Students make a study of two contrasting stream habitats (erosional and depositional zones).  They identify the aquatic insects by using a key, and describe important body structure adaptations for survival in each habitat.

TOPICS: Invertebrates, insects, adaptation, ecology

LEVEL: High school and advanced placement

TIME:  One class period for field work, one or more class periods for lab work.

ADVANCE PREPARATION:  Locate a small stream, which is ice free, rapidly flowing, and shallow with a rocky, erosional substrate.  Do not use a slow moving, deep water stream with a depositional substrate; it will have fewer aquatic insects and will be more dangerous to sample. You may wish to make extra copies of the procedure and data pages for students to use outdoors .

MATERIALS: Aquatic nets with rectangular or D-shaped frames are preferred.  Label collecting jars "riffle" and "pool" .

PREDICTION: In class discussion, encourage creative, logical, and critical thinking. We recommend that you introduce the following ideas if they are not raised by students: Greater diversity and abundance of insects are usually found in the riffle area, since the bottom offers a greater area for secure attachment of both plants and insects.  The depositional zone has a shifting substrate, which is poor for attachment.  Oxygen depletion may occur in the decomposing bottom sediments, further limiting diversity and abundance. Students may think that a pool will have more insects because they will be flushed into the area due to turbulence in the riffle area; in fact, dislodged insects simply drift downstream until they reach another riffle area .

STEP A4. The riffle zone is characterized by shallow fast moving water and turbulence as the water flows over rocks.  The water is typically clear and free of surface ice all winter.  Depending on temperature, the water may be slushy due to the presence of frazil ice; the bottom rocks may be blanketed with anchor ice under supercooled conditions. The riffle zone substrate is made up of large rocks or pebbles typically smooth and rounded by erosion, and slippery to the touch due to attached algae and mosses .

STEP B1 & D: Students should collect insects over approximately equal areas and time periods for both habitats .

STEP B2 & B3. The insects collected by the kicking method are more active, wriggling and crawling.  The insects picked off the rocks are passive, often curling up .

STEP C. The pool area has slow moving, deeper water, which may or may not be ice covered. The pool substrate is made up of fine sediment of sand, silt, clay and dark colored organic debris.  There is typically no rooted or attached vegetation .

STEP D2. Pool organisms are slow moving, such as snails that withdraw into their shells.  The insects move back and forth trying to burrow into the sediment .

STEP F: If insects are difficult to count alive, preserve the specimens by adding 1 part 70% isopropyl alcohol to 6 parts H₂O.

QUESTIONS 7-11 require higher order thinking skills.  We recommend that you discuss these questions in class before having students write out their answers .

2. The water in the erosional zone is fast flowing and turbulent.  The water in the depositional zone is sluggish and calm.  The temperatures in both zones are about the same .

3. The insects collected by the kicking method are more active, wriggling and crawling.  The insects picked off the rocks are passive, often curling up .

4. Pool organisms are slow moving, such as snails that withdraw into their shells.  The insects move back and forth trying to burrow into the sediment .

5. Snails, leeches, worms (round, segmented, or flatworms), or crustaceans (pill bugs, scuds, crayfish) may be collected .

6. Answers will vary according to habitat sampled, but may include caddisflies, mayflies, and stoneflies .

7. Greater diversity and abundance of insects are usually found in the erosional zone, since it has greater area for attachment of both plants and insects.  The depositional zone has a shifting substrate, which is poor for attachment.  Fewer organisms are adapted for burrowing.  Oxygen depletion may occur in the decomposing bottom sediments, further limiting diversity and abundance .

8. Riffle insects typically have flattened streamlined bodies, sticky bodies, legs with hooks or sucker-like attachments, or attachment cases.  These body structures aid in survival by allowing them to stay attached to the substrate in the fast current.  The depositional substrate has few rocks where attachment adaptations are useful .

9. The riffle (erosional zone) is characterized by photosynthetic production.  Since sunlight easily penetrates the shallow water, attached mosses and algae are able to photosynthesize .

10. The pool (depositional zone) is characterized by decomposition.  It contains fine organic matter that is decomposing.  The soft, shifting sediments do not allow for plant attachment even though light penetration may be sufficient for photosynthesis .

11. Fish are usually secondary consumers.  They migrate to the deeper pools in winter, become relatively inactive, and probably will not be observed

.

Insects	Adaptations
stoneflies, damselflies, dragonflies	hooks on appendages; flat streamlined bodies
mayflies, some beetles (waterpenny), Dobsonflies	flat, streamlined bodies
blackflies	sucker-like attachments
mothflies, soldierflies	sticky or slimy surfaces
caddisflies	cases or nets for attachment; hooks on appendaages