Beetles’ choice—proline for energy output: control by AKHs

Many beetle species use proline and carbohydrates in a varying ratio to power flight. The degree of contribution of either fuel varies widely between species. In contrast, dung beetle species investigated, thus far, do not have any carbohydrate reserves and rely completely on proline to power energy-costly activities such as flight and, probably, walking and ball-rolling. While the fruit beetle, Pachnoda sinuata, uses proline and carbohydrates equally during flight, proline is solely oxidised during endothermic pre-flight warm-up, as well as during flight after prolonged starvation. Thus, proline seems to be the essential fuel for activity in beetles, even in flightless ones and in those that use proline in combination with carbohydrates; the latter can be completely substituted by proline in certain circumstances. It is apparent from the rapid decline of energy substrates in flight muscles and haemolymph after the onset of flight that mobilisation of stored fuels of the fat body is necessary for prolonged flight periods. This task is performed by AKH-type neuropeptides. In beetles, like in other insects, these peptides mobilise glycogen via activation of glycogen phosphorylase. They also stimulate proline synthesis from alanine and acetyl-CoA in the fat body. Acetyl-CoA is derived from the β-oxidation of fatty acids and we propose that the neuropeptides activate triacylglycerol lipase.