Abstract: The generation of adult rectum and functional differentiation of rectalpapillary cells in the black cutworm (Agrotis ypsilon ) were studied with light andelectron microscopes. Attention has been paid on their histological and cytotogiralchanges in sixth instar larvae, prepupae, pupae and adults. The results are summarized as follows: 1. Adult rectum was generated after the larval cryptonephridial complexdegenerated. Larval rectal cells rapidly divided, reconstructed and differentiated during the prepupal and early pupa 1 stages so that adult flat rectum cells and discal rectum papillae were formed. 2. Adult rectum papilla was discal and consisted of five layers of cuticle intima, papillary cells, basal lamella, muscle and peritoneal membrane. Papillary cells were divided into four types: cap cell, tracheolar cell, netty cell and discal base cell. 3. In our study it is found that adult rectum expands owing to the papillary volume enlarging. A series of delicate constructure enable papillary cells to enlargein volume and provide oxygen and nutritent supply

Key words: Agrotis ypsilon, metamorphosis, rectum, rectal papilla