Abstract: In order to further understand the biological activity of digestive system and the origin of cocoonase of Bombyx mori during the pupal-adult metamorphosis, the ultrastructure of the midgut and salivary glands was observed and the hydrolase activity was assayed. The results showed that active secretory characteristics such as a mass of secretory vacuoles, secretory granules and microvilli were observed in the midgut from the 7th day after pupation to one day before emergence and in salivary glands of newly emerged adults. High hydrolase activity existed in the midgut and salivary glands of pharate adults. The activities of protease, esterase and fibrinolytic enzyme were 726 U/mg, 751 U/mg and 263 U/mg in the midgut, and 603 U/mg, 523 U/mg and 147 U/mg in salivary glands, respectively. This suggests that the midgut and salivary glands were the possible origin of cocoonase. Weight and content analysis of ingredients in the pupal midgut during the pupal-adult metamorphosis showed that protein, carbohydrate and fat are major components, accounting for over 95% of the total weight, of which the proteins accounted for 78.8%－80.2%. The dry weight of the contents in the midgut was 20.1－21.9 mg/head in the early stage of pupation, without obvious change during the 7 days after pupation, but reduced by 63.01%－66.17% on the 9th day after pupation, and was extremely low in the newly emerged adult. The possible reason is that the contents were gradually assimilated and absorbed during the pupal-adult metamorphism. It was so inferred that during the pupal-adult metamorphism the midgut still keep the function of storaging and releasing nutrients, and cocoonase probably has another function of decomposing and digesting contents of the midgut.

Key words: Bombyx mori, metamorphosis, digestive organs, ultrastructure, secretory function