PURPOSE:  To identify aquatic insects and study their adaptations for survival.

MATERIALS: Field -- suitable winter clothing, waterproof boots or waders, rubber gloves, aquatic net, collecting jar, clipboard or other firm writing surface, pencil.  Lab -- white enamel pan, forceps, watch glass or petri dish, 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 of these insects live in the stream all their lives. But most aquatic insects live in the water only as larvae or nymphs. They emerge from the water as adults during the warmer months. 

Winter is the best season to observe aquatic insects. In spite of the cold temperatures, most aquatic insects are active. They continue to feed and grow all winter.  

Small streams often have riffles of fast-flowing shallow water, and pools with deeper, slow-moving water.  Riffles and pools have different bottoms due to the water current.  As a result, different kinds of plants and animals live in these two habitats.

In this activity, you will first observe the characteristics of riffles and pools in a small stream. Then you will collect and identify insects from the riffle zone. You will describe how their body shape and structures help them survive in their habitat.

PREDICTION: Use what you know about riffles, insects, and aerodynamics to make a prediction.

1. Do you think aquatic insects in the riffle zone will have round, bulky bodies or flat, streamlined bodies? Explain your ideas.

FIELD PROCEDURE: Dress properly for winter weather. You should wear waterproof boots and rubber gloves.  If you use bare hands in the cold water, be sure to dry and warm them often!

A. Stand on shore and examine the riffle zone of a small steam. Record your observations on the data table as follows:

B. For comparison, examine a deeper pool area using the same methods in step A.

C.  In a riffle zone, put the aquatic net on the stream bottom facing the current.  Stir the bottom in front of the net with your feet or a stick to dislodge the organisms.  The moving water will sweep them into the net.  Transfer the organisms from the net into a collecting jar.  Repeat the procedure in another riffle area until you get several different kinds of aquatic insects.

D.  In the riffle zone, pick up a rock from an undisturbed area, turn it over, and look closely for any attached insects.  Pick off any organisms that you find and put them into your collecting jar.  Repeat the procedure using other rocks until you get several different kinds of aquatic insects.

LAB PROCEDURE:

E.  Transfer the contents of your collecting jar to a white enamel pan so you can see the organisms better.  Look for any invertebrates that are NOT insects, and separate them from the rest.  Use forceps to put individual insects into a petri dish or watch glass for close examination with a stereoscope or hand lens.

F.  Use the Key to Aquatic Insects to identify the order and the common name of each different kind of insect you collected.  Describe the body shape (round bulky or flat steamlined) and distinctive structures of each.  Look for hooks on appendages, suckers, sticky or slimy bodies, and cases or nets.  Record this information on the data sheet.

GOING FURTHER:  You may wish to use the samples you collected to do a more detailed study of pond life, such as found in many life science laboratory manuals.

CONCLUSIONS:

2. How is the water in the riffle zone different from the water in the pool area?

3. How is the stream bottom in the riffle zone different from the bottom of the pool?

4. What non-insect invertebrates did you find in the stream?

5. Which insects (order and common name) were most common in the riffle zone of the stream?

6. Did the riffle zone insects have round bulky bodies, or flat streamlined bodies? Use your data to describe some other insect adaptations for attachment to the rocks on the bottom.

DISCUSSION:

7. Why was it necessary to use two different methods for collecting stream insects?

8. Do you think that the water in the riffle area of the stream has much oxygen or only a little oxygen?  Explain your ideas.  What conditions could cause decreased oxygen for aquatic insects?

9. How does the water current affect the stream bottom?  Use your answer to question 2 as evidence.

10. Why do scientists call the bottom of a riffle zone an erosional substrate?  (Hint: Look up "erosion" and "substratum" in a dictionary.)

11. Energy is supplied to stream insects by photosynthesis or by decomposition of dead plants and animals.  Which of these two processes is the main source of energy for insects in the riffle zone?  Use your data to support your ideas.