IN medical and veterinary entomology specific and generic determination of flies is dependent upon the structure of the genital apparatus. There is no recognized standard for naming the modified terminal segments and appendages (terminalia) of the abdomen in the Cyclorrhapha. Morphologists have found no conclusive evidence to settle the nature of, nor have taxonomists managed to agree upon the naming of, these sclerites and other parts concerned. Nevertheless there are four papers that have, between them, covered the groundwork required to solve this problem. Together they make an entirely new approach and I have co-ordinated the data. The following conclusions that are drawn from this study are surprising, but logical.