There are two groups of insects. In the first group, including insects such as cockroaches, grasshoppers, dragonflies and bugs, the wings develop gradually on the outside of the body and get larger at each moult until they are fully formed. The young stages of these insects are called nymphs and they usually resemble the adults in general appearance, often inhabiting the same places and eating the same kinds of food. This group has a 3-stage life cycle.
In the second group, including beetles, butterflies, moths, flies, bees, and wasps, the young stages are very unlike the adults. These young stages are called larvae (or grubs and caterpillars). They often occupy completely different habitat niches and exist on quite different food sources from those of the adults. Instead of undergoing a series of small changes to reach the adult form, the larvae undergo one very dramatic change that requires a special resting and non-feeding stage during which the transformation can occur. This resting stage is called the pupa. In this group, the wing buds develop internally and are not visible until the pupae stage. This group has a 4-stage life cycle.
Unfortunately, this has to be memorised for exams – a chart will help better with visualisation and memory recall.
Fish, mammals, reptiles, birds
They don’t undergo any metamorphosis. Their young are typically similar to the parent, just smaller. They are born (either alive or hatched from eggs) and then they grow up. This group has a 3-stage life cycle: before birth, young and adult.
Amphibians (eg. Frogs)
They undergo a metamorphosis. Their young is dissimilar to the parent. They are born alive or hatched from eggs, but their young spend their childhood under water, breathing through gills. They grow into adults and move onto land. This group also has a 3-stage life cycle.
Here’s a summary of the above.