Diapause state in adult female mosquitoes is usually characterized by 1) enlargement of the fat body, 2) reduction in biting activity and 3) delayed development of the ovarian follicles. All these properties must be derived from some changes in the ordinary pattern of metabolic processes, of which, however, no accurate information has yet been obtained. In Anopheles maculipennis of the hibernating generation, the fat body may be elaborated at the expense of blood meals, which are to be used for developing eggs in active generations (Swellengrebel, 1929; Hecht, 1932, 1933a, b ; Weyer, 1937; Roubaud and Treillard, 1937) . Another cause of fattening is also suggested by Roubaud (1932), who maintains that the enlarged fat body in hibernating C. pipiens females is not caused by the sugar or blood meals, but is derived from the histolyzed larval muscle material, which disappears from the imaginal abdomen shortly after emergence. Existence of such an autotrophic fattening, however, is denied by de Buck (1935) in the case of typical C. pipiens. Nieschulz and Bos (1931) report that the activity of hibernating C. pipiens increases with the loss of fat. It is true that fattened mosquitoes usually show decreased desire for blood sucking, but there still remains ambiguity as to whether fattening is the cause of the inactivity or inactive females are likely to become fat. Relation between biting activity and growth activity of the ovarian follicles is quite obscure at present; mosquitoes such as C. pipiens pallens show gonotrophic dissociation on some occasions, although they are subject to gonotrophic concordance in nature (Hosoi, 1953a) . Further experiments, therefore, were designed at finding some clues to the above problems.