Metamorphosis
Some animals go through a radical change in form or structure, a process called metamorphosis – during the development from egg to adult, resulting in a series of distinctive stages. Animals that undergo one or more metamorphoses as they develop are said to have complex life cycles. Early developmental stages are often termed larvae; sexually reproductive stages are termed adults.
The life cycle of the frog provides a simple example of hatches from the eggs is limbless and possesses external gills and a large tail. Tadpoles feed on aquatic algae and have a long, coiled intestine to process this type of food. After a period of time ranging from a few months to several years, depending on the species involved, tadpoles metamorphose to frogs (adult stage) over a few weeks. They absorb their tails. Hind limbs and then forelimbs soon develop. The eyes enlarge and the skeleton is altered. As the frog emerges from the pond, it breathes with the aid of lungs and feeds primarily on insects. To accommodate this shift from a herbivorous to a carnivorous feeding habit, the intestine of the frog is short and straight. The beginning of metamorphosis is triggered by the release of thyroid hormone.
The higher insects (such as beetles, flies, butterflies, and wasps) complete a more complicated series of metamorphoses as development proceeds from egg to adult. For the butterfly or moth the larva that hatches from the egg is called a caterpillar. The caterpillar is vermiform and feeds voraciously on plant matter. After completing four molts the larval butterfly has grown tremendously in size and undergoes a metamorphosis to the pupa (or chrysalis) stage. As its final act the caterpillar secrete a cocoon about itself. The larval growth and metamorphosis are controlled by juvenile hormone, which is produced by the corpora allata in the larval brain. Inside the cocoon the tissue of the Pupa are recognized. Larval structure are broken down, and adult structure such as wings develop. When this metamorphosis is complete, the winged adult emerges from the cocoon.
Each of the stages is highly specialized for a limited series of functions. The slow moving caterpillar is the feeding stage. Essentially all the increase in size (weight) is confined to this stages. The pupal stage is complicated period of larval tissue breakdown and differentiation of the adult structures. Periods of inclement weather, such as winter, are often endured in the pupal stage. The adult stage does little feeding and does not grow, but it provides dispersal, selects an appropriate habitat to lay its eggs, and reproduces sexually.
Metamorphosis is considered complete when a clear distinction exist between larval and adult stage, such as from tadpole to frog and from larva to pupa to adult. Some animals undergo gradual, or incomplete, metamorphosis. The young resemble adults. Their form gradually changes into an adult through a series of molts. They undergo no pupal stage and no destruction of immature tissues. Examples of such animals are lobsters and cockroaches.
Many marine organisms, both invertebrates and fishes begin life as minute planktonic larvae. These larval stage are responsible for dispersal and ultimately for habitat selection. The larvae must settle out of the water at a location suitable for growth of the juvenile to the adult.
Parasitic organisms are also characterized by complex life cycles with sticking metamorphic changes. For parasites the different stages may be specialized for dispersal, host seeking, attachment and penetration, feeding, and reproduction.
