Morphological and Ultrastructural Characterization of the Venom Apparatus of the Predatory Stink Bug, Arma custos

The predatory bug Arma custos (Hemiptera: Pentatomidae) is a natural enemy insect capable of preying on over 40 types of agricultural and forestry pests. Here, we describe the characteristics of the morphology and ultrastructure of its venom apparatus visualized using light and electron microscopy. Light microscopy revealed that the venom apparatus of A. custos consists of a pair of main gland and tubular accessory gland. The main gland consist of two lobes, the anterior main gland (AMG) and posterior main gland (PMG). Between the two lobes of the main gland, there is a strong constriction, characterizing a hilum (Hi) where two separate ducts, the venom duct of the main gland (VD) and the duct connecting the accessory gland to the main gland (AMD), are inserted. The VD extends toward the head and connects to the venom pump (VP), while the AMD extends toward the thorax and connects to the accessory gland (AG). Ultrastructural examination of the venom glands reveals that the AMG and PMG consist of a layer of cubic or spherical glandular cells forming a large circular lumen, while the AG exhibits two narrow lumens. The secretory cytoplasm of AMG, PMG, and AG contains a well-developed rough endoplasmic reticulum, along with mitochondria, nuclei, secretory vesicles, autophagosomes, and secretory granules. However, significant differences exist in the ultrastructural characteristics among the three glands. Unlike glandular secretory cells in the venom glands, the ultrastructure of VD, and AMD reveals only well-developed nuclei, mitochondria, and elaborate plasma membrane folds. These results indicate that venom proteins are synthesized and stored by the AMG, PMG, and AG, while the VD and AMD ducts are responsible for transporting the venom.