No less than heat, water is necessary to living beings, for it constitutes the greater portion of their protoplasm and plays a part in almost all their internal changes. Also, all organisms are sensitive to variations of humidity in the space surrounding them, and with a great number this sensitiveness takes the form of a directive orientation which is called hydrotropism.
We know the famous experiments made by Stahl, in 1884, on the hydrotropism of the fungi of the genus 2Ethalium (tanning fungus). The plasmodial mass of these plants, during the vegetation period, enters the tan, making for the humidity necessary to it, and remounts to the surface in a dry milieu when it is going to form its spores which serve its multiplication. Its hydrotropism is positive. This is the case with the beetles of the genera Haliplus and Hydro-porus. Wheeler (1899) had taken from a pool a tuft of aquatic plants where these insects swarm. He says:
As soon as the beetles could come out and disengage themselves from the plant they turned, with a common accord, toward the sea and to it directed their steps. As this was a distance of about twenty feet, the little creatures could not see the water, and I was led to believe that they had some means of perceiving a source of moisture and acted accordingly.
Aquatic bugs act the same way when they are taken from the place in which they live, and we know that the land crabs go a long distance to water when they are ready to place their progeny. The proper degree of humidity differs, moreover, with different species. Wheeler reports that Bembidium, Elaphrus, Omophron, and other small Coleoptera which bury themselves in the sandy beaches, leave their burrows and come out into the open air when one throws a little water on their strand. This is negative hydrotropism. It is well known to collecting entomologists, who use it in making captures.