448 



WILLIAM G. VAN DER KLOOT 



time of pupation, the development of the silkworm abruptly stops. For weeks 

 or for months growth, differentiation, and even cell division are suspended; 

 the pupa is in diapause. Diapause goes on for many months if the pupa is 

 stored at 25 °C. But after a few weeks at a low temperature, similar to the 

 cold of winter, if the pupa is returned to 25"C, development is soon resumed. 

 Twenty-one days after the onset of development, the adult moth emerges. 

 Diapause is obviously the means by which the pupa passes the winter. The 

 usual regulation is by the annual temperature cycle. A mistake by the control 

 mechanism within the insect leads to premature development and to death. 



The control mechanism, as Williams (1946, 1956) showed, lies in the brain 

 of the silkworm. Diapause results from the failure of twenty-six specialized 

 neurons in the brain to release a neurohormone. The neurohormone from the 

 brain acts by stimulating a second endocrine organ, the prothoracic glands. 

 As summarized in Fig. 1, the prothoracic gland hormone, in its turn, acts on 

 the tissues to promote cell division, growth, and molting. 



NEUROSECRETORY CELLS 

 in the brain 



brain hormone 



•release inhibited during diopouse 



XT 

 PROTHORACIC GLANDS 



p.g. hornnone 



\i 



BODY TISSUES 



(promotes growth, differentiotion, and molting) 



Fig. 1. The control of diapause in the Cecropia silkworm. For explanation see text. 



) > Excitatory hormone. 



Why do the neurosecretory cells not release the hormone during the months 

 of diapause, and how do low temperatures act to restore hormone release? 

 The scant evidence so far available suggests that neurosecretory cells release 

 hormone when they conduct action potentials (Knowles et ai, 1955; Van 

 der Kloot, 1960; Hodson and Geldiay, 1960). It therefore seemed reasonable 

 to study the electrical activity of the neurosecretory cells during diapause and 

 development. 



The surprising result is that in the diapausing pupa all attempts to record 

 either spontaneous or evoked electrical activity from the brain have been 



