Using the adult Calliphora bioassay the tanning hormone, bursicon, has been found in the blood of pupal and adult Tenebrio only at the time of ecdysis. Contrary to other workers, no evidence could be found for the presence of the hormone in the haemolymph during pharate adult development, before ecdysis begins. When newly ecdysed pupae were ligated about the neck, post-ecdysial tanning of the adult cuticle was almost completely prevented. This failure to tan was not due to a lack of bursicon as the hormone was released normally in the ligated insects at the time of ecdysis. This suggests that a pre-ecdysial signal may be required for the development of epidermal competence to respond to bursicon. Other events were found with similar critical periods for the head to that for bursicon responsiveness e.g. pre-ecdysial plasticization of the elytral cuticle. The results are discussed in terms of a single endocrine effector. However, attempts to demonstrate the presence of a factor in blood and nerve cord of pharate adults that promotes competence to respond to bursicon failed. A study has been made on the control of digestive enzyme secretion in young adult Tenebrio. A spontaneous increase in midgut a-glucosidase activity was observed in the first few days of adult life. This was prevented by neck ligation >1 day before ecdysis. Ligation experiments revealed a critical period for the head in post-ecdysial midgut enzyme activity during the 24 h prior to ecdysis comparable to that for competence to respond to bursicon. Transfusing neck-ligated insects with pharate adult blood promoted a significant increase in midgut a-glucosidase activity suggesting that a hormone is involved. A fine structural study has been made of the enterocytes in the midguts of insects starved or fed from emergence for 4 days. The cytological features of the enterocytes are related to the processes of enzyme secretion and peritrophic membrane formation. Cytoplasmic extrusions were only found in the fed insect and it is concluded that they are the result of cell breakdown associated with a high rate of enterocyte turn-over. Although 3 "types" of enterocyte are described, it is suggested that they may simply reflect different stages in the ageing of a single population of cells.
|Date of Award||1984|