The 'lesser dung fly', Archicollinella penteseta (Richards), previously known only from the Scilly Isles of Great Britain, was discovered on three of Namibia's off-shore guano islands, Ichaboe, Mercury and Possession islands, in 2003, from where the species was presumably introduced to Great Britain. Adults were found to feed on the guano of seabirds, in the nests of Jackass penguins and on carcasses and scats of Cape fur seals. The third instar larva and puparium are described, this being the first description of immature stages for the genus. Species dominance of guano-producing seabirds and Cape fur seals has altered on these islands during the last 180 years or so, since the onset of human exploitation. Changing larval food resources are discussed by drawing on historical accounts of the islands. Introduction of the species to Great Britain could have resulted directly from the guano trade between Namibia and Great Britain in the 1840s or later, probably as a result of mass fly emergence, dumped ballast from sailing ships or shipwrecks off the coast of the Scilly Isles. Keywords: Africa, Namibia, guano islands, Scilly Isles, immature stages, guano trade, species introduction.