The effect of fasting and of fasting followed by refeeding and heat shock was studied in Malpighian tubules of fourth instar nymphs of the blood-sucking hemipteran, Panstrongylus megistus (Burmeister). The aim was to detect different frequencies of nuclear changes (apoptosis, necrosis, heterochromatin decondensation) under conditions assumed to be stressful in blood-sucking hemipterans. The insects were fasted for up to 90 days at 28 degrees C and their survival was followed daily. Groups of nymphs were separated each month, with part of the group being refed and the other part kept fasting. Insects in each of these subgroups received either a heat shock at 40 degrees C for 1 h or were maintained at 28 degrees C (control for heat shock). The Malpighian tubules were removed one and seven days after each assay and subjected to the Feulgen reaction for identification and counting of the various nuclear phenotypes. Insect survival was high (90%) even after 40 days of starvation but decreased thereafter. Necrosis rather than apoptosis, increased with fasting. Feeding after fasting increased the frequency of apoptosis but not of necrosis. The short heat shock as used here did not additionally affect the responses induced by fasting and refeeding. P. megistus nymphs could withstand relatively long periods of fasting although individual variation in the mean length of cell survival had been found especially after a three-month fasting. The results related to cell necrosis suggest that part of the Malpighian tubule cells may not have developed highly efficient mechanisms for dealing with fasting. For those cells resistant to fasting, feeding subsequent to fasting acted only as a mild stressing agent and heat shock was well tolerated. The ability of P. megistus nymphs to withstand and recover from periods of inadequate or poor nutrition inclusive in association to a short heat shock as demonstrated here is certainly an important adaptation for the survival of the species.592144152