Functional and evolutionary aspects of mouthpart structure in parasitoid wasps

Abstract

This paper considers specializations in mouthpart structure among parasitoid wasps. I commence with a brief survey of mouthpart specializations (mostly involving the mandibles only) that serve functions other than feeding: facilitating emergence of the parasitoid adult from its place of pupation; facilitating grasping of the partner in mating (phoretic copulation); facilitating excavation and/or protecting vulnerable mouthpart components during host searching; facilitating handling of the host; and facilitating nest excavation and construction. I then consider in detail mouthpart specializations for feeding (mostly involving the labiomaxillary complex), and place them in an evolutionary context. Whereas the digitate labrum of Perilampidae and Eucharitidae and the stoutly setose labrum of chrysolampine Pteromalidae are purported to be devices for filtering pollen grains from nectar, I conclude that they are not a feeding-related specialization whatsoever. I recognize seven functional types of mouthpart specialization relating to the extraction of floral nectar from long, narrow, tubular corollas (‘concealed nectar extraction apparatus’ [CNEA]) (Braconidae, Ichneumonidae, Leucospidae, Chrysididae), and postulate a transformation series for them. An eighth type of CNEA possibly occurs in Scoliidae. No such specializations occur in either Orussoidea, Trigonalyoidea, Megalyroidea, parasitoid Evanioidea, Stephanoidea, Cynipoidea, Proctotrupoidea, Ceraphonoidea or parasitoid Aculeata other than Chrysididae and Pompilidae. From examination of published cladograms I conclude that the evolution of CNEA has occurred several times independently within the parasitoid Hymenoptera. So far as Ichneumonoidea are concerned, possession of CNEA is an autapomorphy for taxa at least below subfamily level. Possession of CNEA appears to be a synapomorphy for the family Leucospidae (Chalcidoidea) as a whole, and the same applies to the subfamily Parnopinae (Chrysididae). Hitherto not noted in the literature is the strong degree of evolutionary parallelism, with respect to CNEAs, between the Ichneumonoidea (Ichneumonidae, Braconidae) and the Aculeata (Chrysididae, Pompilidae, Vespidae [Vespinae, Masarinae, Eumeminae], Apidae, non-parasitoid Sphecidae). Also, Apocrita and Symphyta have two types of CNEA in common. Functional comparisons are made between the CNEAs of the parasitoids and those of other Hymenoptera. Compared to parasitoid flies, very few parasitoid wasps possess CNEA. Mouthpart specialization for conveying a nuptial gift of nectar or honeydew to the female occurs in the males of thynnine Tiphidae, while in the females of regurgitation feeders there is a trend towards reduction in labiomaxillary components, constituting a specialization for receiving and processing the gift. Mouthpart specialization for pollen feeding occurs in Mutillidae and Scoliidae. No mouthpart specialization for host feeding occurs in any parasitoid wasps, in contrast to parasitoid flies, despite host feeding being more common among the former. Sexual dimorphism in feeding-related mouthpart specializations is rare among parasitoid wasps; where it occurs, both sexes share the same type of CNEA, and the dimorphism is attributable to allometry.