ACID-BASE STATUS AND SPIRACULAR CONTROL DURING DISCONTINUOUS VENTILATION IN GRASSHOPPERS

Many insects ventilate discontinuously when quiescent, exhibiting prol onged periods during which little or no gas exchange occurs, We invest igated the consequences of discontinuous ventilation (DV) on haemolymp h acid-base status and tested whether spiracular opening during DV is due to changes in internal gas tensions in the western lubber grasshop per Taeniopoda eques, At 15 degrees C, resting T. eques exhibited inte rburst periods of about 40 min, During the interburst period, haemolym ph P-CO2 rose from 1.8 to 2.26 kPa, with minimal acidification of haem olymph, Animals in atmospheres in which C-O2 was 2 kPa or below contin ued to exhibit DV, while atmospheres in which P-CO2 was 2.9 kPa or abo ve caused cessation of DV. These data indicate that accumulation of in ternal CO2 to threshold levels between 2 and 2.9 kPa induces spiracula r opening in grasshoppers, In contrast to the situation in lepidoptera n pupae, variation in atmospheric P-O2 had no effect on interburst dur ation, Relative to lepidopteran pupae, the internal P-CO2 of grasshopp ers during DV is threefold lower, the P-CO2 required for triggering sp iracular opening is also threefold lower, and the open phase spiracula r conductance is at least tenfold higher, demonstrating that considera ble diversity exists in these aspects of insect respiratory physiology .