What is Invertebrates?

Invertebrates: About 99% of all the animals lack backbones, and are invertebrates! Invertebrates include the phylum Arthropoda, or the animals with jointed feet - the spiders, scorpions, crustaceans, insects, millipedes and centipedes, among others. Most arthropods undergo dramatic changes in their body forms during development from an embryo. This process is called metamorphosis. Click on the button to see a Monarch butterfly undergo metamorphosis during its life history.

The Monarch butterfly undergoes complete metamorphosis - which means it goes through 4 distinct stages: egg, larva, pupa, and adult.

The life of a Monarch butterfly begins with an egg, laid on a tree twig by an adult Monarch butterfly. The egg contains the embryo and a yolk - its food supply. The case protects the embryo and allows gases and wastes to diffuse across.

The egg develops into a larva, which in this case is commonly called a caterpillar. The larva has an exoskeleton for protection and it crawls around, feeding on vegetation and storing up energy. As it grows, the larva goes through several moltings, shedding its skin, to allow for the increase in size.

After a while, the larva stops feeding and goes into a period of apparent inactivity within a protective shell. This stage is called a pupa. Sometimes it is called a cocoon. It appears to be inactive, but a total rearrangement of its body parts - a metamorphosis - is really happening.

Metamorphosis results in the reorganization of the caterpillar body into an adult butterfly body. When the adult finally emerges, it has a completely different body from the wormlike larva. While sole purpose of the caterpillar was to feed, the goal of adult butterflies is to find a suitable mate and reproduce to start the life cycle over again.