A Little Bit More
Although crab spiders are quite common, they’re so well camouflaged on flowers that without careful observation, one would never know they were there. I found my first crab spider, not because I saw the spider, but because I saw its prey. I saw a large bee on a flower (like the one in the illustration) and couldn’t understand why it wasn’t moving. When I looked more closely, I saw that the immobilized bee was in the grasp of a small spider that blended in so perfectly with the flower, I hadn’t seen it. Some crab spiders select flowers that are the same color as themselves, often moving from one flower to another over time, while others are actually able to gradually change their body color to match the flower on which they wait.
Crab spider don’t produce a web for capturing prey; most lie in ambush for passing insects, with their legs at the ready. Their eyesight is particularly good at detecting movement and they are adept at catching flying insects that approach their perch, searching for nectar. Crab spiders cling to the flower with their back legs, and grapple with their prey using their crab-like front legs. Once an insect is caught, it is bitten and injected with a strong venom. If the prey is caught by its abdomen (back end), the spider manipulates it to turn it head-on, then bites it in the head or thorax. Apparently, the toxin reaches the central nervous system more quickly if it is injected near the insect’s head. Once the prey is subdued, it can be fed upon without further struggles.
Activities
Camouflage Game
Objectives: See camouflage in action
Materials: Paper punch, 5 or 6 different colored papers, and 2 to 4 different colored blankets or similar sized substrates (plain or patterned)
This is a very simple way for students to see how camouflage works. Talking about camouflage and seeing it “in action” are two different things - this exercise should make a lasting impression. This activity can be done in almost any room, even outside if you wish, with a minimum of prep.
The camouflage game provides a wonderful opportunity for an art teacher to be involved and discuss color with the students. Why some colors stand out quite obviously on a given background while others do not can be discussed and related to the color wheel.
1. Secure an assortment of 5 or 6 different colored papers and use a paper punch to punch out 12 plugs of each color – you won’t need to keep the plugs separate.
2. Select one student per colored blanket or substrate and have them stand aside so they can’t see your setup area. This exercise works best if these students don’t know the “reason” they’ll be asked to pick up the paper plugs.
3. Place your first background on the ground and loosely scatter all of your paper plugs so there is a random assortment spread across the background.
4. Tell your selected students that they’re going to be asked, in turn, to pick up as many paper plugs as they can find within a 15 second period – don’t tell them why. They’ll be eager to pick up as many as possible and should automatically act appropriately for this exercise.
5. Once they understand the instructions, lead your first student, with his/her eyes closed, to the first setup. At a given signal, allow them to open their eyes and begin picking up as many paper plugs as they can within 15 seconds. Once time is up, have that substrate placed aside and count how many plugs were picked up and what their colors were. (it’s important to keep track of the colored background and the actual colors chosen during that exercise)
6. Return ALL of the plugs to the same container so they can be randomly spread out on the next substrate and repeat the same process with the second (third and fourth) student in turn until finished with all substrates. (Again, be sure you note which plugs were chosen from each of the backgrounds)
Very quickly, you should be able to see that with each substrate used, plug selection is NOT random but tends to be selective. In fact, those colored plugs that do not blend in well with the background (are not as well camouflaged) are more likely to be seen and picked up by the students (we could call them our predators). Those plugs that are camouflaged (blend in better with the substrate color) are less likely to be selected and picked up by the students and you should see this in the assortment of colored plugs at the end of each exercise.
This exercise is a quick, over-simplified way for students to see how camouflage actually works to the advantage of an organism. As each student “predator” selects plugs under the 15 second time pressure, they automatically reach for those most obvious and therefore tend to choose the “easier to see” plugs and overlook the “less obvious” ones. In nature, those animals that are better camouflaged will tend to be preyed upon less often than their more poorly-camouflaged brothers and sisters.
It’s important that you mention that camouflage works both ways – whether the camouflage helps a predator, such as a crab spider, or its prey (as with this exercise)!
Key Concepts
Structures and Functions, Predator/Prey Relationships, Identification
Questions
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