TRACTOR PULL

Objectives:

  1. observe and quantify the pulling power of a bess bettle
  2. compare relative pulling strengths of an insect and a human
Background
Many insects are capable of lifting and moving many times their body weight. Observe, for example, ants carrying of a large piece of sandwich at your next picnic. How can they do this? Why can't humans? Basically it is a matter of "form and function". The body function of lifting weight is determined by the form or structure of the levers involved . In otherwords, strength depends on the arrangement of skeletons (exoskeleton or endoskeleton) and the muscles involved. Human beings have to use simple machines with mechanical advantages to approximate the strength of insects.

Materials:
bess beetles
scrap of carpet
cellophane tape
thread
digital balance
bag of pennies
petri dish

Beetle Movie
PROCEDURE:
Work in pairs. Quietly so as not to agitate the beetles

  1. Weigh the petri dish and record its weight on the data sheet
  2. Weigh a penny and record its weight
  3. Place a bess beetle upside down in the petri dish and weigh both. Subtract the weight of the petri dish and record the difference in weight
  4. Predict how many pennies the beetle will be able to pull on the petri dish sled and record it.
  5. Cut a piece of thread three about 20 cm ,long. Make a slipknot harness on one end. Slip the harness over the bess beetle and gently tug it around the middle of the body between thorax and abdomen. Tape the other end to the front of the petri dish so that the line of pull is parallel to the table.
  6. Place the beetle on the carpet and the sled on a smoth laab table top.
  7. When the beetle begins walking add pennies to the pertri dish, one at a time, until you find the maximum number the beetle can pull. (you may have to reposition the beetle periodically, but do not prod or push it)
  8. Remove the harness and return the beetle to the "woodpile"
  9. Compute the relative pulling power of the beetle by dividing the weight of the pennies by the beetle's weight
  10. Make the remaining calculations and answer the questions.
DATA
What is your prediction for how many pennies the beetle can pull? ________________
Complete:
a. weight of the petri dish ___________ grams
b. petri dish plus beetle ______________ grams
c. Beetle weight: a - b =___________ grams
d. penny weight ______________ grams

Experiment results:
a. maximum number of pennies pulled: ______________
b. Number of pennies x weight of a single penny = total penny weight pulled: _____________- grams
Total penny weight + weight of peetri dish = grand total weight pulled________________.

Calculations:
a. compute the pulling power of the beetle by dividing the grand total weight by the beetle's weight = _______________________ factor
b. If you had the strength equivalent to a beetle,  how many pounds could you pu?  Multiply your weight times the factor above. How many tons is this (2000 lbs/ton)?

Discussion:
1. How did your prediction compare to your results?

2.What explanation can you suggest for the huge difference between relative sttrengths in humans and the beetle?

3. Get the class data and examine the class average. What are some pissble reasons for the variation you see?

Based on a demonstration by: Robert Matthews, Dept of Entomology, Univ of Georgia, Athens GA 30602
NABT meeting Nov 7, 1991, Nashville, TN