Comparative Ultrastructure Of The Larval Midgut And Malpighian Tubules Of Panorpidae And Bittacidae(Mecoptera)

Midgut is the significant part of digestive system and plays an important role in digesting food,absorbing nutrition,and regulating ion balance.Malpighian tubules are the main excretory and osmoregulatory organs of insects and are responsible for releasing primary urine,reabsorbing solutes,and maintaining osmotic homeostasis.The midgut and Malpighian tubule ultrastructure have been investigated in many insect orders,such as Diptera and Hemiptera.However,the ultrastructure of midgut and Malpighian tubules have only been briefly described in Mecoptera.Mecoptera is one of the primitive lineages of holometabolous insects,with Panorpidae and Bittacidae as the two largest families.The larvae differ in external morphology and habits between the two families.The larvae of Bittacidae also have an intriguing soil-spraying habit.The larvae spray their body surface with soil that has been swallowed and excreted from the anus after passing through the digestive tract.The fine structure of larval midguts and Malpighian tubules,however,remains largely unknown in the larvae of Panorpidae and Bittacidae to dateIn this thesis the ultrastructure of the larval midguts and Malpighian tubules of the hangingfly Bittacidae and the scorpionfly Panorpidae were compared using light,scanning,and transmission electron microscopy.The midguts of both families are simple tubes and without segments and diverticula.The midgut consists of a muscular layer,a single layer of epithelium encircled by a basal lamina,and a central lumen.The epithelium is commonly comprised of digestive and regenerative cells.The basal plasma membrane of epithelial cells exhibits infolding in Bittacidae,but is closely apposed to its basal lamina and infolded in Panorpidae.Lymph spaces are present between adjacent epithelial cells in Bittacidae,but are absent in Panorpidae.The regenerative cells are scattered among the digestive cells in Bittacidae,but are aggregated and scatted in Panorpidae.The longitudinal muscle bands are compact in Bittacidae,but are sparse in Panorpidae.The larvae of both families have six elongate Malpighian tubules at the junction of the midgut and hindgut.The tubule comprises a basal lamina,a single-layered epithelium,and a central lumen.The basal plasma membranes of the epithelial cells are conspicuously infolded and generate a labyrinth.The epithelium consists of two types of cells: large principal cells and small stellate cells.Spherites are abundant in principal cells in Bittacidae,but are absent in Panorpidae.Microvilli of principal cells are swollen and contain mitochondria in Bittacidae but are slender and short in Panorpidae.The differences of basal plasma membrane and microvilli are likely related to the larval physiological period in the two families.Spherites are only abundant in the larval digestive cells and principal cells,and the compact longitudinal muscle bands of midgut are likely associated with their soil-spraying habit in Bittacidae.