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

## TL;DR

This paper describes the structure and function of the venom system in the predatory stink bug Arma custos using microscopy techniques.

## Contribution

The study provides a detailed ultrastructural characterization of the venom apparatus in Arma custos, revealing differences in glandular cell structures and functions.

## Key findings

- The venom apparatus of Arma custos consists of glands and ducts with distinct ultrastructural features.
- AMG, PMG, and AG are responsible for venom synthesis and storage, while VD and AMD are involved in venom transport.
- Differences in organelle composition among glands explain variations in venom composition and function.

## Abstract

In this work, we characterized the morphology and ultrastructure of the venom apparatus in Arma custos using light and electron microscopy. Our study found that the venom apparatus of A. custos consists of glands and ducts. The glandular cells in the AMG, PMG, and AG display typical secretory cell architecture and function to secrete venom, whereas the duct cells in the VD and AMD lack secretory structures and are primarily involved in venom transport. However, the differences in organelle type, shape, size, and electron density among the three glandular cells are the real reason why the venom composition and function of the AMG, PMG, and AG differ.

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.