Contacts and the pressure that results may irritate the protoplasm and, at least by contraction or extension, modify the physiological state of its substance.
When contacts are produced by currents of air, the animal which reacts to their stimulation orients itself in their direction, even, and sometimes goes against them, sometimes with them.
In the two cases one finds oneself in the presence of anemotropism and we say that anemotropism is positive in the first case and negative in the second. Positive anemotropism is common with flying insects, above all with the flies and the midges. Wheeler has noticed numerous Diptera of these two groups:-in Bibio albipennis, which comes out in clouds in the spring, in the chirono-mids which gather in swarms to dance in the sunlight, with the syrphids, and with the male of an anthomyid, Ophyra leucostoma. All these Diptera face the wind and change direction with it. The males of Ophyra, especially, react to the aerial excitant with a military precision. As with all the tropisms, the orientation changes direction according to the intensity of the stimulus. The migratory locusts of the Rocky Mountains, and without doubt also those of other countries, face the wind when it is very strong, while their ane-metropism becomes negative when the breeze is weaker.
The reaction to currents of water is not less evident, but is almost always negative. This form is seen in the plasmodes of the tannery fungus, in many aquatic plants, in the spercnato-zoids of certain animals and notably in the Palu-dinas, and finally in numerous fish, especially in those which normally swim up the rivers from the sea (salmon, anguillas, lampreys, etc.). These phenomena of rheotropism are common with the articulates, which group Dewitz (1899) has studied in this regard with great care. They are very indefinite with the water-bugs of the genus Nepa and with May-fly larvae, and they manifest themselves in negative form with Gammarus and in Notonecta glauca, which is a water-bug which swims upside down. Caddis-fly larvae always react in the same way; since they live in fixed cases, they cannot mount the current and they con-tent themselves with holding the opening of the case against it. But it is above all with the water-bugs of the surface that rheotropism is especially accentuated. Paddling upon the liquid mirror with their great legs, these Hemiptera, (Hydrometras, Velias, etc.) face the current and direct themselves in other ways only in places where the water is still. Dewitz writes:
When I saw a company of Hydrometras I sometimes carried out the following natural experiment, in order to put their rheotropism to the proof. In front of them I made a barrage with a plank, which turned the current and sheltered the water-bugs from its influence. Instantly there were no more efforts to swim against the current; behind the protecting barrier they turned irregularly in all directions. But, the plank lifted and the current liberated, the animals arranged themselves like soldiers and all together worked their legs again to react against the current.
These are observations and an experiment which it is easy to repeat anywhere.
With solids the result of the stimulus is to render the contacts shorter or to avoid them as much as possible. That is called stereotropism. It is negative with the amoebas, whose pseudopods are drawn in at the approach of a solid. As Dewitz has shown with cockroaches and Massart with frogs, it is positive for the spermatozoids; for in the fluid in which they move these reproductive elements rest against solid surfaces.
The researches of Miinsterberg (1906) on an American crawfish, Cambarus affinis, have shown the positive stereotropism of this crustacean which seeks contact with solid bodies and holds it naturally in the galleries it has dug or under stones. Our European crawfish (Astacus fluvia-tills) is surely endowed with the same tropism, for it has similar habits. Positive stereotropism is equally wide-spread among the nocturnal moths. In repose these insects hold themselves almost always close to walls and tree trunks in a plane on which it is not easy to see them. Ordinarily this attitude is attributed to negative phototropism, but Loeb has shown that it is not due to this. He says:
I placed some of these animals (noctuid moths of the species Amphipyra pyramidoides) in a box of which one-half was covered with an opaque substance and the other with transparent glass. I covered the bottom with fragments of similar glass which rested on little blocks and which wer(e just far enough from the bottom to allow the Amphipyra to creep under them. But the moths got under these little bits of glass, where their bodies were entirely in contact with the solid bottom, and this not only in the dark parts of the box, where they were hidden from their enemies, but in the same way when exposed to the direct light of the sun. The same reaction occurred when the whole box was shaded. Nothing but stereotropism could bring this about.