ENERGY METABOLISM IN LOCUSTS DURING FLIGHT

In locusts, energy metabolism during flight is initiated by octopamine and regulated by adipokinetic hormone (AKH). Trehalose serves as the major fuel at the onset of flight, but as the hemolymph trehalose levels decline with activity, octopaminergic neurons within the corpus cardiacum stimulate the release of AKH. The AKH activates an adenylate cyclase that increases cAMP levels and subsequently activates a protein kinase. The protein kinase then phosphorylates and activates a lipase that induces the release of diacylglycerols from the triacylglycerols stored in the fat body. AKH also induces the production of a lipoprotein carrier from the fat body that transports these diacylglycerols through the hemolymph to the flight muscles. The metabolism of carbohydrates that are stored in flight muscle during this lipid mobilization is also inhibited by AKH so that the lipid reserves are used exclusively. Octopamine has several other effects on flight behavior, stimulating the interneurons involved in maintaining flight, the power output of the flight muscles themselves, and the proprioreceptors on the wing that monitor flight behavior. This amine may be the functional equivalent of flight-or-fight hormones in vertebrates, released during stress and causing an increase in the insect’s arousal levels.