly distribution than H. diffinis, H. aethra occurs from Manitoba to Nova Scotia and adjacent parts of the United States, the two species occurring in strict sympatry in eastern Ontario and likely other regions. Co-mimicry of Bombus Latreille bumblebee models has likely resulted in phenotypic convergence of H. diffinis and H. aethra, as the two do not appear to be sister taxa, the latter instead being more closely related to the western species H. thetis (Boisduval). The larvae of H. aethra are illustrated for the first time, together with diagnostic images and comparisons of adults. Lectotypes are designated for Hemaris tenuis Grote and Hemaris marginalis Grote.