ABSTRACT
A complete picture of the locomotory mechanism of any animal involves a knowledge of the forces required to overcome the external resistance of the animal’s environment. In the case of certain terrestrial animals (the earthworm in particular) it is possible to measure these forces by means of a modified form of the apparatus originally designed by one of us (H. W. L.) for the study of locomotion of the snail.
The apparatus (Fig. 1) consists essentially of a horizontal bridge (B) mounted on knife-edges (K) in such a way as enables it to be displaced either forwards or backwards in respect to two fixed platforms (P). The bridge is provided with counterpoise weights (W) so that, when a displacing force is applied, the bridge reaches a position of equilibrium when the applied force is equal and opposite to the restoring couple of the weights; the sensitivity of the balance can be varied by changing the masses of the weights and their distances from the point of suspension of the bridge. In most experiments a horizontal movement of 1 mm. was equivalent to an applied tension of 3 g. : the horizontal movement of the bridge was magnified by a hinged pointer (p), and a complete record of the forces exerted on the bridge, when an earthworm moved over the gap between the bridge and the fixed platform, was obtained by allowing the end of the pointer to write on a smoked drum. A typical record is shown in Fig. 2. The analysis of such records necessitates an exact correlation between the magnitude and direction of the forces exerted on the bridge with particular positions of the “foot” of the worm relative to the edge of the bridge and to the edge of the fixed platform. In order to provide such a correlation, cinematograph records were taken in such a way that the tension recorded by the bridge was known for successive positions of the locomotory wave relative to the gap between the bridge and the platform. The interpretation of such records is, in the case of an intact worm, slightly complicated by the presence, at certain phases of the movement, of two regions of fixation to the ground,1 and it is therefore convenient to consider, first, a record, such as that shown in Fig. 2 a, derived from a worm whose posterior segments had been amputated two days prior to the experiment, and which therefore possessed only one point d’appui during the whole of its contractile cycle. Such records illustrate the following facts: (i) when the anterior end of the worm moves forward, owing to the passage of a wave of circular contraction, a frictional force of the order of 3 ·0 g. is generated, the bridge being displaced in the direction of motion of the worm ; (ii) as soon as a point d’appui is established on the bridge, the bridge begins to be displaced in a direction opposite to that of the worm relative to the ground. This displacement is due to the contraction of the longitudinal muscles of segments lying immediately posterior to the point d’appui, and the extent of the displacement is a measure of the frictional resistance between the posterior part of the worm and the fixed platform. The propulsive thrust of the circular muscles and the tractive force of the longitudinal muscles vary with the size and activity of the worm, but in all cases they are approximately equal to each other and vary from 2 to 8 g. Records from intact worms (e.g. Fig. 2b) do not differ from those obtained from posteriorly amputated preparations, and it may, therefore, be assumed that data derived from the latter are applicable to intact worms moving under normal conditions.
By the use of a bridge approximately equal in length to a complete set of fully contracted segments (i.e. about 7 ·0 cm.) between two fixed platforms, and observing the displacement of the bridge when the “foot” is completely on the bridge, it is possible to measure the total friction of the whole worm when moving over the ground. This force was found to be of the order of 8 g. for a worm of 25 cm. in length (when fully extended) moving at an average velocity of 0 ·4 cm. per sec.
The “efficiency” of a worm’s movement is greatest when the external friction between the body and the ground is at a minimum. Such conditions exist in the anterior region of the body and in the neighbourhood of the clitellum, since these regions often move forward without being in contact with the ground at all; in the more posterior regions external friction is reduced by the secretion of mucus. The essential elements for progression are (i) the fixation of the fully contracted segments, and (ii) the passage of waves of muscular contraction.
Records such as are illustrated by Fig. 2 obviously give no indication of the total force exerted by the muscles of the worm, and it is extremely probable that most of the energy of contraction is utilized in overcoming the internal resistance of the body.
After suitable modification of the sensitivity of the bridge, the apparatus can be used for measuring the propulsive forces of a great variety of animals; reference to the results so obtained will be made in subsequent publications.
SUMMARY
An apparatus is described whereby it is possible to measure the propulsive forces generated during the locomotion of small terrestrial animals.
It is shown that the propulsive thrust of the circular muscles and the tractive force of the longitudinal muscles of an earthworm are approximately equal to each other and vary from 2 to 8 g. according to the size and activity of the worm.
The total frictional resistance overcome by a worm 25 cm. in length (when fully extended), moving at an average velocity of 0 ·4 cm. per sec., is approximately 8 g.
Since each wave of longitudinal contraction starts at the anterior end before its predecessor has passed over the tail of the animal, there are, during this phase of activity, two point d’appui or “feet”—whereas at all other phases there is only one.