Studies on the initiation of growth and moulting in Locusta migratoria migratorioides R. & F.—I : The time and nature of the initiating stimulus

Abstract

Studies were made to determine the time during a stadium at which the growth and moulting hormone is released from the brain and/or corpora cardiaca of the locust. The criterion used was the appearance of mitotic figures in the prothoracic gland. Samples were taken at known ages from just before the second ecdysis until the middle of the fifth instar. The locusts were fixed in Bouin's fluid, the prothoracic glands dissected out, stained, and mounted, and the absolute number of mitotic figures present in each gland counted. These results show: (A) that the appearance of mitotic figures in the prothoracic gland precedes their appearance in the epidermis; (B) at 28°C and 43°C the maximum number of mitotic figures present occurs at the moment of ecdysis; no figures were found during the late middle stages of an instar; (C) at 25°C, the maximum number of figures occurs after some 20 per cent of the stadium has passed; (D) at all temperatures the first mitotic figures appear during the 12 hr period preceding ecdysis. It is therefore suggested that the release of the growth and moulting hormone coincides with the onset of ecdysis, and it is possible that the emptying of the gut at this time may provide the stimulus for its release. This would enable one moulting cycle to follow another independently of any external stimulus and as rapidly as possible. It further points to studies on the relationships of the stomatogastric and neuroendocrine systems for the elucidation of mechanisms of the initiation of growth and moulting in the locust.