Function of the expanded hind leg in gasteruptiid wasps

Among some of the most unusual traits of the carrot wasps (Hymenoptera: Gasteruptiidae) is their unique hovering flight and the expansion of their hind tibiae. Tibial expansions in parasitoid hymenopterans often involve an enlarged organ for vibration detection, the chordotonal organ, thus enabling refined substrate-borne detection of concealed hosts. In gasteruptiids, the presence of this structure atypically in both males and females leaves open the question as to whether the enlarged tibia may serve a similar purpose in detection of bee and wasps hosts for these predator-inquilines. 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 this expanded structure is filled largely with fat body, a substance thought not typically to occur in insect legs, and observe a broader presence of fat in the legs of other Hymenoptera. The gasteruptiid fat body is associated with a chordotonal organ, where it may function to amplify vibrational signals. Other aspects of its structure, such as interior tissue connections and flexible cuticle sensitive to movement, provide additional evidence for its role in mechanoreception. We reveal 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. 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.