Invertebrates are animals without backbones. Insects are the most common invertebrates. Invertebrates are the oldest known animals on earth. Their fossil record goes back at least 600 million years. Insects, the most successful invertebrates, originated about 400 million years ago.
Invertebrates occur in a tremendous variety of shapes and colors.
These are adaptations to specific habitats. For example, burrowers, such as earthworms, usually have long tubular bodies and are darkly colored, similar to the soil in which they live. Invertebrates that are active during the day, such as butterflies, are often brightly colored. Others have colors and patterns which serve to hide them. Immature forms of mayfiies and stoneflies occur in fast-running streams.