Metabolic recovery from drowning by insect pupae

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 days. All pupae survived up to 5 days 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 approximately 1 day in the group immersed for 1 day and for at least 2 days in the 3- and 5-day 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 models of Förster 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 12-48 h. Pupae in the 3- and 5-day 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.