Ultramorphology of the proventriculus in Panorpidae and Bittacidae (Mecoptera)

The proventriculus is a specialized part of the foregut in the alimentary canal in some insects, varying greatly in its structure across different insect groups. The ultramorphology of the proventriculus in Mecoptera was investigated in eight species of Panorpidae and one of the Bittacidae using light microscopy and scanning electron microscopy. The proventriculus consists of the outer circular muscles, the inner longitudinal muscles, an epithelium, and a sclerotized intima with dense acanthae. The proventricular acanthae could be divided into three categories: the anterior acanthae (AA), the main acanthae (MA), and the posterior acanthae (PA). In Panorpidae the AA are rounded in cross section and could be subdivided into two subtypes, the long AA with a polytomy apiculus and the short AA with a V-shaped or spine-like apiculus. The MA, covering the majority of the proventricular inner surface, are dense, uniform, and hollow rectangular structures in cross section with forked apiculi. Each hollow base of the MA was first found to be inserted with a cone-shaped process of the corresponding epithelial cell. The PA are very thin, with a spine-like apiculus. In Bittacidae the proventriculus is specialized by a ring of cuticular thickening within its anterior orifice, and the AA are shaped like a ring of thin spines. Six cardiac valves were observed in Panorpidae and ten in Bittacidae. The functions of the various types of acanthae in digestion and extra-oral digestion and their significance in insect systematics are briefly discussed.