Comparative morphology of the labium and external efferent system in the Janeirona clade (Hemiptera: Pentatomidae: Ochlerini)

Abstract The labium and the external scent efferent system (ESES) are important characters in Heteroptera systematics. This study provides a comparative morphological analysis of these structures in genera within the Janeirona clade of Ochlerini (Pentatomidae: Discocephalinae), characterized by a subdivision in labiomere II. Using light and scanning electron microscopy (SEM), 15 species were investigated, including five outgroups. Results confirm that the subdivision in labiomere II is a functional intercalary segment, exhibiting articulation and intrinsic musculature. Two distinct morphological types of intercalary segments were identified: a dilated type with a wide range of motion, and a flat, short and less mobile type. Regarding the ESES, the clade shares an elliptical, ventrolaterally oriented ostiole and a spout-shaped peritreme. Detailed SEM imaging further revealed specialized scale-like microsculptures on the peritremal surface and complex spiracular filter systems. These findings provide the first high-resolution characterization of the labial intercalary segment and significantly expand the morphological data available for the tribe Ochlerini. This study offers relevant insights for future taxonomic and phylogenetic research aimed at refining the classification of Pentatomidae. Introduction The suborder Heteroptera is distinguished from other insect groups by the combination of the external scent efferent system (ESES), and the labium forming an elongated, segmented tube (Grazia et al. 2015; Schuh and Weirauch 2020). The labium is highly conserved within the Heteroptera infraorders, varying mostly in the number of segments (labiomeres); most families have four labiomeres, but three are seen in Reduviidae (Weirauch 2008). In contrast, the ESES morphology is highly variable between and within groups of Heteroptera, being useful for delimiting characters for phylogenetic inference and for taxonomic identification in different levels (Kment and Vilímová 2010; Barão et al. 2017; Weiler et al. 2017; Schuh and Weirauch 2020; Genevcius et al. 2021). The ESES plays an important role in the chemical communication of true bugs, being indispensable for their defense against predators. The system is composed by paired metathoracic scent glands, in the adults, which release repellent secretions (allomones) through specialized cuticular structures such as the ostiole, peritreme and the evaporatorium. The ostiole is the opening for the draining of the secretions of metathoracic scent glands; the peritreme consists of a cuticular expansion or excavation adjacent to the ostiole of the thoracic gland, responsible for conducting secretions to the evaporatorium, which is composed by cuticular mycoid microsculpture present on the meso- and metapleural surface, responsible for spreading and containing the secretion in a delimited pleural area (protecting the insect of the secretion toxicity) and helping volatilization (Staddon 1979, 1986; Aldrich 1988; Kment et al. 2012; Weber et al. 2017). Although studies of the ESES microsculpture under scanning electron microscopy (SEM) dates from the 1960s (for a review see Kment and Vilímová 2010), it was only recently that SEM techniques began to be employed in systematized comparative studies of groups within Pentatomoidea and Pentatomidae. In the seminal work by Kment and Vilímová (2010) the ESES in Pentatomoidea was characterized in detail, and standardized terminology was proposed. They distinguished six types of peritreme in the superfamily: auricle, spout, groove, ruga, disc and lobes; and highlighted the different parts composing the mushroom bodies of the evaporatorium: cap, stem, bridges, alveoli and trabeculae. This work served as a basis for studies in Pentatomidae (Carpocorini) (Barão et al. 2017), Aphylidae (Kment et al. 2012) and Scutelleridae (Weiler et al. 2017), however, the ESES fine morphology of many other groups remain unstudied. The labium has been considered of taxonomic value in Pentatomidae, frequently used in keys and diagnoses to genera and species (e.g., Rolston and McDonald 1979; Grazia et al. 2015; Rider et al. 2018; Schuh and Weirauch 2020), being highly variable in length in most subfamilies. However, in the Janeirona Distant, 1911 clade (sensu Sampaio and Campos 2023) of Discocephalinae, the proximal portion of labiomere II presents what previous authors have named as “pseudo-segment" (Kormilev 1949, 1955, 1956), “pseudo-base” (Ruckes 1958), or "intercalary segment” (Rolston 1992), what suggest different interpretations of the morphology and functionality of the structure. Discocephalinae is one of the 10 subfamilies of Pentatomidae, consisting of a Neotropical group with more than 300 species and 77 genera divided in two tribes, Discocephalini and Ochlerini (Grazia et al. 2015; Rider et al., 2018); in the latter belongs the Janeirona clade composed by Adoxoplatys Breddin, 1903, Neoadoxoplatys Kormilev, 1956, and Paropsocolis Sampaio and Campos, 2023 (Sampaio and Campos 2023). In this study we investigate the morphology of the labium and ESES of the genera in the Janeirona clade, following a comparative approach with species of other pentatomid genera.