Comparative Morphology of the Mouthparts in Three Predatory Stink Bugs (Heteroptera: Asopinae) Reveals Feeding Specialization of Stylets and Sensilla

SIMPLE SUMMARY: As the only predatory group in the family Pentatomidae, Asopinae are a diverse group of specialized soft-bodied insect predators, which have the potential for use in controlling pests of orchards, forests, and field crops. However, the feeding behavior remains poorly known for Asopinae, especially how the mouthpart structures relate to various functions in feeding. The fine structure of the mouthparts, including distribution and abundance of receptor sensilla, was observed in three species of Asopinae using scanning electron microscopy and structural details are described for the first time. The morphology of the mouthparts is similar to those of other Heteroptera. The four-segmented labium and labrum in all studied species have fourteen types of sensilla. Unlike phytophagous pentatomids, two types of olfactory sensilla with nanopores (St1, Sb3) were observed in these three predatory insects, which probably function to locate prey by smell. In predatory stink bugs, each mandibular stylet tip has five irregular teeth and three long, pointed hooks; the apices of the right maxilla have small teeth and few short barbs along the edge of the food canal. The structure and function of the mouthparts are adapted for predatory feeding in the three studied species. The detailed structure of the predatory pentatomid’s mouthparts shown in this study can provide more data for the morphological differentiation of the mouthparts in the future. ABSTRACT: Mouthpart structures were observed in three species of Asopinae using scanning electron microscopy to investigate their morphological disparity. The examined species attack mainly slow-moving, soft-bodied insects, primarily larval forms of the Lepidoptera, and are the natural enemies of many pests. This is the first detailed description of their external mouthparts. The triangular and elongated labrum and four-segmented tube-like labium are longer in Picromerus species (Picromerus bidens (Linnaeus, 1758) and Picromerus lewisi Scott, 1874 than in Cazira bhoutanica Schouteden, 1907. The labrum of P. lewisi and C. bhoutanica appear to be equipped with olfactory sensilla basiconica Sb3, a special type of sensilla with nanopores. The labium surface in all studied species bears 14 types of sensilla (St1–St4, Sb1–7, Sst, Sca1–2). A new characteristic of sensilla trichodea is represented in sensillum St1; in both Picromerus species, it is classified as an olfactory sensillum with nanopores. The tripartite apex of the labium consists of two lateral lobes and a central membranous lobe having microtrichial extensions. Each lobe has one sensory field, including sensilla basiconica (Sb7), sensilla styloconica (Sst), and sensilla trichodea (St4). In the three studied predatory stink bugs, each mandibular stylet tip has five irregular teeth and three long, pointed hooks. The two opposing maxillae, which are held together by a tongue-and-groove system, form a food canal and a salivary canal. The apices of the right maxilla have small teeth and few short barbs along the edge of the food canal. In P. bidens and P. lewisi, there are 5 teeth, while in C. bhoutanica there are 2. Based on structural differences, we inferred that the hook-shaped mandibular teeth, right maxilla with small teeth, and few short barbs along edge of the food canal are more adapted for a predatory lifestyle. Predatory stink bugs use sharp recurved hooks and irregular teeth penetrating, tearing, or filing devices that aid in the mechanical disruption of host tissue. Stiff bristles in the food canal may indicate their possible adaptation to feeding on insect larvae. The evolution of mouthpart morphology and the putative functional significance of sensilla are discussed, providing insight into the sensory mechanism.