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Animal Life Cycles and Dispersal

Incomplete Metamorphosis

Amongst the other insects, such as dragonflies, stick insects and grasshoppers, the transformation from larva to adult is much more gradual. The immature stages look quite like the adults. We call the immature stages of these insects nymphs.

In the insect world the largest number of moults is shown by the silver fish. It can change its exoskeleton 50 times. It is a primitive wingless insect which keeps growing throughout its life.

Each time they moult they look a bit more like the adult. The last parts to develop are the wings and the reproductive organs. Not only do the nymphs look like the adults but they also tend to eat the same kind of food. The nymphs also have senses which are just as good as those of the adults.

This type of development is called incomplete metamorphosis because the change in shape of the animal is more gradual. The diagram below shows the life cycle of the grasshopper which develops in this way.

Mayflies are unusual in the insect world because they are the only group of insect which moults after the wings are fully developed. The adult stage of the mayfly only lives for about 24 hours in some cases. The nymph of the mayfly, however, takes two years to grow and develop in the streams where it lives.