ABOUT METAMORPHOSIS: Butterflies change into very different forms as they grow, due to a process called metamorphosis. It begins when a butterfly lays an egg on the bottom of a leaf near the top of the host plant. The eggs hatch a few days later, then go through three stages: larva (caterpillar), pupa (when the larva hibernates in a sac), and adult. Monarch butterflies go through the entire process in about three months, but some species of butterfly can take as long as several years to go from the egg to adult stage.
HOW DO HUMANS SEE COLORS? The human eye works in much the same way as a camera captures images on film. Its "film" is the retina, a thin layer of neural tissue lining the back of the eye, made of photoreceptor cells that receive light, and other cells that interpret this information and send the signal to the brain via the optic nerve. The retina contains two kinds of photoreceptor cells: cone cells and rod cells. Cone cells are sensitive to bright light and can perceive colors. The human eye has three types of cone cells, each sensitive to a particular primary color of light: blue, green and red. These three primary colors can mix in the eye so we can see more complex shades, such as violet or orange. Objects absorb some colors of light and reflect others, and this determines the colors that we see. When light hits a bright red apple, for instance, the apple's surface absorbs all the wavelengths except red, which is reflected to the eye. So we perceive the apple as being red. In contrast, rod cells work best in low light and can perceive black and white images.
The American Society of Civil Engineers contributed to the information contained in the TV portion of this report.