Cretaceous Chikuniidae fam. nov. (Diptera: Brachycera) illuminates the evolution of blood-feeding in lower brachyceran flies.

With over 80 000 extant species, brachycerans are among the most diverse insect lineages, exhibiting a wide range of feeding strategies and ecological adaptations. Originating in the Early Jurassic, this megadiverse clade then underwent rapid speciation and morphological diversification during the Mesozoic, profoundly shaping its modern diversity. However, the early evolutionary history of many lineages remains poorly known due to a sparse fossil record. Here, we describe a new brachyceran family, †Chikuniidae Feng, Yang, Engel, Ren et Wang fam. nov., from mid-Cretaceous Kachin amber. Phylogenetic analyses place it as the earliest-diverging lineage of the Asiloidea + Eremoneura clade within Muscomorpha, providing a critical fossil calibration for estimating the crown age of Brachycera and refining the early diversification scenario. Exceptional preservation allows functional reconstruction of the mouthparts, corroborating specialized haematophagy in both sexes. This discovery reveals previously unknown ecological diversity among Mesozoic brachyceran flies and provides direct fossil evidence for multiple independent origins of haematophagy within Brachycera. Furthermore, evolutionary analyses of haematophagous brachycerans and their potential hosts indicate that Mesozoic blood-feeding lineages likely exploited different host resources from their modern descendants, suggesting a major host transition during the replacement of Mesozoic and modern ecosystems.