The length of discontinuous gas exchange cycles in lepidopteran pupae may serve as a mechanism for natural selection

Gas exchange is studied in diapausing pupae of Mamestra brassicae L., whose larvae are reared under identical conditions. The release of CO₂ gas is recorded with infrared gaseous analyzers. Oxygen convective uptake into the tracheae and oxygen consumption rates are recorded by means of a constant-volume coulometric respirometer. Outputs from both of these respirometry systems are combined with infrared actographs. All 3-month-old pupae of M. brassicae display a pattern of discontinuous gas exchange (DGE) cycles of CO₂ gas release by bursts, although the lengths of these cycles varies between individuals. Some pupae exhibit long DGE cycles of at least 20 h in duration, with negligible CO₂ gas release during interburst periods, and there is presumed to be a convective gas exchange at this time. As a result of a partial vacuum inside the tracheae, a large oxygen convective uptake always occurs at the start of the spiracular opening phase. Other pupae have short DGE cycles of less than 3 h in duration, with elevated CO₂ gas release during the interburst period, when gas exchange is predominantly diffusive. The spiracular open phase in these pupae consists of frequent separate convective bursts of CO₂ gas release, with the opening–closing rhythms of the spiracles, which are considered as O phase fluttering. The pupae with long DGE cycles exhibit extremely low metabolic rates and very low total water loss rates, whereas those with short DGE cycles have higher metabolic and total water loss rates. The pupae with long DGE cycles live approximately twice as long as those with short cycles; thus, the present study demonstrates that long DGE cycles confer a fitness benefit on pupae as a result of a lower metabolic rate associated with water economy, conferring on them a longer life.