The evolutionary pattern of host use in the Bombyliidae (Diptera): a diverse family of parasitoid flies

The larval host associations and mode of parasitism of Bombyliidae (bee flies) are summarized and analysed within an evolutionary framework. We discuss difficulties in extracting information from the (almost 1000) host records, noting that most observations are made by chance, often imprecise, and distributed unevenly across bombyliid taxa. These caveats aside, the vast majority of Bombyliidae are ectoparasitoid; endoparasitoids are known in only three tribes belonging to two distandy related subfamilies, the Toxophorinae (Gerontini and Systropodini) and Anthracinae (Villini). The recorded host range of Bombyliidae spans seven insect Orders and the Araneae; almost half of all records are from bees and wasps (Hymenoptera). No Bombyliidae have evolved structures to inject eggs directly into the host as is the case in many hymenopterous parasitoids. Bombyliid larvae usually exhibit hyper-metamorphosis, and contact their host while it is in the larval stage. Bee fly larvae consume the host when it is in a quiescent stage such as the mature larva, prepupa or pupa. Records of hyperparasitism by Bombyliidae are uncommon, most occurring in genera of the Anthracinae. All bombyliids recorded as hyperparasitoids do not appear to have evolved in any close association with the primary host, and are best termed pseudohyperparasitoids. Both facultative and obligate pseudohyperparasitism has been recorded. Bombyliidae are difficult to place in the koinobiont/idiobiont classification used most extensively in Hymenoptera but they share most features of koinobionts. Provision-directed cleptoparasitism has been recorded in one genus. We propose an evolutionary scenario progressing from an ancestral substrate-zone free-living predator to ectoparasitoid, a broadening of host range to include the consumption of orthopteran egg pods, and the independent development of endoparasitism in two lineages. The suggestion that host range narrows as the intimacy of encounter between female parasitoid and host increases is supported in the Bombyliidae. Amongst the basal subfamilies which are parasitoids, host range is narrowest in the Toxophorinae. In the more derived subfamilies host range is generally broad, and is dictated by ecological context rather than host phylogeny. Bombyliidae violate the prediction of increased species richness in parasitic groups, and the broad host range of most bee flies is a possible explanation.