Fergusoninidae is a fascinating but little-known family of Australian flies. In a unique mutualism with a nematode, these flies induce galls on plants in the family Myrtaceae, most commonly on the eucalypts, and develop in darkness surrounded by worms. Despite the abundance and diversity of Fergusoninidae and its tight association with Australia’s most iconic flora, the host specificity and coevolutionary relationships of Fergusonina with its eucalypt hosts have not previously been examined in depth.
I used a molecular phylogenetic approach to examine the evolutionary relationships of over seventy putative Fergusonina species from four genera of plant hosts. Over the course of the study, I discovered nearly a hundred previously unrecorded host plant/gall site associations, indicating that the potential number of species in this family is huge. The definable morphological and ecological differences among the lineages of Fergusonina argue for a revision of the genus along these lines, but the type species, described from a single poorly preserved specimen, cannot be identified, posing an interesting taxonomic puzzle.