ABSTRACT. Insect photoperiodic responses are complex and variable, but all involve a ‘clock’ which is used to determine the qualitative difference between long and short nights, and a ‘counter’ mechanism which accumulates successive long (or short) nights quantitatively up to an internal threshold at which induction (of diapause, seasonal morphs, etc.) is ‘complete’. The clock is circadianbased in at least seven species; in others a non-oscillatory ‘hour-glass’ device is indicated, but negative ‘resonance’ experiments may not necessarily rule out a circadian involvement in time measurement. The counter is temperature-compensated. In some species it adds up long nights, in some short nights, and in others long and short nights. The proportion of the population entering diapause is dependent on an interaction between the temperature-compensated counter and temperature-dependent rates of development; this interaction explains many of the known effects of temperature, diet, and latitude in insect photoperiodic responses. In formal terms, the accumulation of long nights is seen as an increase in a ‘diapause titre’ which is compared with an individual internal threshold. In concrete terms, this ‘titre’, and the counter mechanism, might be represented by the accumulation of neurosecretory granules within the neuro-endocrine system.