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SUMMARY Abdominal appendages in male sepsid flies are a complex novel structure of unknown developmental and evolutionary origin. Although these abdominal appendages superficially resemble serially homologous insect appendages, they do not develop from imaginal discs like other dipteran appendages. Cauterization of the genital disc and ventral abdominal histoblasts in Themira biloba (Sepsidae, Diptera) revealed that these abdominal appendages develop from the ventral histoblast nests of the fourth abdominal segment. Cell counts of the histoblasts in males and females revealed that the ventral histoblast nests on the fourth abdominal segment in males were significantly larger than other histoblast nests, indicating that the specification of that segment as the location of the abdominal appendages occurs before the last larval instar. The recruitment of histoblasts to produce appendages has not been documented before, and implies a developmental and evolutionary potential for histoblasts that was previously unknown.