Evolutionary novel structures are structures with no known homology. Novel structures often provide a novel function to the organism. Some species of sepsid flies (family Sepsidae) have evolved novel abdominal appendages on the fourth segment in males and are thought to be used during mating to stimulate the female abdomen. However, the necessity of these appendages for the sepsid flies has not been determined, and a little is known about the genetic basis of these structures. Controlled laboratory experiments were conducted to identify the necessity of these appendages in sepsid Themira biloba. Surgical manipulation of the bristle length of the appendages prevented successful mating, and this was observed when the longest bristles were manipulated; reducing the length of short bristles had no effect. These results indicate that bristle length is important for mating success and that the longer bristles are critical. The genetic regulation of the initial specification of the abdominal appendages in sepsids was also investigated. In situ hybridization was done to examine the expression of abdominal-A and Abdominal-B genes in sepsid T. biloba, to determine whether their expression has deviated in forming the appendages. However, the expression domains of these genes are conserved in T. biloba, and are not different from those observed in D. melanogaster. Overall my research emphasizes the necessity of the abdominal appendages, and provides insight into the genetic basis of these novel structures.