- The movements of many species of animals are influenced by gravity. Some simple experiments on this type of behavior can be performed with terrestrial snails, slugs, isopods, and other slow-moving invertebrates.
MATERIALS:
Board, Tape, Ruler, Protractor, Stopwatch
Several sheets of typing paper, compass, pencil
Books to incline board
Several terrestrial snails or other slow-moving invertebratesPROCEDURE:
1. Make several circle graphs to use in measuring the movement of the animal. Draw a circle eight inches in diameter in the center of the paper with the compass. Divide the circle into four equal sections with a horizontal and a vertical line.
2. Attach a circle graph to the board with tape. Make sure the horizontal line is level with the board.
3. Tilt the board at a 20 degree angle from horizontal using books as supports. Mark the vertical axis of the circle graph that is up hill with an arrow head at its end. Measure the angle of the board to your desk top with the protractor and adjust the books as necessary to bring this angle to 20 degrees.
4. Place your test subject, designated animal number 1, at the intersection of the lines on the graph paper in the center of the circle. If you use a subject that may roll off, such as an isopod, stick 2 pins just below the center point to rest it on till it takes off. Start timing the subject when it starts to move, using the stopwatch. Observe the path of the animal as it moves out of the circle. If the subject is a snail or slug, it will leave a slime trail. Make a mark at each point of the slime trail where the subject changed direction or angle of movement in relation to the horizontal line. If the subject is one that does not leave a slime trail, make a mark just behind the subject each time it changes direction or angle of movement in relation to the horizontal line. Be careful not to touch or bother the subject, so you will not negatively effect its movement. Record the time required by the subject to reach the edge of the circle on the DATA TABLE
5. Using the ruler, draw a pencil line from the center of the circle where the subject started, to the perimeter of the circle approximately the mean (average) path of the animal. The line of travel of the subject will almost always be irregular and wavy. Your "mean path" line should divide this irregular path so that about half the total area between the line and the actual path of the subject will be above the line and half below the line. This is called Finding the best line of fit, and is a simple form of averaging.
6. Measure the angle of the mean path from the horizontal line down through the origin, using a protractor, and record this angle in the DATA TABLE. Because the force of gravity is down, angles below the horizontal line are positive while angles above the horizontal line are negative.
7. Replace the circle graph with a new one and re-adjust the angle of the board to 40 degrees from horizontal and repeat the experiment using the same subject. Record your results as before on the DATA TABLE.
8. Replace the graph paper again, re-adjust the angle of the board to 60 degrees, and repeat the experiment with the same subject. Record your results.
9. Repeat with an 80 degree angle and the same subject and Record your results. 10. Repeat the experiment for animal number 2 with a new subject, of the same species, and using angles of 20, 40, 60, and 80 degrees. Record your data in the DATA TABLE as before.
DATA TABLE:
Animal No.____
Angle of Board__________Mean Angle of Movement__________Time to Escape Circle___
20 degrees_________________________________________________________________
40 degrees_________________________________________________________________
60 degrees_________________________________________________________________
80 degrees_________________________________________________________________
GRAPHING:
- When you have collected all your data, prepare a graph for the first subject with the angle of movement plotted as the ordinate (Y axis) against the angle of the board as the abscissa (X axis). Remember: If you have negative values on your Y axis, zero can not be at the origin. Make a similar graph of the time required to reach the perimeter as the ordinate, against the angle of the board as the abscissa. Repeat graphs for subject two.
QUESTIONS:
1. Did your subjects move toward or away from the pull of gravity?
2. Does the angle of the board influence the angle of movement? How?
3. Does the angle of the board influence the amount of time required to reach the perimeter of the circle? In what way?
4. Was the behavior of the subjects the same? How did they differ? Why? If they differed, were their responses basically similar or basically different?
5. Does your data indicate that the subjects you tested are positively geotactic (move toward gravity) or negatively geotactic ( move away from gravity)? Why or why Not?
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- When you have collected all your data, prepare a graph for the first subject with the angle of movement plotted as the ordinate (Y axis) against the angle of the board as the abscissa (X axis). Remember: If you have negative values on your Y axis, zero can not be at the origin. Make a similar graph of the time required to reach the perimeter as the ordinate, against the angle of the board as the abscissa. Repeat graphs for subject two.
RESPONSES OF SOME
INVERTEBRATES
TO GRAVITY
