Windows into Wonderland

Pre-Trip Lesson Plan: Mystery of the Missing Bugs




The student will:

  • Perform a simulated survey that demonstrates how biological indicators are used to gauge the health of a stream.
  • Perform six different calculations with 100% accuracy.
  • Assess the “water quality” of their sample with 100% accuracy.
  • Comparatively determine the sample site that indicates high pollution.


  • Attachment A - Mystery of the Missing Bugs Worksheet (56KB pdf)
  • Large bag of M&Ms
  • Bag of macaroni noodles
  • Box of Cheerios
  • Bag of beans
  • Small plastic bags
  • Paper
  • Pencils


Animals are often more finely-tuned than humans to changes in an environment. At one time, coal miners used canaries to detect the presence of dangerous gases because these birds are sensitive to shifts in air quality. Similarly, aquatic macroinvertebrates have differant tolerances to water pollution. The health of a stream can be judged by the macroinvertebrate species that are present and may indicate shifts in an environment before such changes are easily recognizable to humans.

Both macroinvertebrate species that are sensitive to and those tolerant of pollution can live in a stream with good water quality. With increased pollution however, fewer sensitive species will be found. If the stream’s water quality is very poor, only those macroinvertebrates tolerant of pollution will survive. EPT is an abbreviation for Ephemeroptera, Plecoptera, and Trichoptera (mayflies, stoneflies, and caddisflies), insect groups that are largely intolerant of pollution. If many EPTs are present, the water quality is probably good. Macroinvertebrates capable of surviving in very polluted water include aquatic worms, leeches, and midge larvae.

The Hilsenhoff Biotic Index, or HBI, measures the health of a stream by assigning tolerance values to macroinvertebrates which range from 0 for organisms that are very sensitive to organic wastes to 10 for organisms that are very tolerant of organic wastes. The biotic index of a stream site is found by collecting a sample of benthic macroinvertebrates, then multiplying the number of individuals within each separate species by the tolerance value of the species. The resulting products are then added together and this sum is divided by the total number of organisms in the sample.

Students will be given the problem of locating the most polluted site. M&Ms will represent species very sensitive to pollution, Cheerios will represent species semi-sensitive to pollution, noodles will represent species semi-tolerant of pollution, and beans will represent species tolerant of pollution


The instructor will:

  1. Prepare small plastic bags (one for every two students) filled with a variety of M&Ms, Cheerios, noodles, and beans. There should be at least 30 pieces in each bag. All but one of the bags should be composed of each of the four groups, though care should be taken to insure that the bags do not contain a majority of beans or noodles. One bag, however, should contain very few M&Ms, few Cheerios, and be composed of a majority of noodles and beans.
  2. Tell students that the health of a stream is often measured using macroinvertebrates as bio-indicators and give an explanation of the Hilsenhoff Biotic Index as provided in the Background section.
  3. Tell students that pollution has been detected in a small stream near their home and samples have been taken from a number of different sites. Their job is to locate the most polluted site based upon the “macroinvertebrates” collected from the different sample sites.
  4. Divide the students into groups of two and distribute a copy of Attachment A and one bag to every group. Tell students that each bag represents a different sample site and the M&Ms, Cheerios, noodles, and beans represent different macroinvertebrates found in the sites.
  5. Write on a blackboard that for the purposes of this exercise, the M&Ms represent species that are very sensitive to pollution and have a tolerance value of 1. Cheerios represent species semi-sensitive to pollution and have a tolerance value of 4. Macaroni noodles represent species semi-tolerant of pollution and have a tolerance value of 6. Beans represent species that are tolerant of pollution and have a tolerance value of 9.
  6. Direct the students to empty their bags and divide their contents into four different groups: Group 1: Sensitive to pollution, Group 2: Semi-sensitive to pollution, Group 3: Semi-tolerant of pollution, Group 4: Tolerant of pollution. Write the numbers in each group on Attachment A.
  7. Direct the students to multiply the number of items in each group by their respective tolerance values, then add the four products together. Students should record this information on Attachment A.
  8. Instruct students to calculate the biotic index of their sample by dividing the sum of the products in Step 6 by the total number of “macroinvertebrates” in their sample. This number should be recorded on Attachment A.
  9. Ask students to compare the biotic index of their sample with the chart on Attachment A and identify the water quality of their sample site.
  10. Direct each group to share the biotic index and water quality of their sample site with the rest of the class.
  11. Ask students to record on their worksheets which group has the sample from the most polluted site based on information provided from all the groups.


Mystery of the Missing Bugs Rubric (41KB pdf)


If time and resources allow, use this exercise as an introduction, then conduct an actual macroinvertebrate stream assessment. Numerous assessment plans and identification keys are available on the web, for example, Biosurveying 101 from the US Environmental Protection Agency.

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