Grasshoppers are insects of the order Orthoptera, suborder Caelifera. They are sometimes referred to as short-horned grasshoppers to distinguish them from the katydids (bush crickets) which have much longer antennae. They are typically ground-dwelling insects with powerful hind legs which enable them to escape from threats by leaping vigorously. They are hemimetabolous insects (do not undergo complete metamorphosis) which hatch from an egg into a nymph or "hopper" which undergoes five moults, becoming more similar to the adult insect at each developmental stage. At high population densities and under certain environmental conditions, some grasshopper species can change colour and behaviour and form swarms. Under these circumstances they are known as locusts.