Utilisation of substrates during tethered flight with and without lift generation in the African fruit beetle Pachnoda sinuata (Cetoniinae).

We have investigated the pattern of metabolic changes during tethered flight with and without lift generation in the African fruit beetle Pachnoda sinuata. Two distinct metabolic phases occur during lift-generating flight. The first phase is characterised by a high rate of oxygen consumption and a rapid change in proline and alanine levels in the haemolymph and flight muscles and in glycogen level in the flight muscles. Carbohydrates are released from the fat body into the haemolymph. These carbohydrates are oxidised during the second phase. Changes in proline and alanine levels in the haemolymph and flight muscles and in glycogen level in the flight muscles are minor during the second phase and the rate of oxygen consumption is reduced. During lift-generating flight, metabolic changes are rapid. Proline concentrations in the haemolymph and flight muscles fall dramatically during the first 30 s of flight, while alanine concentrations rise concomitantly. While haemolymph concentrations of proline and alanine remain virtually unchanged thereafter, further changes in the levels of these amino acids occur in the flight muscles during 5 min of flight. The initial levels of the two amino acids in the flight muscles are re-established over 1 h of rest following a 5 min flight, while this process takes longer in the haemolymph. The concentration of haemolymph carbohydrates increases during the first 30 s of flight and declines thereafter during 5 min of flight. The pre-flight levels are restored after 1 h of subsequent rest. The stores of glycogen in the flight muscles are rapidly diminished during the first 10 s of flight and decrease at a lower rate during further flight lasting up to 5 min. A subsequent 1 h of rest is sufficient almost to restore pre-flight levels. Haemolymph lipid levels are slightly but significantly increased during 11 min of flight and after 1 h of subsequent rest. During flight without lift production, the metabolic changes are considerably slower and beetles fly approximately seven times longer than during lift-generating flight. Resting basalar (BM), dorso-ventral (DVM) and dorso-longitudinal (DLM) flight muscles show no differences in levels of proline, alanine and glycogen. After different periods of flight, during which lift and wing loading were minimised, the DVM was found to have the highest level of proline after 5 min of flight. Levels of alanine in the DVM were lower than in the DLM. There was no evidence to suggest that different flight muscles are specialised for either proline or carbohydrate utilisation. Proline and carbohydrates participate equally in supplying energy to the flight muscles during lift-generating flight. The contribution to the energy supply by the flight muscles is 54 %, while that of the haemolymph is 46 %.