Independent variation of the dark and light components of the daily photocycle has shown that the linden bug,Pyrrhocoris apterus, unlike other species, ‘measures’ daylength rather than nightlength. Greatly extended dark periods coupled with a short photophase (a Nanda-Hamner protocol) shows peaks and troughs of diapause at about 16-h intervals, an extremely short period for a circadian clock. If circadian oscillations are involved in photoperiodic time measurement in this species, a photoinducible phase might lie in the early rather than the late subjective night.