Among vertebrates, bats are able to perform a, perhaps, even greater feat. As was known already to the naturalists of the i8th century, even blind bats are able to avoid all obstacles which may be placed in their path.
An opposite condition to that discussed hitherto prevails in not a few cases in which the animal seeks to establish contact with a solid object, and is uneasy if it cannot do so. This phe nomenon has been given a special name, and is known as "thig motropism." In most animals the soles of the feet are usually placed in constant contact with solid bodies. The vigorous at tempts at "righting" which are made by almost every animal if it falls upon its back, until it regains its feet once more, are due, in many cases, to the effort to bring the feet again into contact with solid bodies. This may be seen particularly well in insects. If we put a caterpillar upon its back it immediately rights itself. If, however, we place between its feet a twig to which it can adhere, it remains peacefully upon its back, and even eats in this position. The following experiment is easily carried out, and shows this every clearly. A thread is gummed firmly on to the back of a fly, and is then attached to a stand, the feet of the insect being allowed to hang down free. If, now, a small ball of paper is placed between them, the fly begins at once to run about upon it.
The normal running movement, therefore, follows contact of the feet with the solid object. In leeches, contact stimuli are of particular importance in locomotion. Uexkuell showed that in these animals a quite definite co-ordination exists between the action of the suckers and the contraction of the muscles of the body. When the leech is adhering by means of its posterior sucker, it stretches out its body at full length, searching thus for a suitable surface on which to fasten its anterior sucker. When the anterior sucker is attached, the posterior sucker relaxes its hold, and immediately a reflex contraction of the longitudinal muscles of the body takes place. Both phases, taken together, give rise to the characteristic looping motion of the leech. (See
fig. 4.) If, however, we throw the leech into water without giving it an opportunity of attaching itself by one of its two suckers, swimming movements at once take place.
Thigmotropism, however, appears also in quite another form. It was thought at first that many animals which are in the habit of dwelling in narrow tubes (such as many worms), or in crevices in wood, or under bark are led to seek out their hiding places through an aversion to light. The truth of the matter is that such an animal feels comfortable only if its body is in con tact, as much as possible, on all sides, with solid surfaces. Thigmotropic worms will also creep into glass tubes, and some insects hide between glass slides, in spite of being fully exposed to daylight when in these retreats. Under natural conditions, the thigmotropic animal thus avoids the covetous eyes of its enemies, since in nature there are no transparent solid objects.
Sensitiveness to mechanical stimuli has a very important use in the sense which in higher animals makes them aware of the relative position of the parts of the body. To the majority of human beings it certainly appears quite as a matter of course that they are able to take hold of their noses with their hands. Science teaches, however, that this is pos sible only through stimulation of numer ous sensory cells in the skin, which are pulled or pressed when the arm or the hand is moved. If the skin is narcotized, the power of touching any particular part of the body is lost.
It is of interest that some of the lower animals, particularly arthropods, possess a similar sense. Insects and Crustacea show this in the very vigorous way in which they clean the limbs and the body, mak ing use of legs specialized for this purpose. The precision with which the decapod Crustacea, after moulting, place fresh statoliths in their statocysts is astonishing.
(See also HEARING.) They seize grains of sand, or similar ma terial in their delicate pincers, and are able to introduce them into the statocyst through its minute opening.