Abdominal appendages in male sepsid flies are a complex novel structure of unknown developmental and evolutionary origin. Sepsid abdominal appendages superficially resemble the serially homologous insect appendages in that they are jointed and have a short segment that can be rotated. However, as appendages have been absent from the insect abdomen for many millions of years, these abdominal appendages are novel structures not strictly homologous to other appendages. The development of the abdominal appendages in Themira biloba (Sepsidae, Diptera) was investigated to determine whether (A) the abdominal appendages develop from imaginal discs like other dipteran appendages and (B) whether the same patterning genes are expressed in the sepsid abdominal appendages as in other insect appendages. Cauterization of the genital disc and ventral abdominal histoblasts in T. biloba revealed that these abdominal appendages develop from the ventral histoblasts of the fourth abdominal segment, and not an imaginal disc. Cell counts of the histoblasts in males and females revealed that the ventral histoblasts on the fourth abdominal segment in males were significantly larger than other histoblasts, indicating that the specification of the fourth segment as the location of the abdominal appendages occurs prior to 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. Immunohistochemistry was used to determine the expression patterns of the genes extradenticle, Distal-less, engrailed and Notch in the developing appendage. The expression of extradenticle, engrailed and Notch was consistent with the conclusion that the appendage patterning pathway has been co-opted to pattern the developing abdominal appendages. However, Distal-less was not expressed in the developing appendages indicating that sepsid abdominal appendages are patterned as only a proximal part of an appendage.