Some rearing experiments were carried out to determine the stages sensitive to photoperiod inducing or completing the diapause in the green rice leafhopper, Nephotettix cincticeps UHLER. Rearings were carried out in a room of constant temperature of 20°C. 16hr illumination was given in the long day photoperiod and 8hr in the short day.<br>1. For the induction of diapause in all larvae subjected the short day treatment is required since the 1st larval stage. Even the long day during the 1st stage causes 60 to 70 per cent of the larvae to become non-diapause type.<br>2. The larvae of all stages which are reared under the short day photoperiod retain the sensitivity to the long day photoperiod, the sensitivity being shown in the acceleration of development thereafter.<br>3. The 4th and the 5th stage larvae which present the most clear elongation of development under the short day photoperiod react to the long day treatment in the similar way for each other, namely, the development is not accelerated in such a degree that the non-diapausing level is attained in that stage the treatment is commenced, but attains to the level provided the treatment is commenced in the preceding stage. It is difficult to determine which is the true diapausing stage the 4th or the 5th stage from the reaction to photoperiod.<br>The 3rd stage also reacts in the similar way but in a lesser degree.<br>4. The adults obtained from the various treatments mentioned above were measured in the length of the tegmen, the tibia and femur of the hind leg, and the head width. The typical summer form is obtained under the long day photoperiod throughout the development and the typical spring form under the short day. In the other treatments intermediate forms in the size and pigmentation are obtained, the longer the developmental period the more approaching to the spring form are the ensuing adults. A hyperbolic relation is found between the two variables, the length of tegmen or other body parts and the developmental period.<br>A ratio, length of tegmen/length of femur of the hind leg, was taken to study the relative growth of body parts. The ratio presents a similar trend with that of the length of tegmen itself with elongation of the developmental period. This fact means that the length of tegmen is most sensitive to the effect of photoperiod.<br>5. Elongation of the larval period due to low temperature shows no such a relation to the size of ensuing adult as in the former case due to the diapause.<br>6. Corpora allata are assumed as a common proximate centre of action inducing the diapause and the reduction of adult size.