Morphology of the larval midgut of the longhorn beetle Rhytidodera bowringii White, 1853 (Coleoptera: Cerambycidae: Cerambycinae).

The midgut of insects originates from the endoderm. It is located in the central part of the digestive tract and serves as the primary site for chemical digestion and nutrient absorption. The larvae of Cerambycidae are the most destructive life stage. However, the ultrastructure of the larval midgut has been reported in only a few cerambycid groups. This study employs light, scanning electron, and transmission electron microscopy to observe the midgut ultrastructure of the Rhytidodera bowringii White, 1853 larvae. From outside to inside, the midgut of R. bowringii consists of a muscle layer, a basal membrane, an epithelium, and a lumen. The external muscles of the midgut are arranged in an outer longitudinal muscle and inner circular muscle. The epithelial tissue mainly comprises two types of cells: digestive cells and regenerative cells. The arrangement of regenerative cells in the midgut forms nidi, where multiple cells cluster together. The nucleus occupies most of the cytoplasm, which contains only a small number of organelles. The basal plasma membrane of the midgut epithelial cells is conspicuously infolded in R. bowringii, and around the nucleus, with a large amount of rough endoplasmic reticulum. At the apical regions of these digestive cells, numerous tightly arranged microvilli and mitochondria can be observed and many vesicles are localized near the lumen. This indicates that the digestive cells in the midgut have a strong secretory activity of digestive enzymes and other proteins, which may facilitate the larvae of the cerambycid in digesting recalcitrant plant tissues. It is the first time that the ultrastructure of the midgut of the R. bowringii larvae has been studied. The results can provide foundational insights into the ultrastructural organization of the Cerambycidae larval digestive system and the toxic mechanisms underlying prevention strategies for this pest.