The question of why insects are not as dominant at sea as they are on land is ideal for teaching how to form and evaluate scientific questions (and other sorts of questions, come to that). More specifically, it provides a great opportunity to explore how evolutionary arguments are made and assessed. Because the hypotheses involve questions about chemistry, evolution, entomology, and marine biology, this problem illustrates how many subjects and perspectives may be necessary to address a single issue. Of course, given that we haven’t been around to study this issue over the past 300 million years, we can’t know the correct answer. But we can look at different hypotheses (or guesses, if you prefer), and decide which are most likely to be true. In fact, just as scientific theories advance or fail based on consensus in the scientific community, your students can decide themselves which hypotheses fail and succeed.
We’ve used the marine insects question with various student groups (elementary, high school, undergraduate, and graduate classes). The key issue in dealing with these different educational levels is to determine appropriate educational goals, provide sufficient background so students can address the question, and engage students in the process of forming and evaluating hypotheses. Here, we provide some of these goals, support materials (beyond what are on the rest of the web site), and some ideas for teaching activities.
Secondary educational objectives might include understanding concepts such as osmoregulation, water pressure, evolution, and (ecological) competition.
Insects are the most numerous, most diverse, and most ecologically important terrestrial animals. They occur in virtually all terrestrial habitats, excepting the most extreme (the arctic, Antarctic, and peaks of the highest mountains). Over much of the earth insects also form an aerial sea up to many of thousands of feet into the atmosphere.
Key features of insect biology are that insects have an exoskeleton, three body regions (head, thorax, and abdomen), and six jointed legs. Insects must molt as they grow, and immature forms may resemble the adult (incomplete metamorphosis) or be completely distinct in appearance and habits (complete metamorphosis). Insects have passive respiration (air diffuses into the body through a series of tubes called tracheae and the insect "blood" has no oxygen carrying capacity). Despite passive respiration, many insect species have cool mechanisms for breathing under water, including siphon tubes, gills, and schemes for diffusing oxygen from water into a bubble the insect carries on its body (called a plastron).
Understanding the biological classification of insects is important, because it helps in denoting ecological and evolutionary commonalties among species. Key levels for insects are the Order and Family, as these divisions often provide useful levels for generalizing about insect morphology (structure and function), physiology, and ecology. The levels of classification are:
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Vocabulary
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A scientific hypothesis is an explanation that’s a guess. The value of any hypothesis is how well it accounts for what we know to be true. There’s nothing wrong with wild hypotheses, but their longevity depends on how well they stand up to facts. If they don’t fit the facts, they die (as they should).
Testing hypotheses is big business for scientists. When you do experiments, where you (at least in principle) control every factor except the one you are examining, hypothesis testing usually involves using statistics. Statistics are important, because they provide a mathematical statement based on probability theory of how likely a given outcome is. By convention, scientists tend to say that unless an experimental result could have occur only 1 time in 20 (5% of the time), it probably is not a real effect.
Unfortunately, given that we don’t have planets and hundreds of million of years to experiment with, we have to take a different tack with the marine insects question. Here, as in much evolutionary argumentation, we try to form plausible hypotheses, and then try to find evidence that supports or disproves these guesses. Once a hypothesis is formed, we look for current examples that would contradict it. For example, the argument that insects can’t survive in the ocean because of water pressure doesn’t seem so good when you realize one insect species survives at a depth of 1,300 meters! This is a form of counter example. Because at least one species can survive a great depth, it implies that other insects could have evolved to do so. Eliminating hypotheses by counter examples is a powerful approach in assessing hypotheses.
Counter examples are a type of comparison (comparing one species’ biology with what might be possible for the group). Often comparison provides a mechanism for supporting a hypothesis. In our marine insects example, we compare insects in the oceans with insects in fresh water. We find that lots of insect species live in fresh water, but almost none do in the oceans. Compare: what is different about the two habitats? If it isn’t something physical (such as salinity or water pressure), maybe it has to do with biology. There are lots of small crustaceans in the ocean, but not so many in fresh water. OK, maybe the crustaceans beat out the insects. Is there any other evidence? Well, the fossil record shows that the crustaceans appeared many millions of years before insects. Like shopping for Tickle-Me Elmo (or whatever is the current faddish toy), those who get there first, win. And, if the crustaceans are out-competing insects, this fits with another theory (a type of strong hypothesis), the competitive exclusion principal. (Competitive exclusion is an ecological theory that two species can’t both have identical ways of making a living [occupy identical niches], because one will inevitably displace the other.) Competitive exclusion really isn’t evidence for or against our competition hypothesis with marine insects, but it does show our hypothesis doesn’t contradict a widely held principal, which is good. Does any of this prove competition with crustaceans is the reason for the lack of marine insects? Not really, but (to us) it is the best explanation to fit all the available facts.
This last point is very important. Theories are really hypotheses that have stood the test of close examination and time. It is possible to disprove theories, but in most instances it is not logically possible to prove a scientific theory. We can get close, but that is not the same as certainty. Consequently, the (ignorant) argument that something (like evolution) is "only a theory," ignores how the entire business of science works.
One other way we can test hypotheses is by making predictions. This is harder with evolutionary questions, because typically we can’t look at evolutionary processes over the long term. However, we might design small experiments where we look at competition among fresh water crustaceans and aquatic insects, and in these experiments we can predict how competition should produce different results. Again, such experiments wouldn’t prove or disprove our hypothesis, but they might support aspects of it or cause us to modify our ideas. In a nutshell, that’s science.
The marine insect question works best by engaging the students in a process of active inquiry, developing hypotheses, and then putting their hypotheses to the test. For this exercise, the process is more important than the specific outcome, because it is the process that teaches the students how to approach scientific questions. Listed below are a few of the procedures you might try.
We’re still experimenting and adding to the marine insects site. As you try the marine insect question in your teaching, we’d be interesting in hearing how it went and in any ideas you and your students have in making it a successful educational experience. Good luck.
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