Illinois Natural History Survey - University of Illinois

The Naturalist's Apprentice: Floating Flora and Fauna

Floating Flora and Fauna

 Objective: to learn about some of the remarkable adaptations organisms have that enable them to live on the surface of water


Materials: multiple copies of The Water's Surface


Vocabulary: predaceous, surface film, surface tension


Comments: The surface of a quiet marsh or pond presents no barrier to a raccoon searching for crayfish or a heron attempting to spear an elusive tadpole. To the small creatures of the world, however, the water's surface presents a firm but flexible surface, and many plants and animals are well adapted for living at or near this surface. Surface tension results because water molecules are more strongly attracted to each other than to the air above. The surface of the water, therefore, is held in place from each side and from below and results in a dense surface film of water molecules. Each of the plants and animals in this activity is adapted in some way for living at or near the water's surface.



1. Pass out copies of the handout The Water's Surface to students.

2. Discuss the material in the comments section and ask students to try to match the brief descriptions with the drawings.

3. Go over the correct answers after students have completed the activity and ask if they are aware of any other plants or animals that live on the surface film.


The Water's Surface: A Ceiling for Some, a Floor for Others 

Write the number of the description below the picture of the plant or animal it describes.

1. The American featherfoil floats on the surface of quiet waters by means of a spongy, inflated stem. Its feathery leaves are partially submerged or float on or near the water's surface. Small blue flowers occur along the stem. Although the American featherfoil occurs mostly in the southern United States, it ranges north into the Midwest.

2. Fishing spiders occur along the shores of many wetlands and move about on the water's surface with considerable agility. They feed primarily on animals that have fallen into the water and become trapped in the surface film. Even though these spiders do not spin silken webs, the surface film of the marsh functions as a web.

3. Marsh treaders, also called water measurers, are slender, sticklike predators or scavengers that live in vegetation found around the edges of ponds and marshes. Unlike their fast cousins, the water striders, marsh treaders can walk very slowly on the water's surface. As a result, they seem to hunt in slow motion.

4. Duckweeds are small flowering plants that live on the surface. Tiny rootlets less than half an inch long dangle from the undersides of these plants. Duckweeds reproduce rapidly and can cover the surface of a wetland within a few weeks.

5. The gyrating, spinning whirligig beetles are familiar inhabitants of most quiet waters in the Midwest. Adults are specially adapted for living on the surface film. Their eyes are divided into upper and lower halves--the upper portion sees the world above the water while the lower portion keeps tabs on what's happening below the surface! Whirligigs often congregate in large schools in which they catch small prey or feed on dead organisms. Only the upper half of the beetle is water repellent; thus, it swims half above and half below the surface.

6. The water strider, sometimes called a pond skater, does not float on water like a piece of wood. Instead, it skates along on the surface. Its body is so light that its six legs do not break through the water's thin surface. Tiny waterproof hairs on its feet prevent it from sinking.

Michael Jeffords, Center for Economic Entomology

Illinois Natural History Survey

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Champaign, IL 61820

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