Daily peak temperature is tracked by daily peak locomotor activity. Locomotor activity was recorded for Canton-S flies subjected to one of eight different daily temperature gradients in constant darkness (Z1-8 as indicated in Additional file 2 and Additional file 3). The time of entrained daily peak activity was determined from median activity records and plotted as a function of the time associated with the middle of the peak temperature phase either relative to external time (Eastern Standard Time; panel A) or relative to the time of the middle of the trough temperature phase (panel B). The data is plotted separately for each gender along with trend lines and associated correlation coefficients and their significance (2-tailed). Examples of entrained daily activity profiles are represented in panel (C). In a separate experiment Canton-S flies were exposed to a temperature protocol that violates the normal arrangement of the subjective daily phases of temperature ascent and descent relative to the midpoint of the temperature maximum. Median 6-day activity profiles representing groups of 8 males and 8 females synchronized to this protocol are shown in panel (D). The dotted lines in the activity profiles as well as the shaded bars underneath for panels (C) and (D) represent the daily temperature gradient, while the closed and open arrowheads at the top of the diagrams in panel (C) correspond to the middle of the peak and trough temperature phase, respectively. The numbers of individual flies (n) and experimental days (d) are indicated in panel (C).