Among some of the most unusual traits of the gasteruptiid wasps is their unique hovering flight and the expansion of their hind tibiae. Tibial expansions in female parasitoid hymenopterans often involve an enlarged sensory structure for vibration detection, the subgenual organ, thus enabling refined substrate-borne detection of concealed hosts. In the present paper, we utilize a combination of microscopy, chemical analysis, gene expression, and behavior to explore the function of the expanded hind tibia of gasteruptiid wasps. We find that the expanded hind tibia of gasteruptiids is filled largely with fat body, a cell cluster thought not typically to occur in insect legs. Based on its position relative to flexible tibial structures and the subgenual organ, the gasteruptiid tibial fat body may function to amplify vibrational signals. We show the tibial fat body to be filled with both trophocytes and, depending on gasteruptiid lineage, oenocytes. Transcriptomics reveals enrichment for fat-related genes more than expected in an insect leg and raises several additional possibilities for functions of fat in the leg including detoxification. Finally, our flight observations support the hypothesis that this structure may also function in balance and maneuvering in the unusual leg-dangling flight behavior of this wasp.