Abstract: By comparative studies of the follicle development and mosquito diapause, the major criterion of diapause in adult C, pipiens pallens has been demonstrated to be the retardation of the first follicle at N-stage, which occurred 24 hours after emergence, with a length around 50 micrometers. The almost equal length of the first follicle and the germarium (i.e. second follicle plus third follicle) may also be considered as a secondary evidence of the diapause. Developing mosquitoes are indicated by the appearance of la stage which is the most prosperous period of the follicle development in its early stage when the mosquitoes start blood-feeding. In the absence of blood supply; development may continue up to and then stop at the resting stage of IIb.The process of follicle development in diapaused adults was carefully compared with that of decapitated individuals which were taken as an alternative of brain inhibition to juvenile hormone (JH) secretions of the corpora allata (CA). Results strongly inferred that follicle retardation may be attributed to lack of JH. It is suggested that the develpment of the first follicle from stage No.1 at emergence to N-stage may be unrelated to JH, while further development from N-stage may depend on the function of JH from CA. The critical point of JH secretion from CA was demonstrated at around 24 hours after emergence of the female adult.