No animals can live without water, but some can get by with the water created in their own metabolic processes. The vast majority of terrestrial animals will drink water whenever they have the opportunity. This applies to both insects and vertebrates. The black rat, for example, does not have to drink, but does so anyway. The brown rat, however, cannot endure without water and always stays in the vicinity of some form of free flowing water. Among insects, there are several species that can live and multiply in rather dry locations. Mealworm beetles can survive in very dry flour where the water activity is only 0.01, corresponding to a relative humidity (RH) of 1%.
The insects that can survive in very dry places have developed various protective measures to prevent them from drying out. They may have vapour retardant barriers in the form of thick, wax impregnated exoskeletons. There may be a rational water economy, where the water is used for the dietary digestion in the intestines and the excretion of excess nitrogen will be absorbed and recycled. Opening and closing the breathing holes, depending on whether it is wet or dry in the ambient air is also a method. Finally, the insects react to moisture, or rather the lack of moisture, by seeking out places where evaporation corresponds to the surplus water, which they dispose of. One of the consequences of the latter is that hungry confused flour beetles will choose to stay in more humid air than confused flour beetles that have fed recently.
Pests that live in food do not react to the water rates in goods. Their reactions in a room are determined by the relative humidity, and in the product itself it is the water activity which they respond to. Minor differences in water activity have no significant influence on development time, but the differences can be perceived by insects and can determine where in the products or consignments that the insects gather.