Insect flight muscles perform their work completely aerobically, and working flight muscles are known to be the most metabolically active tissue in nature with respect to oxygen uptake. Various substrates can be oxidised and utilised as fuels for flight. Insects such as Diptera and Hymenoptera power their flight muscles by the breakdown of carbohydrates, whereas lipids are the predominant fuel for the contracting flight muscles of Lepidoptera and Orthoptera during long-distance flight. The amino acid proline can also be used as a substrate for flight, especially in tsetse flies and beetles (Colorado potato beetle, blister beetles, certain dung beetles). Neuropeptides from the corpus cardiacum are well-known to be responsible for carbohydrate and lipid mobilisation from the fat body. In this short overview, we show that peptides belonging to the large adipokinetic hormone/red pigment-concentrating hormone family are also thought to be the chemical messengers for initiating proline homeostasis. The peptides isolated and sequenced so far from glands of beetles from the genera Pachnoda, Scarabaeus and Onitis all have a tyrosine residue (at position 2 or 4) and seem to be related to each other.