Action of the Photoperiod in Controlling Insect Diapause

Abstract

Although there is much yet to be discovered about the photoperiodic control of insect growth and diapause, a number of basic facts concerning insect development are available. The researches of Williams (1947, 1952, 1961) show that continued development in many insects depends on the action of the brain in activating the- prothoracic glands causing them to secrete the growth hormone, ecdysone. Without the presence of ecdysone, growth cannot proceed. The direction of development to larva, pupa or adult is mediated by a third hormone, the juvenile hormone. It appears certain from results of Williams (1947), Van der Kloot (1955) and Cloutier et al. (1962) that in the diapausing pupa or larva arrested development is caused by the failure of the brain to "switch" on the prothoracic glands. This inhibits the secretion of ecdysone and further development is prohibited. It may then be surmised that the action of the photoperiod is to control the synthesis or release of the brain hormone, "switching" it on when growth is called for and "switching" it off when the environment demands the diapause condition. The photoperiodic control of diapause must also invoke some mechanism for the measurement of the relative duration of light and dark in each solar day since the induction or prevention of this phenomena within a species typically revolves around a light-dark period of certain critical length. As to how insects are able to measure time is not known. The fact that they do, and with remarkable precision, will be demonstrated in this paper.