As regards the lower animals, we do not yet know anything for certain about the localization of the sensory cells which func tion in these movements. In Crustacea, this faculty is distributed apparently among sensory hairs in the region of the joints; in insects we know that there are free nerve endings in the skin of the joints.
To the mechanical sense belongs also the sense of "strength." When a man lifts a weight, he notices exactly if it is heavy or light, and, according to this sensation, he regulates the amount of energy to be expended. The leech shows that a similar sense is present also in the lower animals. It has been mentioned already that this animal contracts when the anterior sucker is attached and the posterior sucker is free. When the animal is in this position we can force it to lift fairly heavy weights, which, under certain circumstances, it will support for hours together. (See fig. 5.) If we cautiously hold up the weight with one hand, the worm experiences a sensation of considerable relief, and alters the impulse it sends to its muscles. This is clearly proved by the fact that, as soon as we withdraw our hand again, the worm is drawn out by the weight which it formerly had supported.
Stimulation of the Nervous System by Mechanical Influ ences.—Every organism, in order to be able to move vigorously, requires constant stimulation from without. To man this state ment does not appear very credible, but it can occasionally be proved quite clearly in the lower animals. In the article on HEARING, we have described the statocysts of the lower organ isms, which usually act as balancing organs. It has been generally proved by experience, that the animals fall into an enduring state of debility after these statocysts have been removed by an opera tion. They are no longer able to make powerful movements; we can express mathematically the amount of diminution in the gripping power of the chelae of a crustacean caused by the removal of the statocyst. It is not yet determined for certain how this connection between the statocysts and the muscular strength is to be explained. Apparently, the mechanical stimula tion of the sensory cells of the statocyst by the statoliths affects the central nervous system of the animal, thus, as we may say, rousing it into activity in a somewhat similar manner to that in which coffee stimulates a tired man.
Flies furnish a particularly remarkable instance of this. In these insects, the posterior wings are transformed into oscillating clubs or "halteres," which, during flight, move simultaneously with the wings, with great rapidity (figs. 6a, 6b). If these clubs are removed or stuck down firmly, the insect is no longer capable of flight. It can neither rise from the ground nor main tain itself in the air. It appears from this that the active move ment of the halteres is essential. Formerly, it was thought that the halteres were a kind of balancing organs, and it was believed that the fly lost its power of balance after they had been removed. This, however, is excluded by the extremely minute size of the diversity. The rule that the cuticle is particularly thin where the sensory cells are situated (see fig. 1o) applies to all these creatures (insects, crustaceans, spiders). When a sensory cell is quite separated from the exterior by a layer of chitin, it is im possible that it can perceive chemical stimuli, and we may assume, with some degree of probability, that it is an organ for the perception of mechanical stimuli.
Among Crustacea the most general form in which these occur is the sensory hair; in insects we find in addition sensory cones halteres. Now, however, numerous sensory cells affected by mechanical stimuli have been found on the basis of the halteres. These are so arranged that they must be vigorously stimulated by the movement of the halteres, which, apparently, has this stimulation as its object. We may assume that this affects the nervous system of the fly in a similar way as stimulation of the statocyst affects crustaceans. The fly is able to send the necessary impulses to the muscles of flight only if this constant stimulation is present.
Wille has recently discovered a similar, and, considered as a whole, certainly still very problematical contrivance in the Brazil ian locust, Rhipipterix cizopardi. This insect bears sense organs on the under side of its hind legs; if these are put out of action by being stuck down, the insect is prevented from springing and flying.