Étude de la diapause de Gryllus campestris (Orthoptera)

Abstract

This paper demonstrates the existence of two distinct types of larval development in an insect which diapauses in the next to the last larval stage. One type consists of rapid growth followed by a diapause and the other of slow growth without a diapause. The diapause is linked to the speed of development of the larval stages which precede it and, indirectly, to external factors which may, in turn, affect this speed. The following parameters are used to modify the speed of prediapause development: temperature, photoperiod, and group effect. The speed of prediapause development may be raised progressively to a maximum which occurs at 32°C with a photoperiod of 12 hr in insects reared in crowded conditions. The diapause remains unchanged, as well as the treatment necessary for reactivation. This is why until now diapause in this species was considered obligatory whereas it is facultative. A slowing down of the speed of development by low temperature (20°C) is sufficient to eliminate diapause providing that the first two larval stages are subjected to an elevated temperature (30°C): this sequence is necessary in order to programme the normal succession of moults. The insects obtained by this procedure are much smaller. The larval development of this species thus seems to be the site of a sort of feedback mechanism which allows the final imaginal emergence to occur at an equivalent period whether the path is by way of a rapid growth period followed by a diapause, or by a slow continuous growth period without a diapause. The ecological aspect of the phenomenon is examined.