What makes an insect an insect and separates insects from other small hard-bodied creatures?
First, all insects are arthropods, animals that lack backbones and have jointed legs and external skeletons, or exoskeletons. Arthropods include such diverse animals as roaches, millipedes, tarantulas, and lobsters. Second, all insects have three distinct body parts: a head, a thorax, and an abdomen; many other arthropods have either more or fewer body areas. Third, almost all insects have wings. No other group of arthropods can fly. Fourth, all insects have two antennae and six legs. Most other arthropods have eight or more legs. Their tiny size, quick reproduction rate, and ability to fly enable insects to explore and exploit almost every environmental niche imaginable.