Beetles are the most diverse group of organisms on Earth. More than 350,000 species of beetle have been described, making one in every five species of organism a beetle. Beetles got an early start; the first beetles appeared during the Triassic (about 240 million years ago). Munching on plants, algae, and fungi, beetles were well-positioned to exploit the first flowering plants as they arrived on the scene. Beetles were among the first pollinators, visiting flowers to eat nectar and other plant products. Flowers became specialized for attracting (and feeding) beetles, such as magnolias whose appearance and smell continues to attract beetles. While bees certainly have the monopoly on pollination these days, the win-win relationship (mutualism) between beetles and flowering plants may have contributed to beetles becoming an exceptionally diverse group. Their sturdy body plan, with a hardened set of front wings (elytra) protecting the softer, hind wings underneath, may also have contributed to their success.

An insect has a brain, connected to bundles of nerves (ganglia) in each of its three body segments. Like us, they have sensory nerves that receive information from their environment and send it to the brain. Information comes from many types of touch receptors. Touch-sensitive hairs all over an insect's body are sensitive enough to detect vibrations in the air, such as from an approaching predator. Insect hairs also do chemical detection (chemoreception). Pores at the end of chemoreceptors on mouthparts or other body parts allow odors to reach the nervous system. Insect antennae may have thousands of chemoreceptors, used to detect chemicals in the air (pheromones) emitted by members of the opposite sex. An insect, like us, has a pair of eyes on its head. But, insect eyes are compound, with hundreds or thousands of lenses, together making an image consisting of spots of light, like pixels. An insect's sound receptors are on its legs.

Insects are by far the most diverse and abundant group of organisms on Earth. More than one million insect species have been identified, and estimates of how many species exist range into the tens of millions. Insects got their start way back, in the Paleozoic (about 500 million years ago) and have evolved in tandem with flowers (coevolution). Insects have become specialized on particular flowers, leading to complex sets of adaptations that couple them in a feeding and pollination relationship (a mutualism). Specialization allows for many types of insects to live in the same habitat, accommodating their exceptional biodiversity. In a backyard in the temperate zone, one might find several thousand species of insects. Specialization can occur within one plant species as well, with different insects using different parts of the plant. In just one species of tropical tree, Dr. Terry Erwin of Smithsonian's National Museum of Natural History found about a thousand species of beetles.