Desiccation tolerance

Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or drought-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or behavioral adaptations to withstand these periods are necessary to ensure survival. In particular, insects occupy a wide range of ecologically diverse niches and, so, exhibit a variety of strategies to avoid desiccation.

In general, desiccation resistance in insects is measured by the change in mass during dry conditions. The overall mass difference between measurements before and after aridity exposure is attributed to body water loss, as respiratory water loss is generally considered negligible.

There are three main ways in which insects can increase their tolerance to desiccation: by increasing their total body water content; by reducing the rate of body water loss; and by tolerating a larger proportion of overall water loss from the body. Survival time is determined by initial water content, and can be calculated by dividing water loss tolerance (the maximum amount of water that may be removed without resulting in death) by water loss rate.

Insects with a higher initial body water content have better survival rates during arid conditions than insects with a lower initial body water content. Higher amounts of internal body water lengthen the time necessary to remove the amount of water required to kill the organism. The way in which body water content is increased may differ depending on the species.

The accumulation of glycogen during the insect larval stage has been linked to increased body water content and is likely a source of metabolic water during dry conditions. Glycogen, a glucose polysaccharide, acts as an oxidative energy source during times of physiological stress. Because it binds up to five times its weight in bulk water, insects with increased levels of body glycogen also have higher amounts of internal water. In general, insects selected for desiccation resistance also exhibit longer larval stages than those sensitive to desiccation. This increase in development time is likely a response to the environment, allowing larvae more time to accumulate glycogen, and therefore more water before eclosion.

Another possible source contributing to higher levels of initial body water in insects is hemolymph volume. The insect equivalent to blood, hemolymph is the fluid found within the hemocoel, and is the largest pool of extracellular water within the insect body. In the fruit-fly Drosophila melanogaster, flies selected for desiccation resistance also yielded higher amounts of hemolymph. Higher hemolymph volume is linked to an increase in carbohydrates, in particular trehalose, a common sugar found in many plants and animals with high desiccation resistance.Drosophila melanogaster flies selected for desiccation resistance show a 300% increase in hemolymph volume compared to control flies, correlating to a similar increase in trehalose levels. During periods of aridity, cells dehydrate and draw upon hemolymph stores to replenish intracellular water; therefore, insects with higher levels of this fluid are less prone to desiccation.

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