gland is enlarged to nearly the size in В period queen larvae, but it has an 

 irregular shape; the cell boundaries are difficult to see and the cytoplasm 

 is fibrous and lies close to the nuclei and many of the nuclei show conspicuous 

 condensations of chromatin but do not appear to be dividing. Can this be 

 a phase of growth and regeneration? Five days' incubation is sufficient to 

 restore normality. 



Discussion 



These histological facts do not fit as easily into existing knowledge of caste 

 differentiation as one would wish but it is perhaps worthwhile to attempt 

 a provisional interpretation. For diapause larvae in autumn, growth is difficult 

 but metamorphosis is easy. During winter they acquire an increased ability 

 to sustain high growth rates and to depress development of the adult system. 

 This suggests that the purple activity of the brain cells which is absent in au- 

 tumn but appears during winter, promotes growth and that metamorphosis 

 can be started by other means. Perhaps the neuro-secretion stimulates protein- 

 ase production in the gut (T ho m s en and Moller, 1959). The ca which are 

 enlarged and reorganised during winter may suppress the development of the 

 adult system which is presumably even in the A period under the influence 

 of low concentrations of ecdyson. 



Worker determination is at present conceived as a sharp rise in concentra- 

 tion of ecdyson: so sharp that the relatively small ovaries and wings are incap- 

 able of response. Even in worker determined forms there is some growth in the 

 В period for which perhaps the residual purple coUoid may be responsible. 

 Queens arise from larvae that are able to prevent worker determination 

 and in these one presumes that ecdyson increases gradually in concentration 

 so that all imaginai buds grow and differentiate normally. Perhaps the grey 

 state of the brain cells is connected with this, but as there is considerable 

 growth in this period in queens, it is difficult to understand why there is no 

 purple activity. 



Acknowledgements 



It is a pleasure to acknowledge the advice of Dr. A. Fraser and the help 

 of Miss A. Sams. 



LITERATURE 



Fraser A., 1957, Nature London, 179: 257 — 8. 



Gabe M., 1953, Bull, Micro, appliq., 3: 153. 



Thomsen E. and Moller I., 1959, Nature London, 183: 1401—2. 



170 



