A53 Accelerated dehydration

Topic

An insect subjected to thermal stress loses water much faster than it would through the mere passage of time, because this accelerated dehydration results from two simultaneous effects rather than just one: on the one hand, the permeability of the insect's cuticle increases with temperature; on the other, the vapor pressure driving water out of the insect's body rises.

The first of these effects occurs within the insect's cuticle itself: as the temperature rises, cuticle permeability increases exponentially—following the Arrhenius equation—primarily because the epicuticular wax coating the insect melts and becomes less effective at blocking water loss through the cuticle.

The second effect, independent of the first, occurs in the air surrounding the insect: as the temperature rises, the air's saturated vapor pressure also increases—following the Antoine equation—thereby amplifying the vapor gradient between the insect's moist interior and the outside air, which drives water out of the insect's body with greater force.

These two effects do not merely add up; they multiply one another: the increased cuticle permeability and the wider vapor gradient act simultaneously on the same water-loss pathway, and this multiplicative combination means that water loss at forty degrees can be ten to one hundred times greater than at twenty degrees.

As this accelerated water loss depletes the insect's available extracellular water, its cells undergo progressive osmotic shrinkage; when the volume of these cells drops below a critical threshold—close to sixty percent of the original volume—the cell membranes collapse irreversibly. How close an insect is to reaching that point of cellular collapse depends largely on its habitat: insects from arid environments possess a cuticle with very low permeability—thanks to more resilient epicuticular waxes—and can tolerate water losses of up to half their body mass before hitting that critical threshold, whereas insects from humid environments may die after losing as little as one-fifth of their body mass.

By combining the rate at which an insect loses water through these two processes with the fraction of that loss it can tolerate based on its native environment, one can calculate the time until lethal dehydration occurs—a key parameter for predicting the vulnerability of each insect species to a heatwave accompanied by low humidity.

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