Ultrastructural characterization of the midgut in the short-faced scorpionfly Panorpodes with mysterious feeding habit (Mecoptera: Panorpodidae)

The midgut of insects as the function core of the alimentary canal usually exhibits diverse morphology depending on their different food types. Mecoptera is an ideal model for comparative studies of the midgut because this small primitive holometabolous group possesses diverse feeding habits. In this study, the midgut of Panorpodes kuandianensis adult was investigated using light and transmission electron microscopy. The result shows that the midgut of P. kuandianensis is a simple straight tube. The single-layered epithelium is composed of regenerative cells and digestive cells. The regenerative cells are grouped in nidi along the epithelium base with high electron-dense cytoplasm and limited organelles. The regenerative cells undergoing differentiation elongate toward the lumen and accumulate numerous mitochondria in the apical region, gradually forming new digestive cells. The columnar digestive cells are the predominant cell type, with distinct regionalization in organelles distribution. Based on the ultrastructural characteristics of the different cells, the functions of the midgut epithelium of P. kuandianensis and the possible feeding habit are discussed. The results could provide a foundation for further understanding the digestive physiology and phylogeny of Panorpodes.