1. A method is described which makes the registration of body temperatures possible without disturbing the animals. Thus it is possible to observe and quantify the effect of basking on the development of a population. 2. In crickets (Gryllus bimaculatus) the time of development is drastically reduced by the possibility to bask in sunshine at self-determined intervals. The old rule of temperature-sums is demonstrated to be invalid in this case; the changing body temperature acts as a signal to unknown processes which speed up the development of the animal.