Circadian rhythms are particularly ubiquitous in insects and, as in other organisms, have been the best analysed. Physiological circadian rhythms in insects have been well- documented in relation to hormone production, particularly hormones controlling postembryonic development. The expression of rhythmicity in insects has been largely documented at the cellular level, in organs, as changes in the physiology and behaviour of individuals, as well as in population synchrony. Fruit flies, cockroaches, butterflies, honey bees, and other insects cast much light on the physiology and molecular basis of the circadian clock. Insect adaptation and success in the colonisation of the most diverse environments is also associated with the temporal organisation of their daily life. Circadian locomotor rhythms of the adult flies reflect endogenous, self-sustained oscillations with a temperature compensated period. The free-running rhythms become synchronised (entrained) to daily light:dark cycles, but become arrhythmic in constant light above a certain intensity. Some flies show fragmented rhythms (internal desynchronisation) suggesting that overt rhythmicity is the product of a multioscillator (multicellular) system.