SHOW 303: Spider Web-making Stratagems
On the tropical island of Barro Colorado in the Panama Canal, Yale biologist Catherine Craig is studying the unique web-building strategies of the Argiope argentata, a spider that weaves a particularly alluring trap for bees. How and why does the spider decorate her web with UV-reflecting silk? And why are bees (and other insects) attracted to the web, yet eventually learn to avoid it? FRONTIERS offers an intimate look at this predator-prey relationship.
Activity: L-UV To Catch Spiders
Notes & Discussion
Report From the Field: Catherine L. Craig, PhD., Biologist at Yale University
ACTIVITY: L-UV TO CATCH SPIDERS
"Bug-zappers" are used by people to electrocute (zap!) insects. Some spiders, like the ones you saw on FRONTIERS, use ultraviolet light to confuse insects into thinking a web is a flower. Can you design a "Bug Trapper" that will confuse insects?
Build a device that traps flying insects. Your device must be built so that it mounts easily on the outside of a window and can be easily removed. The inside of your trapper must be visible through the window.
You may choose a variety of materials to build your Bug Trapper. Here is a list of suggested materials to start with. You may have some better ideas.
- mosquito netting or fiberglass screening
- empty wax-coated paper container
- posterboard or construction paper
- plucked flower petals or small flowers
- fine monofilament thread
- shoe box
- cotton thread
- coffee can
- rubber bands
- duct tape
- camera (optional)
This is just one suggested procedure. You may invent your own procedure, but remember to keep careful notes and record everything you do and observe.
- Cut both ends from an opaque container. One end will be attached to a window; the other will have a trap door or one-way door attached to it.
- Design the trap door from fiberglass screening or mosquito netting. Attach the door to your bug trapper with a rubber band or waterproof tape. You may also want to decorate the door with real flower petals.
- Attach the bug trapper to the outside of a window using duct tape. Choose a window out of direct sunlight to confuse daytime insects. (If you want to attract nighttime insects, any window will do.) Test the trap door to be sure it works after the bug trapper is mounted to the window.
- Except for the space taken by your bug trapper, cover the window with construction paper. Set up the light source on the inside of the window so that it shines through your bug trapper.
- Check your bug trapper every hour to see if you have trapped any insects. Cover the trap door of your bug trapper and turn off the light when you are not trapping insects. Try using different types of lights on different days.
- Carefully observe the insects by looking into your bug trapper. Catalog the captured insects. If you can identify the proper names of all the insects, do so. Descriptions should be as detailed as you can make them. Each day, release the captured insects carefully (some may have stingers) after you have observed and recorded them. As an alternative to a written insect catalog, you may wish to take photographs of the captured insects and post them with information on the day, time and kind of light that was used to attract the insects.
- Did you capture the same types of insects every day? If so, which insects seem to be most attracted to your trapper?
- How did different kinds of light affect the types of insects attracted to your trapper?
- Does day or night trapping seem to be most effective? Why?
- If you used a trap door decorated with newer petals, did it attract more of any one kind of insect? If so, identify the insect.
- In this activity, students will build a UV bug trapper. Give students the list of materials on the worksheet and invite them to brainstorm about the best way to build the trapper.
- Even though the level of UV is low, students should not stare at this or any other UV light source. Direct students to find light sources that produce relatively safe, low-level ultraviolet radiation. If a UV light source cannot be obtained, fluorescent lights, grow lights or black lights may have the same effect, especially at night.
- One light source may be sufficient for a whole class if students pack their trappers closely together. A fluorescent light in a classroom is excellent for trapping night-flying insects.
NOTES & DISCUSSION
- Spider thread (silk) is produced by glands in the spider's abdomen and is either sticky or dry. Spiders produce different kinds of thread, each with unique properties. New research indicates that at least one type of silk is stronger than steel! Biochemists are researching possible applications for commercially produced spider silk.
- Only female spiders construct webs. So that the male spider is not mistaken for a fly when he visits a female, he taps a special code on the web.
- Ask students to reconstruct the sequence in which a spider spins her web. Explain that the spider weaves a spoked support web with non-sticky dry silk, also used by the spider to drop to the ground and climb back up to the web. After she spins the spokes, she spins a spiral of sticky silk around them. Finally, she spins a "telegraph thread" that detects motion on the web. Since her sight is very poor, she locates prey with her sense of touch, feeling the tautness of each thread until she finds the one on which her prey is stuck. As she moves around the web, she avoids the sticky silk and stays exclusively on the dry threads. Note: not all spiders decorate their webs.
- Research other examples of predator-prey or co-evolutionary relationships. One example of a unique relationship was featured on FRONTIERS Show 202, which profiled an ant species that enslaved another species.
- Invite an entomologist to bring some live spiders to class to show students. Use a field guide to learn more about spiders, especially those in your region.
- Researchers have found evidence of UV sensitivity in insects, birds (it helps them navigate), fish, amphibians and, recently observed, rodents. UV light (the same component of sunlight absorbed by the ozone layer) is invisible to humans. Review a chart of electromagnetic radiation for greater understanding.
- Ask students to find out why spiders are not considered insects.
- A "bug-zapper" is an electronic version of a deadly spider web. The trap is a UV light surrounded by several electric screen grids. When a bug flies toward the light, it comes in contact with the grids. If it touches two or more grids, it is electrocuted, resulting in a loud "zap."
REPORT FROM THE FIELD: CATHERINE L. CRAIG, PhD., BIOLOGIST AT YALE UNIVERSITY
Biologist Catherine L. Craig first became fascinated by spiders while working in Costa Rica for an environmental assessment company and has been studying them closely for the past 15 years. Since the experiments seen an FRONTIERS were filmed, Craig has begun new work that explores the relationship between insect behavior and the color of webs. To insects, some webs appear blue, and others appear white, yellow or blue/green, depending on the structure of the protein molecules in the web. Craig hopes to learn how the differences in color affect a web's visibility to prey and how prey respond. She has observed that bees seem to have a hard time avoiding yellow webs, possibly because yellow is frequently paired with a positive stimulus in their environment, such as yellow flowers.
We were curious how Craig came up with the practical idea of using nail polish to identify bees, as demonstrated in the show. "There are two ways to mark bees," she explained, "either with numbers or with colors. The numbers that are available are too big for the bees I'm working with, so that left color. The usual way to do this is to mix pigments with shellac. I decided to use nail polish because it's readily available and easy to use. I simply mix pigments in with the polish to make different colors I need."
Craig conducts her field work in Panama because the country offers a high diversity of spiders, and large numbers of both primitive and derived spiders can be found in the tropical rainforest regions there. As a biologist who relies on an abundance of species to conduct her work, Craig is acutely aware of how these environments are endangered. "I'd really like to encourage students to respect the biological world and take an active interest in preserving all kinds of organisms," says Craig with conviction.
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