This exercise examines the correlation between gravitational-force (g-force) tolerance and the sizes of organisms, emphasizing differences between vertebrates and invertebrates, particularly the effects of size and scale on biological processes. Students form a hypothesis based on background information and then test it by spinning subjects in a centrifuge. Class results can be graphed, analyzed, and compared to human tolerance. The activity engages students in scientific process while investigating the effects of physical forces on structure and function.

Introduction

Every day, we experience gravity that pulls us toward the center of the Earth at a rate of ~9.81 m/second2 (32 feet/second2). The force varies slightly with elevation; the force of gravity (G) is defined as 1 at sea level. Our heart works against gravity by generating pressure needed to pump blood to our brain. As we move rapidly, we experience increasing g-force and the heart has to work harder. At about 5× G, the heart cannot supply the brain with oxygen and we lose consciousness. Of course, we don's lose consciousness if g-force only lasts a few seconds, and humans survive much higher instantaneous g-forces. Just sitting down in a chair generates a g-force of ~10 (Cameron et al., 1992).

Exposure to modest g-forces during acceleration can cause unconsciousness and even death, posing problems for aircraft pilots, roller coaster riders, and space explorers. For this reason, large centrifuges are used to train astronauts. Air Force pilots, wearing special gear, can withstand ~9 G (Sevilla & Gardner, 2005); however, it is difficult to find the maximum forces that humans can withstand without putting lives at risk. Some g-force testing can be done with other vertebrate animals, but it is difficult to ethically find maximum g-force tolerance. Lukatch et al. (1997) found rats to tolerate 12 G, which suggests that smaller organisms may be less affected by G-force.

Their relatively small size allows insects to be less affected by gravity, giving them proportionally greater strength and ability to survive the forces generated by propelling their bodies over relatively large distances. Modern insects range in size from <1 mm (0.04 inches) to ~50 cm (22 inches) in length. Insect size is a result of constraints on the weight of their exoskeleton and their system of acquiring oxygen. Insects have a tracheal breathing system in which blood cells are not used to transport oxygen. Instead, oxygen and carbon dioxide are directly supplied or removed via diffusion (Hagner-Holler et al., 2004). Some small jumping insects can survive instantaneous g-forces of 500 to 700 (Burrows, 2006, 2009), but few tests of their ability to survive sustained g-forces have been conducted.

Investigating sustained g-force tolerance using invertebrates can lead to insights into physiology and physics. The diversity of insects allows comparisons across body types and life stages. Insects are also preferable to other test subjects because research suggests that invertebrates are unable to feel pain (see http://insects.about.com/od/insects101/f/Do-Insects-Feel-Pain.htm) and because they are not subject to the protocols necessary for experimentation with vertebrates. This experiment can be conducted on either live insects or insects that have been euthanized (using a kill jar or by placing in a freezer for a few hours). Additionally, experiments can be conducted with a maximum g-force of 1000 G, which is not lethal to most insects.

After conducting these experiments, students will be able to explain g-force and the relationship between centrifugal acceleration and g-force. The experimental results can be applied to discussion of the effects of g-force on humans and insects, and extended to engineering topics such as space exploration and vehicle design.

Process & Procedure

• (1)

Review basic insect structure and physiology; read materials about insect jumping ability, which can be found on the Internet.

• (2)

Acquire insects for testing of tolerance to g-force (materials can be provided by the teacher or collected by students). Create groups of insects that differ in size. Do you anticipate a difference in tolerance to g-force? Which of the groups do you predict to be the most tolerant to extreme g-forces? Which will be the least tolerant? Why? Create a testable hypothesis. For example, “Large caterpillars will be more resistant to g-force than small caterpillars because they have a thicker exoskeleton.”

• (3)

Review centrifuge operation and settings. If a scale is available, number centrifuge tubes and weigh each tube (first empty, and then with a test subject inside). If the insect is too small to register on the scale, an average mass can be found on the Internet (or based on Table 1). If insects are very active or jump, put them in a refrigerator for a few minutes.

• (4)

Place the tubes in the centrifuge, distributing the weight equally. Use 5 to 10 of each kind of insect as replicates. If you are using an odd number of tubes, use an extra tube with water for balance.

