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The effect of high temperature (35°C) on the metabolism (digestive enzyme activities, fat body glycogen content, haemolymph trehalose and total lipid concentrations) in 5th-instar larvae of Morimus funereus reared from hatching on an artificial diet at 23°C (control) during the intermoult period (7-15 days) has been studied. The high temperature provoked disturbances in the metabolism (reverisible thermal stress) of larvae. The midgut amylolytic activity was significantly increased 6 h after treatment. Its fluctuations disappeared after 72 h and the activity decreased to a negligible value. Up to 24 h, the glycogen content of the controls showed diel changes, disrupted at 35°C. The glycogen concentration which was in inverse correlation with the trehalose concentration, increased from 6 to 72 h of exposure. Thereafter, glycogen reserves were almost exhausted. With the exception of a decrease in total lipid concentration observed in treated larvae after 48 h, other results were similar to those of the controls. The results show that during thermal stress, larvae of M. funereus utilize carbohydrates prior to lipids. Similar midgut proteolytic activities were observed in both groups of larvae but in the treated larvae the enzyme activity was significantly lower. The results are discussed with particular consideration of the modifying effect of diel rhythms and developmental processes. © 1992.