Allometric scaling of discontinuous gas exchange patterns in the locust Locusta migratoria throughout ontogeny

Abstract

The discontinuous gas exchange cycle (DGC) is a three-phase breathing pattern displayed by many insects at rest. The pattern consists of an extended breath-hold period (closed phase), followed by a sequence of rapid gas exchange pulses (flutter phase), and then a period in which respiratory gases move freely between insect and environment (open phase). This study measured CO2 emission in resting locusts Locusta migratoria throughout ontogeny, in normoxia (21 kPa PO2), hypoxia (7 kPa PO2) and hyperoxia (40 kPa PO2), to determine whether body mass and ambient O2 affect DGC phase duration. In normoxia, mean CO2 production rate scales with body mass (Mb; g) according to the allometric power equation , closed phase duration (C; min) scales with body mass according to the equation C=8.0Mb0.38±0.29, closed+flutter period (C+F; min) scales with body mass according to the equation C+F=26.6M 0.20±0.25b and open phase duration (O; min) scales with body mass according to the equation O=13.3Mb 0.23±0.18. Hypoxia results in a shorter C phase and longer O phase across all life stages, whereas hyperoxia elicits shorter C, C+F and O phases across all life stages. The tendency for larger locusts to exhibit longer C and C+F phases might arise if the positive allometric scaling of locust tracheal volume prolongs the time taken to reach the minimum O2 and maximum CO2 set-points that determine the duration of these respective periods, whereas an increasingly protracted O phase could reflect the additional time required for larger locusts to expel CO2 through a relatively longer tracheal pathway. Observed changes in phase duration under hypoxia possibly serve to maximise O2 uptake from the environment, whereas the response of the DGC to hyperoxia is difficult to explain, but could be affected by elevated levels of reactive oxygen species.