Scanning the Hyocephalidae: details of their external morphology with respect to phylogenetic relationships within Eutrichophora (Hemiptera: Heteroptera)

Hyocephalidae (Hemiptera: Heteroptera: Pentatomomorpha) is a small family of true bugs containing two genera and three species endemic to Australia. For the first time, we examine here selected structures of their external morphology using scanning electron microscopy – the external structures associated with the metathoracic and dorsoabdominal scent glands, the strainer organ on the sternite III, as well as the trichobothrial pattern and ultrastructure. The following character states are confirmed (i, ii, iii) or newly recognised (iv, v) as autapomorphies of Hyocephalidae: i) apically projected scimitar-shaped peritreme of the metathoracic scent glands; i) presence of a strainer organ on sternite III in both sexes; iii) trichobothria on sternite V placed immediately ventrad of spiracle, iv) trichobothria on each of abdominal segments III–VII all grouped together within a common trichome, and v) presence of crocus-like structures within the trichome. Of particular interest is the presence of a shallow open bothrium (type B) surrounded by a trichome in Hyocephalidae, a probably derived character state shared with Pyrrhocoroidea and most Lygaeoidea, while Stenocephalidae (previously considered to be closely related to Hyocephalidae and used here for comparison) and the remaining Coreoidea possess a recessed bothrium of type A2, lacking the trichome. The morphology of trichobothria in Hyocephalidae may thus suggest either their closer relationship to Lygaeoidea + Pyrrhocoroidea than to Coreoidea, or a parallel evolution of the open bothrium with trichome in Eutrichophora. We highlight the importance of Hyocephalidae for a better understanding of the phylogeny of Eutrichophora and the urgent need to obtain phylogenomic data for future research. A taxonomic catalogue of Hyocephalidae is supplemented. The neotype designation for Hyocephalus aprugnus Bergroth, 1906 made by ？tys (1964) and supported by Grant & ？tys (1970) is found invalid