Development of respiratory function in the American locust Schistocerca americana. II. Within-instar effects.

We hypothesized that oxygen delivery becomes more difficult for insects and tracheate arthropods as they progress throughout an intermolt period. During this time, body mass can more than double, yet the major tracheae and spiracles cannot be increased in size until molting. Also, tissue growth could compress air sacs used for convective gas exchange. To test these possibilities, we investigated the effect of within-instar growth on respiratory parameters, including CO(2) emission rate, ventilation frequency, tidal volume and critical oxygen partial pressure (P(O2)) for first-, third- and fifth-instar juveniles and adults of the American locust Schistocerca americana. We found that late-stage grasshoppers tended to have 40% higher total CO(2) emission rates but 15% lower mass-specific CO(2) emission rates and 35% higher ventilation frequencies than early-stage animals. Maximal tracheal system conductance decreased by 20-33% at the end of an instar, possibly due to compression of air sacs. In addition, animals nearing the end of an instar had higher critical P(O2) values for abdominal pumping, and late-stage adults had 50% lower tidal volumes, suggesting that increases in tissue mass throughout an instar may hinder the ability of animals to breathe deeply. Late-stage adults had lower critical P(O2) values for CO(2) emission, although this pattern was not found in any juvenile instars, indicating that late-stage juveniles compensate for decreased conductance by increasing ventilation frequency or the use of diffusive gas exchange. Our data suggest that late-stage arthropods are more vulnerable to hypoxia and may have reduced aerobic capacities and lower tissue P(O2)s than early-stage arthropods.