Untapped potential of European natural history collections of megadiverse Diptera

European natural history collections (NHCs) preserve countless specimens across multicellular taxa, representing critical references of biodiversity knowledge for research and conservation. Yet comprehensive surveys of NHCs and their research contributions are lacking for exceptionally species-rich and ecologically diverse taxa, leaving unclear to which extent NHCs provide a realistic representation of biodiversity. We surveyed 16 major European NHCs to examine their role in documenting true flies (Diptera), a large insect order of economic, public health, and ecosystem importance. We estimate that European NHCs house between 26 million and 35 million dipteran specimens, ∼85% preserved in ethanol, with 20,000 to 1 million identified to species level per institution. Dark taxa account for more than 10% of holdings in several families but remain incompletely represented, because taxonomic impediments and funding constraints prevent NHCs from addressing this gap. We propose critical priorities to enable European NHCs to operate at full capacity to advance our knowledge on Diptera.