Many terrestrial insects live in environments that flood intermittently, and some life stages may spend days underwater without access to oxygen. We tested the hypothesis that terrestrial insects with underground pupae show respiratory adaptations for surviving anoxia and subsequently reestablishing normal patterns of respiration. Pupae of Manduca sexta were experimentally immersed in water for between 0 and 13 d. All pupae survived up to 5 d of immersion regardless of whether the water was aerated or anoxic. By contrast, fifth-instar larvae survived a maximum of 4 h of immersion. There were no effects of immersion during the pupal period on adult size and morphology.
After immersion, pupae initially emitted large pulses of CO2. After a subsequent trough in CO2 emission, spiracular activity resumed and average levels of CO2 emission were then elevated for approx. 1 d in the group immersed for 1 d and for at least 2 d in the 3- and 5-d immersion treatments. Although patterns of CO2 emission were diverse, most pupae went through a period during which they emitted CO2 in a cyclic pattern with periods of 0.78 - 2.2 min. These high-frequency cycles are not predicted by the recent model of Foerster and Hetz (2010) and Grieshaber and Terblanche (2015), and we suggest several potential ways to reconcile the models with our observations. During immersion, pupae accumulated lactate, which then declined to low levels over 12 to 48 hours. Pupae in the 3- and 5-d immersion groups still had elevated rates of CO2 emission after 48 h, suggesting that they continued to spend energy on reestablishing homeostasis even after lactate had returned to low levels. Despite their status as terrestrial insects, pupae of M. sexta can withstand long periods of immersion and anoxia and can reestablish homeostasis subsequently.
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