• (5)

Set the centrifuge to 500 rev/min and spin all tubes for 3 minutes. Remove the tubes and lay them on the table. Allow the insects to rest for 1 minute while you make observations. Make a table for your data: tube number, mass, acceleration (rev/min), and whether the insect has visible damage. Repeat the procedures for living subjects, increasing acceleration by 500 rev/min. Continue until no insects are living. If using euthanized insects, record the speed at which rupture occurs. If time is limited, use increments of 1000 rev/min.

• (6)

Measure the radius of the centrifuge rotor in millimeters, and go to http://www.endmemo.com/bio/grpm.php to convert revolutions per minute to g-force. Add g-force to the data table (Figure 1).

• (7)

Record maximum g-force survived and calculate average tolerance by summing speeds of all lethal trials and dividing by the number of insects tested.

• (8)

Compile class data; make a line graph with size on the x-axis and average g-force tolerance on the y-axis. Did insects tolerate more G-force than vertebrates? Was size important?

Table 1.
Size range of insects used in sample experiment.
Average Mass (g)Insect Used
0.001 Fruit fly
0.1  Small cricket
0.152 Small mealworm
0.407 Large mealworm
0.79  Superworm
Average Mass (g)Insect Used
0.001 Fruit fly
0.1  Small cricket
0.152 Small mealworm
0.407 Large mealworm
0.79  Superworm
Figure 1.

Schematic of the experimental procedures. Individual arthropods are placed into centrifuge tubes, weighed, and then tested by spinning for 3 minutes. They are allowed 1 minute for recovery, and then the test is repeated until force is no longer tolerated.

Figure 1.

Schematic of the experimental procedures. Individual arthropods are placed into centrifuge tubes, weighed, and then tested by spinning for 3 minutes. They are allowed 1 minute for recovery, and then the test is repeated until force is no longer tolerated.

Discussion

• (1)

Did the experimental subjects survive higher forces than insects might experience in nature? Explain.

• (2)

What physical observations did you make about the larger insects when they reached or surpassed their maximum tolerable g-force (Figure 2)? What do you think killed them?

Figure 2.

Insects that did not withstand g0 force. Membranes split, and hemolymph is often present.

Figure 2.

Insects that did not withstand g0 force. Membranes split, and hemolymph is often present.

Extensions

Additional hypotheses can be examined, including the orientation of the insects (head up or head down), feeding status, or hydration status. Students can compare different life stages of a species or compare insect tolerance to that of spiders or isopods. The experiment can also test adults of different insect orders (e.g., beetles vs. moths) and morphospecies (e.g., black ants and red ants). Test organisms can be collected from the schoolyard while conducting lessons on biodiversity (Table 1 and Figure 3).

Figure 3.

Relationship of size to mean survived g-force, using the organisms in Table 1.

Figure 3.

Relationship of size to mean survived g-force, using the organisms in Table 1.

References

References
Burrows, M. (
2006
).
Jumping performance of froghopper insects
.
Journal of Experimental Biology
,
209
,
4607
4621
.
Burrows, M. (
2009
).
Jumping performance of planthoppers (Hemiptera, Issidae)
.
Journal of Experimental Biology
,
212
,
2844
2855
.
Burrows, M. (
2013
).
Jumping mechanisms of treehopper insects (Hemiptera, Auchenorrhyncha, Membracidae)
.
Journal of Experimental Biology
,
216
,
788
799
.
Cameron, J.R., Skofronick, J.G. & Grant, R.M. (
1992
).
Physics of the Body
.
:
Medical Physics Publishing
.
Elwood, R.W. (
2011
).
Pain and suffering in invertebrates?
Institute of Laboratory Animal Resources Journal
,
52
,
175
184
.
Hagner-Holler, S., Schoen, A., Erker, W., Marden, J.H., Rupprecht, R., Decker, H. & Burmester, T. (
2004
).
A respiratory hemocyanin from an insect
.
Proceedings of the National Academy of Sciences USA
,
101
,
871
874
.
Haldane, J.B.S. (
1926
).
On being the right size
.
Harper's Monthly
(
March
),
424
427
.
Lukatch, H.S., Echon, R.M., MacIver, M.B. & Werchan, P.M. (
1997
).
G-force induced alterations in rat EEG activity: a quantitative analysis
.
Electroencephalography and Clinical Neurophysiology
,
103
,
563
573
.
Sevilla, N.L. & Gardner, J.W. (
2005
).
G-induced loss of consciousness: case-control study of 78 G-LOCs in the F-15, F-16, and A-10
.
Aviation, Space, and Environmental Medicine
,
76
,
370
374
.