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Benthic Macroinvertebrates

By JoAnne Bartsch


This is an open ended field exercise in which students sample the benthic macroinvertebrate populations of a given stream and determine community structure through the use of two indices. It is part of a semester course in stream ecology in which students survey and describe a stream ecosystem.


By the end of this unit, I hope the students will be able to:

a. identify major structures on a generalized insect

b. use a dichotomous or picture key to classify benthic MACROINVERTEBRATES to the level of family

c. identify by sight the primary indicator organisms in a stream

d. implement a quantitative method of sampling population size

e. apply the Chandler Index and a species diversity index to a stream community

f. analyze water quality in a stream community based on the benthic macroinvertebrate populations


  1. insect collecting nets - kicknets preferable
  2. insect identification guides

Information for the Teacher

A number of methods for determining water quality based on biologic characteristics have been established. Indicator organisms most often used include algae, bacteria, fish and macroinvertebrates. I prefer benthic macroinvertebrates for a number of reasons. They are easily collected and handled by students; they require no special culture protocols. They are visible to the naked eye and their characteristics are easily distinguished by students. They have a variety of fascinating adaptations to stream life. There are a number of excellent references available which are written in lay terms. Certain benthic macroinvertebrates have very specific tolerances and thus are excellent specific indicators of water quality.

Prior to the start of this unit, the class has identified a stream or river for analysis. We have made at least two trips to the stream for general observations and chemical testing.

This unit is divided into three major activities. These are: Macroinvertebrate Sampling and Identification, Macroinvertebrate Monograph and Application of Indices.

Macroinvertebrate Sampling and Identification

1. Introduction to insects:

The students need to be aware of basic insect structures before they can classify any insects they catch. Structures which need to be stressed include head, eyes (compound and simple), antennae, mouth (no emphasis on parts), segments, thorax, legs and leg parts, gills, abdomen,etc. They also need to be familiar with insect metamorphosis - both complete and incomplete - as most of the macroinvertebrates we will meet are larval or nymph stages.

2. Introduction to sampling

I employ a qualitative method of sampling using a kicknet. There are sampling devices and procedures which are more quantitative in nature. The students need to understand the importance of standards in the collection process. We come to a class agreement on the standards we will use.

3. Sampling

We sample at several different stations along the stream. Students should follow the agreed upon procedures for sampling and keep everything they find. Stream insects do not live long out of a stream. We preserve in isopropyl alcohol. This procedure can be modified if you prefer not to kill specimens.

4. Identification

This is accomplished in the lab using references listed in the bibliography. It is necessary to classify some organisms to the level of family in order to use the Chandler index. No classification is needed for the diversity index.

5. Recording of data

It is necessary to know how many individuals of each species were found at each station.

Macroinvertebrate Monograph

The end result of this activity is a descriptive monograph, with drawings and text, of a single stream insect found during the sampling process. The students are required to write/draw their monograph based solely on their own observations. They may not use any outside resource although they may ask questions of other students. I ask my students to look for and describe adaptations for stream life. They are required to draw a dorsal, ventral and side view of the entire insect. They are required to draw adaptations of their insect for locomotion, respiration, food gathering, camouflage and sensation. I ask for closeups of these adaptations with emphasis on specific stream adaptations - gills, hooks, etc.

Students are also required to write about the adaptations they have found and to hypothesize about the natural history of their insect. For example, a student might venture a guess as to an insects diet based on the structure of its mouthparts. Finally, I ask students to propose a list of ten questions they would like answered about their insect.

Application of indices

Students apply two indices to the populations they have found. These are the Chandler Index, which uses indicator species to classify water quality, and a diversity index, which uses community structure to classify water quality. Information on the Chadler Index can be found in the Hellawell or Phillips reference. Essentially, each species is assigned a value based on its tolerance for poor water and its population size. All values are totaled at each station to give a single Chandler value. Higher Chandler values indicate higher water quality.

The diversity index does not require any classification; students need to be able to identify insects as belonging to different groups only. It is not necessary to assign names to the groups. All insects collected at each station are pooled and drawn out, one at a time and at random, and placed side by side.* When all the insects have been placed in a line, the student begins at the left side of the line and counts the number of times that different species lie next to one another. For example, let the letters a - e represent 5 species found in a stream. There are 20 individuals in this sample. The insects are laid out in the following order:

          a d d b a a a c e e d d d b b b b c a a e

Different species lie next to one another 10 times

(ad,db,ba,ac,ce,ed,db,bc,ca,ae). There are 10 runs in the sample. To determine the diversity, divide the number of runs by the number of individuals. In this case, the diversity is 0.5. Higher numbers indicate more diverse communities and - presumably - higher quality water.

We also look at the dominant species at each site and discuss its sensitivity to poor water quality. The best indicators of high water quality are stonefly nymphs, mayfly nymphs and caddisfly larvae. I usually ask the students to measure relative abundance of these organisms and graph their data.

*Note: I have had problems with insects falling apart as students handled them for the diversity index. I have substituted popsicle sticks with identifying letters quite successfully


I use nonobjective methods to evaluate the students on this unit. I evaluate two areas - the monograph and the use of indices.

The monograph evaluation is done by both students and the teacher. I develop an evaluation form which lists the aspects of the project I am most interested in grading and a 1 - 5 scale for evaluating each aspect. Each student will evaluate his/her project and three others. I then evaluate each project using the same scale as the students and compile the scores.

The students are required to write a paper comparing the water quality at the different sites using the biologic information collected. They must discuss the importance of the indicator species as well as the significance of the indices used. I also ask them to suggest areas for further research.


The following are useful references to have handy:

Any insect field guide - Peterson's, Audubon
Bland, Roger. How to Know the Insects. Wm C. Brown.
Chu, ?. How to Know the Immature Insects. Wm. C. Brown
Lehmkuhl, Dennis. How to Know the Aquatic Insects. Wm C. Brown
McCafferty, Patrick. The Fisherman's Guide to Aquatic Insects.
Hellawell, J.M. Biological Indicators of Freshwater Pollution and Environmental Management. Elsevier Applied Science Publishers. New York.
ISBN number 1 - 85166-001-1
Phillips, David. Quantitative Aquatic Biological Indicators. Elsevier.

I also recommend subscribing to The Journal for Freshwater Ecology . Contact Joseph Kawatski, Viterbo College of LaCrosse, Wisconsin.

Check out your local fly fishing store for even better picture guides and other help. Fly fishermen are excellent resources.

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