REGULATION OF PROTHORACIC GLAND ACTIVITY IN DIAPAUSING LARVAE OF THE WAX MOTH, GALLERIA-MELLONELLA L (LEPIDOPTERA)

The development of Galleria mellonella is arrested by a lowered temper ature of 18-degrees-C at the stage of the last instar post-spinning la rva. The haemolymph ecdysteroid titre of these larvae is maintained at a low level of ca 5 pg of 20-hydroxyecdysone per mul. Different types of surgical manipulations including nerve cord-severance, implantatio n of the brain, prothoracic glands (PGs) or fat body accompanied with ecdysteroid titre measurements show that the tow ecdysteroid titre in diapausing larvae is due to nervous inhibition of their PGs. The inhib itory input probably comes from the brain and is transmitted down to t he PGs via the suboesophageal ganglion and paired cervical nerves. Sev erance of the nerve tracts at any level caused rapid activation of PGs of diapausing insects. Nervous inhibition of the PGs ceases about 6-9 h after transfer of diapausing larvae from the lower temperature to t he diapause-terminating conditions of 30-degrees-C. As long as the PGs are under nervous inhibitory control they are refractory to humoral s timulation. The brains of diapausing larvae exhibit in vitro high prot horacicotropic activity which suggests that PTTH is accumulated in the brain during diapause. The role of juvenile hormone in inhibiting the release of PTTH from the brain to haemolymph, and its effect on the P Gs during diapause is discussed.