Age-dependent changes of fat body stores and the regulation of fat body lipid synthesis and mobilisation by adipokinetic hormone in the last larval instar of the cricket, Gryllus bimaculatus

Abstract

Data on the hormonal regulation of the formation and mobilisation of fat body stores are presented and discussed in relation to general parameters of last instar larval development such as growth, food intake, and moulting. Crickets feed voraciously during the first half of the last larval stage. With the onset of feeding, fat body lipid synthesis increases, leading to increasing lipid stores in the fat body with a maximum reached on day 5. Lipid (42% of fat body fresh mass) is the main constituent of the fat body stores, followed by protein (6%) and glycogen (2%). During the second half of the last larval stage, feeding activity dramatically decreases, the glycogen reserves are depleted but lipid and protein reserves in the fat body remain at a high level except for the last day of the last larval stage when lipid and protein in the fat body are also largely depleted. The process of moulting consumes almost three quarters of the caloric equivalents that were acquired during the last larval stage. Adipokinetic hormone (AKH) inhibits effectively the synthesis of lipids in the larval fat body. Furthermore, AKH stimulates lipid mobilisation by activating fat body triacylglycerol lipase (TGL) in last larval and adult crickets. Both effects of AKH are weaker in larvae than in adults. This is the first report on the age-dependent basal activity of TGL in larval and adult insects. In addition, for the first time, an activation of TGL by AKH in a larval insect is shown.