Observations of the early morning behaviour of A. annulosus and Ischnura heterosticta, two species which normally undergo daily physiological colour changes, show that both orient themselves perpendicularly to the direction of incident sunlight when perching and when in flight, as long as they remain in 'dark' colour phase. Accurately controlled experiments on eight A, annulosus show that mean gain in thoracic temperature attributed to dark-phase coloration is 0.23C after exposure to a radiation source similar in intensity and spectral composition to sunlight. However, a thoracic temperature gain of up to 15C may result from orientation to the sun on cloudless, windless mornings; under these conditions an oriented insect is able to forage for food c. 2 h earlier than a similar insect perched in the shade. Field experiments on A. annulosus reveal that receptors other than eyes are used to detect the direction of incident sunlight and that only dark-phase insects are able to do so rapidly enough to remain oriented in flight. It is suggested that dark-phase chromatophores, which have an underlying network of nerve axons insulated by large air sacs, are receptors of incident radiation, and that the temperature increase of dark chromatophores exposed to sunlight is sufficiently rapid to allow the insects to remain oriented in flight.