Digestion in spiders: histology and fine structure of the midgut gland of Coelotes terrestris (Agelenidae)

The fine structure of the midgut gland and its changes according to the nutritional state have been examined in the spider Coelotes terrestris (Agelenidae). In the epithelium of the midgut gland two types of cells are present: secretory cells and digestive cells. The epithelia are connected by an intermediate tissue. Secretory cells, characterized by great amounts of rER extrude enzyme containing granula into the lumen where extracellular digestion takes place. The digestive cells take up predigested food by pinocytosis and the resorbed nutrients are digested in nutritional vacuoles. Fat and glycogen originating from intracellular digestion in these vacuoles are stored in the intermediate tissue. Some nutritional vacuoles serve as an intracellular store in digestive cells. Metabolic wastes are collected in excretory vacuoles and extruded into the lumen. Histochemical examinations reveal that unspecific esterase and aminopeptidase seem to play an important role in digestive processes, whereas alkaline phosphatase is involved in transportation processes.