Bees provide a good example of the mutualism that exists between hymenopterans and angiosperms. Most pollination is by hymenopterans, such as bees. This group includes the bees and ants and all of the eusocial Hymenopterans. Swiftlets are insectivores; hymenopterans and dipterans being the most abundant prey. Unlike most other Hymenopterans, cuckoo wasps cannot sting. Thus, the Halictinae are believed to model the primitive eusociality of advanced eusocial hymenopterans. Some taxa are have been suggested mimics of various hymenopterans, including bees and ants. Meropids are all visually similar and have a diet specialized in Hymenopterans. The species feeds on beetles, caddisflies, flies, hymenopterans, moths, and true bugs. As with all hymenopterans, common sawflies undergo complete metamorphosis. This fact has been used to explain the evolution of eusociality in many hymenopterans. Parasitica also contains groups of phytophagous hymenopterans such as the Cynipoidea (gall wasps). The pupae of social hymenopterans are protected by adult members of the hive. The bats are insectivorous, with flies, beetles, and hymenopterans forming the bulk of their diet. Chrysidid faces, or the front of the head, distinguish them from other Hymenopterans.