Insect Development

After hatching from the egg, a young insect must undergo several molts before it reaches its adult size and becomes sexually mature. Silverfish and a few other insects go through the molting process without changing body form. The majority of insects undergo some type of change in form as they develop into adults. This phenomenon of developmental change in form is called metamorphosis (MET-uh-MOHR-fuh-suhs). There are two main kinds of metamorphosis in insects: incomplete and complete.

Incomplete Metamorphosis

In incomplete metamorphosis, illustrated in Figure 37-6, a nymph hatches from an egg and gradually develops into an adult. A nymph is an immature form of an insect that looks somewhat like the adult, but it is smaller, and its wings and reproductive organs are undeveloped. The nymph molts several times. With each molt, the wings become larger and more fully formed. The final molt transforms the nymph into an adult that can reproduce and, in most species, fly. Insects that undergo incomplete metamorphosis include grasshoppers, mayflies, dragonflies, and termites. Several other examples are listed in Table 37-1.

Complete Metamorphosis

In complete metamorphosis, an insect undergoes two stages of development between the egg and the adult. In both of those stages, the insect looks substantially different from its adult form. Figure 37-7 illustrates complete metamorphosis in the monarch butterfly. A wormlike larva, commonly called a caterpillar, hatches from the egg. Insect larvae may or may not have legs on the thorax and may or may not have leglike appendages on the abdomen. The larva eats almost constantly, growing large on a diet composed mostly of leaves. Thus, it is the larval stage of most insects that causes the most damage to plants.

The monarch larva molts several times as it grows. In the last larval stage, it develops bands of black, white, and yellow along its body. It continues to feed, but soon finds a sheltered spot and hangs upside down. Its body becomes shorter and thicker. Its exoskeleton then splits down the dorsal side and falls off, revealing a green pupa. A pupa (PYOO-puh), also called a chrysalis (KRIS-uh-luhs), is a stage of development in which an insect changes from a larva to an adult. The pupa of butterflies is enclosed in a protective case. Moth pupae are enclosed in a case called a cocoon. Inside the pupa, the larval tissues break down, and groups of cells called imaginal disks develop into the wings and other tissues of the adult. When metamorphosis is complete, the pupa molts into a sexually mature, winged butterfly. Most insects go through complete metamorphosis. Table 37-1 lists several examples besides butterflies and moths, such as beetles, mosquitoes, and bees.

Importance of Metamorphosis

In a life cycle based on complete metamorphosis, the larval and adult stages often fulfill different functions, live in different habitats, and eat different foods. Therefore, the larvae and adults do not compete for space and food. For example, mosquito larvae live in fresh water and feed by filtering small food particles out of the water. When they become adults, the mosquitoes leave the water and feed on plant sap or the blood of terrestrial animals.

Metamorphosis also enhances insect survival by helping insects survive harsh weather. For instance, most butterflies and moths spend the winter as pupae encased in chrysalises or cocoons, which are often buried in the soil.

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Eggs laid by adult

Eggs laid by adult

Pupa

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