Studies on the initiation of growth and moulting in Locusta migratoria migratorioides R. & F.—IV. The relationship between the stomatogastric nervous system and neurosecretion

Abstract

Studies were made on the neurosecretory system in normal locusts from the end of the second instar to the middle of the fifth instar. Samples were taken at 24-hr intervals, fixed in Bouin's fluid, and the whole heads sectioned and stained with Gomori's chrome haematoxylin and phloxine. Special attention was paid to the medial neurosecretory cells, their chiasma, the nervi corporis cardiaci I, and the corpora cardiacum. Histologically no trace of a secretory cycle of any kind could be found; the picture was one of constant production and secretion of neurosecretory material. In locusts from which the frontal ganglion had been removed there was little difference from the normal animal during the first 200 hr following the operation. After this time there was considerable accumulation of material in the nervi corporis cardiaci I, and the corpora cardiaca was shrunken and abnormal in appearance; the cells of the pars intercerebralis differed little from normal. In starved animals after 61.hr the corpora cardiaca was also shrunken and abnormal in appearance. There was, however, no material in the nervi corporis cardiaci I. Based on evidence presented in this and the previous papers, and on facts published by other workers, the following hypothesis of the control of growth and moulting in locusts is proposed. There is a constant production of neurosecretory material in the medial neurosecretory cells and constant release of it from the corpora cardiaca. Its production and/or release are dependent upon impulses arising in the stretch receptors of the pharynx which are conducted via the posterior pharyngeal nerves, frontal ganglion, and frontal connectives to the brain and via the recurrent nerve and hypocerebral ganglion to the corpora cardiaca. Surgical interference in this pathway prevents the secretion of material and leads to the failure of the animal to grow and moult. During an instar the movements of the pharynx associated with the intake of food provide the necessary stimuli, whereas at an ecdysis the swallowing of air into the gut which continues throughout ecdysis will provide the necessary stimuli at this time. This can give rise to a cyclical stimulation of the prothoracic glands in the following manner. During an instar the neurosecretory material is used in the ordinary processes of growth and metabolism; at an ecdysis, when food absorption is absent, these processes will be less. As the material is still being produced, there could be a rise in titre of the hormone in the blood which could result in the stimulation of the prothoracic gland, and consequent upon this the start of a moulting cycle.