Melophorus is an exceptionally diverse ant genus from arid Australia that has received little taxonomic attention, such that just a fraction of its remarkable number of species is described. The Commonwealth Scientific and Industrial Research Organization's Tropical Ecosystems Research Centre (TERC) in Darwin holds by far the most extensive collection of Melophorus, and as of September 2016 this comprised > 850 sorted morphospecies. However, the reliability of such morphospecies is open to question because species delimitation is extremely challenging due to highly generalized morphology and worker polymorphism. Here we use CO1 barcoding of 401 Melophorus specimens from 188 morphospecies in the TERC collection to determine the reliability of morphologically-based species delimitations as a basis for assessing true diversity within the genus. Our CO1 data confirm the extremely challenging nature of morphologically-based species delimitation within Melophorus, and suggest substantially higher diversity than that indicated by morphospecies. We found many cases where combinations of high (>10%) CO1 divergence, polyphyly, sympatric association, and morphological differentiation indicated that single morphospecies represented multiple lineages. Overall, our analysis indicates that the 188 morphospecies barcoded represent at least 225 independent CO1 lineages. We discuss these results in terms of both their limitations and implications for estimating the total number of species in this exceptionally diverse, arid-adapted ant genus.