The significance of the Gariep Centre of Endemicity is noted; this region includes the Richtersveld south of the Orange River in north-western South Africa and the adjacent parts of southern Namibia, where a rich succulent flora exists. A small but distinctive fauna of vennileonid flies is endemic in the Gariep. The new species and member of the Gariep fauna, Leptynoma (Leptynoma) kirkspriggsi sp. n., is descriptionbed from southern Namibia. New records are given of vermileonid species in western South Africa. Cladistic analysis of Leptynoma Westwood s. str. reveals a basal dichotomy, with L. sericea Westwood as sister-species of the other four which form a stepwise clade, with L. appendiculata (Bezzi) basal to L. hessei (Stuckenberg), L. namaquaensis (Stuckenberg) and L. kirkspriggsi sp. n. The two older species (sericea and appendiculata) inhabit the Fynbos Biome, whereas the three younger species occur in the Succulent Karoo Biome. The distribution of Leptynoma s. str. is mapped and its biogeography discussed. Cladogenesis accords with botanical evidence that Fynbos is the older biome and Succulent Karoo a younger biome with many fynbos affinities. It is proposed that the constraints limiting the distribution of these vermileonids relate to selective anthophily and nectarivory, for which the flies show strong morphological adaptations. Studies by other workers are cited, of guild associations between fynbos plant taxa and large species of Nemestrinidae and Tabanidae with an extremely long proboscis. Several striking examples are cited of mouthpart elongation for nectarivory in fynbos representatives of other, unrelated families of Diptera. Less extreme cases of labial elongation often occur among fynbos flies. Cladogenesis in the Succulent Karoo would have been coeval with the evolution of that biome in response to extreme summer aridity which began to develop in the Pliocene. Keywords: Diptera, fynbos biomes, Leptynoma Westwood, succulent karoo biomes, Vermileonidae, Wormlion.