Why don't tobacco hornworms (Manduca sexta) become dehydrated during larval‐pupal and pupal‐adult development

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AbstractStudies of water loss during metamorphosis of insects have usually been confined to the pupal stage where both cuticular and respiratory control of transpiration have been reported. In Manduca sexta and other lepidopterans, preparation for pupation actually begins several days before pupation when the animals stop feeding and begin searching for an appropriate pupation site, a burrow for M. sexta. This study examines some aspects of water balance maintenance from the initial period of searching to the final time of eclosion. Behavior is important for reducing water loss, as animals which fail to find an appropriate pupation site continue to lose water at twice the rate of animals which are burrowed (10 vs. 5 mg H2O/g·h). Part of this is explained by exposure to light as unburrowed animals in constant dark are intermediate in water loss. The epicuticle during the pupal stage reduces water loss rate eight to tenfold, removal causing animals to lose water at 4 mg (g·h)−1. M. sexta does appear to be capable of storing water in preparation for metamorphosis, since individually raised animals consistently have water contents higher than animals reared in large groups. A drop in water loss during the larva‐pupal development is correlated with increased hemolymph ecdysone titer, although how is unknown. Pupal water loss can be divided into cuticular and respiratory pathways, with cuticular loss accounting for two‐thirds of 0.7 mg H2O (g·h)−1 loss rate. Finally, an increase in hemolymph osmotic pressure just prior to ecdysis may aid in reclamation of molting fluid.