In most animals the young are not very different from their parents, and become adult by a series of gradual changes. In some cases, however, the young stages are completely unlike the adult state, and only develop into it after changes which cause considerable alteration in structure and habits. Such a development from youth to maturity is said to be a metamorphosis, and is shown in the life of insects and amphibians. The egg of a butterfly, for example, hatches into a larva, which later becomes a pupa, and this in turn changes into the imago. An insect which passes through these stages in its life history is said to have a complete metamorphosis. Other insects which have a complete metamorphosis are bees, wasps, beetles and flies. Some insects, for example the Cockroach, Earwig, Grasshopper and Dragon-fly, have an incomplete metamorphosis, for there is no pupal stage, and the main difference between the larva and the adult is the absence of wings. The larvas of such insects are termed nymphs. A few small wingless insects, for example the Silverfish, undergo no metamorphosis at all and resemble the adult stage when they hatch from the egg.

A caterpillar has a head, legs and prolegs. A grub has a head and legs only—no prolcgs. A maggot has no head or legs.

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