ENDOCRINOLOGY OF INSECT DIAPAUSE 135 



Stimulated^. This seems to indicate that some substance required 

 for embryonic growth is lacking in the dormant embryo. 



In most locusts and grasshoppers embryonic growth cannot be 

 completed until water is taken up from the environment. Slifer^ 

 has shown that diapause can be terminated in the Qgg of 

 Melanoplus dijferentialis if a waxy layer is removed from the 

 surface of the hydropyle — the special water-absorbing area of 

 the egg-shell. The effective solvents include medicinal paraffin 

 which is so innocuous to living tissues that any stimulatory 

 effect through 'wounding' is extremely improbable. Yet the 

 causal mechanisms which control the ordered appearance and 

 disappearance of such physical and physiological barriers are 

 still unknown. The mode of action of low temperature in 

 bringing about the final dissolution of the wax layer is one such 

 problem. Unlike the egg of Bombyx, which does not take up 

 water, control does not seem to reside in the embryo itself, for 

 Bucklin^^ has found that diapausing embryos, when 'explanted' 

 in a hanging drop of Ringer, immediately resume their develop- 

 ment. 



Although the corpus allatum ceases to secrete juvenile hor- 

 mone just prior to metamorphosis, it often displays renewed 

 activity during adult life. Indeed, its secretion at this time has 

 been shown to be necessary for the maturation of the eggs of 

 many insects. Since adult diapause is characterised by repro- 

 ductive inactivity, it was suspected that the corpus allatum might 

 be involved. This view has been proved correct by De Wilde^i 

 who showed that Colorado beetles (Leptinotcusa) that are 

 about to enter diapause in response to a short photoperiod can 

 be induced to leave the soil, reverse their geotaxis, feed and lay 

 eggs, by implanting active corpora allata. The corpora a/ lata of 

 this insect m.ay also be influenced by the brain. 



MODE OF ACTION OF ENVIRONMENTAL FACTORS 



In my first lecture I gave some account of the way in which 

 environment controlled diapause induction and diapause termi- 



References p. 140 



