Abstract
In order to examine the periodical occurrence theory in mosquitoes, introduced by S. Shibata in 1948, the authors investigated the larval instar-ratio of malaria mosquito Anopheles hyrcanus sinensis Wiedemann in the rice-field at Matsudo, Chiba Prefecture. If Shibata's theory is just, the periodical change of the instar-ratio must be regular as shown in Fig. 1 (black : young stage, oblique line : old stage). But from the investigation data in years 1951 and 1952 (Fig. 2), the authors are unable to recognize the above-mentioned tendency. The variation of instar-ratio is not small, but the authors consider that the percentage of young stage is higher in general except after heavy rains (in Fig. 2 the mark shows the case). Moreover, the authors have to point out the relationship between the seasonal changing of instar-ratio (Fig. 2) and the population density of adult. For example, the falling of young stage ratio toward August may be controlled by the decrease of adults. As the conclusion the authors oppose Shibata's theory so far as the species occurred in the rice-field is concerned.
