Histological investigation of the rectal sac in Bombyx mori L.

The rectal sac (Rs) is formed by enlargement of the rectal portion of the hindgut in Bombyx mori. The Rs consists of epithelium, connective tissue, and many rectal pads (Rps). Rps have a cortex and medulla. In the medulla 2- 5 pyramidal epithelial cells synthesize proteinic secretory material (Ps), and then this substance localizes within epithelial cells and connective tissue. It was thought that lipoid secretory material (Ls) was synthesized by epithelial cells in the midgut and was delivered to the Rs; however, Ls was localized in the epithelium of the Rs and in the periphery of Rps. Both secretions were released into the lumen of the Rs. Thus, the synthesis and accumulation of the secretory material increased as the pupa developed, and the Rs gradually grew. In some insects Rps are covered with a peritrophic matrix, which extends backward from the midgut and epithelium. As the larvae grew, depending on development, it was observed that the Rps decreased in number, but that degradation of the Rps gradually increased. This degradation was also observed as the epithelium that covers the Rs thickened and became corrupted. It was thought that the Rs could be responsible for enabling escape from the pupal case at the end of pupal development.