PURPOSE:  To identify aquatic insects in winter, and describe their adaptations for survival in the riffle and pool areas of a small stream.

MATERIALS: Field -- suitable winter clothing, waterproof boots or waders, rubber gloves, aquatic net, 2 labeled collecting jars, clipboard or other firm writing surface, pencil, meter stick, watch with a second hand or stopwatch, thermometer, white enamel pan.  Lab -- forceps, watch glasses or petri dishes, stereoscope or hand lens, Key to Aquatic Insects.

INTRODUCTION:  In northern climates, there are more insects living in small streams in the winter than there are on land.  In fact, a small stream usually has more insects in winter than it does in summer. Some insects live in the stream throughout their life, but most have immature stages called larvae or nymphs that emerge from the water as adults in the warmer months.  In winter, immature aquatic insects are abundant and active in streams. They continue to feed and grow despite the cold temperatures.  

Aquatic insects live on the bottom or substrate of a stream. Riffles or erosional zones are found where the water current is fast enough to sweep most of the fine particles away from the bottom.  Pools or depositional zones occur where the current is slow and sediments collect on the bottom.  You can often see alternating riffles and pools along the length of the stream or from one side of the stream to the other.  The pattern of riffles and pools along the course of a stream is an example of what scientists call a "mosaic" of microhabitats.  Differences in these microhabitats greatly influence the kinds and numbers of organisms that live there.

In this activity, you will collect, identify, and count aquatic insects from an erosional zone and a depositional zone of a small stream.  You will describe their structural and behavioral adaptations for survival in these microhabitats.

PREDICTION:

1.  Which microhabitat (riffles or pools) do you think has the greatest number and diversity of aquatic insects?  Explain your ideas.

PROCEDURE: 

FIELD PROCEDURE:  Work in groups of three or more. Dress properly for winter weather.  You should wear waterproof boots or waders.  Rubber gloves are recommended. If you use your bare hands in cold water be sure to dry and warm them often!

A.  Go to a riffle area of a small stream where the water is shallow and fast moving.  Approach your sample site from downstream to avoid disturbing the area.  On your data sheet, record the physical characteristics of the riffle as follows:

B.  Collect invertebrates within the riffle, using the following methods:

C. Go to a pool area of the stream where the water is deeper and slow moving.  On your data sheet, record the physical and biological characteristics of this area as you did for the riffle in Step A.

D. Collect invertebrates from the pool using the following methods:

LABORATORY PROCEDURE:

E. Transfer the riffle samples from the riffle collecting jar to a white enamel pan.  Using forceps, transfer similar organisms onto separate watch glasses or petri dishes.  Separate out any organisms that are NOT insects.

F. Use the Key to Aquatic Insects to identify the order and common name for each type of insect collected in the riffle zone.  Record the number of each type.

G.  Describe the body shape and distinctive structures of each insect collected in the riffle zone.  Using a stereoscope or hand lens, look for hooks on appendages, sucker-like attachment structures, sticky or slimy bodies, flat or streamlined bodies, and cases or nets.  Record your observations on the data sheet, and then return the organisms to the collecting jar.

H.  Repeat steps E-G for the pool organisms.

CONCLUSIONS:

2. Among the physical characteristics of the stream that you measured or observed, which ones are most different between the erosional zone and the depositional zone?

3. What behavioral differences did you note in the riffle insects collected by kicking into the nets compared to the ones you picked off the rocks?

4. Did you notice any behavioral differences between the riffle organisms and the pool organisms?  If so, describe them.

5. What invertebrates beside insects did you collect, if any?

6. Which insects, by order and common name, were most commonly found in the stream?

DISCUSSION:

7. Which microhabitat contained the greatest abundance and diversity of insects?  Was your prediction correct? Give one possible explanation for your results.

8. How does the body shape structure of riffle insects help them survive in the erosional microhabitat?  Why are such structures of little help for survival in the depositional zone (pool)?

9. You made observations of plant life and sediments in a riffle area. Does the energy available to insects in the riffle area come mainly from photosynthesis or does it come from decomposition (organic decay)?  What data do you have to support your answer?

10. Does the energy available to insects in the pool come mainly from photosynthesis or does it come from decomposition? What data do you have to support your answer?

11. Did you observe any fish in the stream?  If so, how do they fit into food web of the stream?  If not, where do you think they go during winter?

  AQUATIC INSECT DATA SHEET

Adaptations Table