Insect Develpment and Metamorphosis

Insects are arthropods. They have jointed legs and an exoskeleton. They do not have an internal skeleton like vertebrate animals. This tough outer skin is called the cuticle. The cuticle has a wax layer to prevent the insect from dessication (drying). The exoskeleton provides excellent protection and support of the body, but can limit the growth of the body. Some exoskeletons are inelastic, others have some elasticity. As insects grow, they periodically shed their shell and form a larger one. Some insects may molt only 4 times in their short life, others may molt 16 or more times.

In the photo above, the white cockroach has just emerged from the brown exoskeleton. At first the new exoskeleton is light in color and pliable, but within a few hours it will darken and harden. The new skeleton allows for some future growth. This is the only photo of a non-Kentucky insect, this is a Madagascar hissing cockroach.

Insects are cold blooded. They cannot regulated their temperature as we do. Their body temperature varies with the environment they live in. Below the temperature threshold for that species, little or no development occurs. For many species, this is close to 50°F, but some have lower or higher threshold temperatures for development. As temperatures increases above the insect's threshold, the rate of development increases until the temperature reaches an upper limit.

Insects undergo metamorphosis as they grow and develop. Metamorphosis is a compound word that means 'change in form'. Although we are now focusing on how their body form changes as they grow, we need to appreciate that metamorphosis also affects their behavior as well. An understanding of metamorphosis is key to the understanding of insect biology. While there are four major types of metamorphosis, two of these, gradual and complete, are most important for Master Gardeners to understand.

When insects grow in size, but show little change in shape throughout their life, we call this simple metamorphosis. Simple metamorphosis is found in very primitive groups of insects, in fact some scientists don't even consider these to be insect groups. Pictured below is a firebrat, an insect with simple metamorphosis. Immature firebrats have the same body form as adults, they're just smaller.

Simple metamorphosis occurs with silverfish, firebrats, springtails, and some other groups. One important difference with this type of metamorphosis and that of the upcoming groups is that these insects never develop wings.

Gradual metamorphosis is similar to simple, except that nearly all of these insects will develop wings. Each time they shed their exoskeleton, the wing buds on the outside of their body get larger. With most species, the adult is recognized by the fully developed wings on its back. However, apart from the development of wings, gradual metamorphosis is very similar to simple, the nymphs are similar in shape to the adults, just smaller and without wings. Immature insects with gradual metamorphosis often consume the same types of food as the adults.

With gradual metamorphosis, they are three stages: egg, nymph and adult. As nymphs, they may molt several times (two of those stages are pictured above), but once they become adults they do not molt again. Note the wing pads on the back of the nymphs.

Incomplete metamorphosis is very similar to gradual, except the immature stages that are called naiads occur underwater. In order to breath underwater, the naiads have gills. The adult stage lives out of water and has wings. The naiads look very similar to the adults, except they do not have complete wings, they have gills, and have wing pads on their backs. Examples of insects with incomplete metamorphosis include mayflies, stoneflies, dragonflies, and damselflies.

With complete metamorphosis, the larva and adult appear very different, their shapes are not the same. There are four distinct stages, egg, larva, pupa, and adult. During the larval stage there are usually several molts, between which the insect is actively feeding and growing. The pupa is a resting stage, many of the organs and tissues needed as a larva are reformed into tissues and organs needed by the adult. The adult needs wings to disperse, muscles to power the wings, reproductive organs, and so on.

The complete metamorphosis example above is with the southwestern corn borer. Often the food source for the adult is entirely different than for the larva, so these stages don't directly compete against each other for resources. For example, Japanese beetle grubs feed on roots of grasses, while the adults feed primarily on leaves of various plants. Complete metamorphosis occurs with beetles, flies, butterflies and moths, wasps and bees, and a few other insect orders.

Updated November 2005