Mouthpart evolution in adults of the basal, «symphytan», hymenopteran lineages

Abstract

We review feeding biology and mouthpart structure generally among adults of the basal hymenopteran, or «symphytan», lineages (sawflies, woodwasps, horntails and their relatives). These insects feed on a wide range of materials: floral and extrafloral nectar, pollen, plant (floral and leaf) tissues, plant (angiosperm) sap, the juice of ripe fruit, the spermatial fluid of rust fungi, sternorrhynchan bug honeydew, and insect tissues. Adults show feeding-related mouthpart specialization either for consuming pollen (the Xyelidae only) or for consuming «concealed» floral nectar (several families). Seven functional types of elongated proboscis or «concealed-nectar extraction apparatus (CNEA)» have previously been recognized among Hymenoptera. We identify an additional type, which appears to be unique among Hymenoptera and has probably evolved directly from unspecialized mouthparts (labiomaxillary complex). In total, three types of CNEA are known to occur in «Symphyta». Type 1 occurs in Pamphiliidae, Megalodontesidae, Argidae, Pergidae, Tenthredinidae, Cimbicidae and Cephidae. Type 5 occurs in Pergidae (in two unidentified species of Euryinae). Type 8 occurs in Tenthredinidae (in the genusNipponorhynchus Takeuchi). CNEA of some type or other has arisen at least twice within the family Tenthredinidae and at least twice within the pergid subfamily Euryinae. Evolutionary parallelism in CNEA structure has occurred between the basal, «symphytan», hymenopteran lineages and the Apocrita, a phenomenon hitherto not mentioned in the literature. Within the «Symphyta», possession of Type 1 CNEA appears to be a ground plan feature of each of the following taxa: the pergid genus Eurys Newman, the megalodontesid genus Megalodontes Latreille (the only extant representative of the Megalodontesidae) and the tenthredinid genus Cuneala Zirngiebl, while possession of Type 8 appears to be a ground plan feature of the tenthredinid genus Nipponorhynchus Takeuchi. However, in general among «Symphyta», possession of CNEA is characteristic of only small and taxonomically subordinate groups, suggesting that CNEA has evolved independently many times within the basal hymenopteran lineages rather than being inherited from a common ancestor early in the evolutionary history of the Hymenoptera. In other words, ecological expediency rather than phylogenetic history mainly accounts for its distribution pattern within the basal lineages. The results of a morphological survey of «Symphyta» indicate that the habit of exploiting «concealed» nectar, by means of CNEA, is fairly widespread (CNEA is present in at least half of the extant families), contradicting the view held in the literature on flower visiting that it is altogether absent. We discuss our findings in relation to the occurrence and evolution of «suctorial» mouthparts in other Hymenoptera and in non-hymenopteran insects: Mecopterida, Coleoptera and Hemiptera.