How Insects Grow & Change Form

How Insects Grow & Change Form

When a bird hatches from an egg, it gradually grows and changes day by day until it becomes an adult bird. However, when an insect hatches from an egg it grows and changes by distinct stages. Each time an insect makes a change into the next growth stage, it has to molt (shed) its skin. After each molt, the insect becomes a little larger and somewhat different in form until it reaches the adult stage. After it reaches the adult stage, it does not molt or grow any more. The change in form as an insect grows is called metamorphosis. The exact style of metamorphosis is not the same for all insects, but insects in the same order have the same style of metamorphosis. You can't look at an adult insect and see metamorphosis, but it is one of the characteristics used for putting insects in their orders.

Some orders of insects are said to have no metamorphosis because there is little or no difference in appearance between the young stages and the adult except for size. Insects with no metamorphosis all belong to wingless orders of insects.

Insects with gradual metamorphosis have three life stages: egg, nymphs and adult. Nymphs resemble the adult except that their body parts are out of proportion with each other, and they do not have fully developed wings. With each molt, the nymphs gradually develop wings and take on the body proportions of an adult. Nymphs have the same type of mouthparts as the adult, and they both eat the same kind of food.

Incomplete metamorphosis is somewhat like gradual metamorphosis and also has three life stages: egg, naiads and adult. However, the adult insect with incomplete metamorphosis lays its eggs in or near water and the naiads develop in water. The adults are flying insects that live out of water. Naiads and adults therefore do not eat the same kind of food. Naiads have chewing mouthparts, but the adults have differently shaped chewing mouthparts or no functional mouthparts. The naiad and the adult usually differ a lot in appearance although the naiads gradually develop wings.

The most complex type of metamorphosis is called complete metamorphosis. It has four distinct form stages: egg, larvae, pupa and adult. The larval stages do not look like the adult at all, and they are often worm-like. Larvae often have different mouthparts and food habits than the adult, and they often live in places different from the adult. Larvae molt several times and get a little larger with each molt, but there is no gradual development of wings or other adult characteristics. When a fully grown larva molts, it changes into a pupa. The pupa usually does not eat or move around much, but a lot of internal changes take place. When the pupa has made all its internal changes, its skin splits and the fully formed adult emerges. Most insects with complete metamorphosis are winged in the adult stage. The adults do not molt or grow any more. Little flies or beetles, for instance, do not grow to become larger.

The chart illustrates examples of the four types of metamorphosis and lists the orders of insects that are included in each metamorphosis type. Complete the exercise that follows the chart.