One basic fact of life will become obvious to you and your students when rearing Manduca sexta in the classroom: The caterpillars, especially 5th instars, will produce a large amount of feces. There isn't likely to be any way that you can avoid a discussion of defecation, but you can perform a neat experiment to answer the question, "How fast does diet move through the larva's digestive tract?"

It is probably best to start this activity early in the day, so you will have time to see the results.

Directions

1. Saturate a small piece of diet with food coloring. Try diffrent colors; experiment to determine which works best.
2. Place an individual caterpillar with the piece of dyed diet in a separate container. Start timing as soon as you see the insect begin eating. A young 5th instar is a good choice, because it will be consuming large quantities at this stage.
3. As the caterpillar eats, the dye will enter the digestive tract. You may even see the color when the insect is held up to the light! After the dyed food is consumed, be certain to add fresh diet to keep things moving. Eventually, the students will see the change in color of the feces. At this point you should stop timing and record your observations.
4. You may wish to try variations of this experiment by asking what factors affect the rate of passage: light, temperature, age, length, weight, individual variations, etc.