Metamorphosis worth considera
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.
Metamorphoses may be accompanied by shifts in habitat, feeding behavior,
and other habits. Animals that undergo one or more metamorphoses as they
develop are said to have complex life cycles, with distinctive stages being
initiated by a metamorphosis from the preceding stage. Each of these stages
is usually specialized for a particular function such as feeding, growth,
habitat selection, or differentiation of distinctive tissues. Early
developmental stages are often termed larvae; sexually reproductive stages
are termed adults.
The life cycle of the frog provides a simple example of metamorphic
development. The tadpole (larval stage) that hatches from the egg 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
remaining tadpoles for 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. Hindlimbs 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. Accompanying 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 asÜjÜ 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 pupal
(or chrysalis) stage. As its final act the caterpillar secretes a cocoon
about itself, firmly cementing the cocoon to a branch or twig. Inside the
cocoon all the tissues of the PUPA are reorganized. Larval structures are
broken down, and adult structures such as wings differentiate from little
groups of cells called imaginal disks. When this metamorphosis is complete, the winged adult, capable of sexual reproduction, 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 stage. The pupal stage is a
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 provides dispersal, selects an appropriate habitat to lay its eggs,
and reproduces sexually. If the adult selects the correct site to lay its
eggs, the slow-moving caterpillar will hatch near a good source of food.
Metamorphosis is considered complete when a clear distinction exists
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, beginÜjÜ life as minute planktonic larvae. These larval stages are responsible for dispersal and
ultimately for habitat selection. They must settle out of the water at a
location suitable for growth of the juvenile to the adult. For many marine
invertebrates the transformation from planktonic larva to adult is very
rapid. Sea urchins can complete this metamorphosis in about one hour.
Crustaceans have complicated life cycles sometimes involving four separate
metamorphoses.
Parasitic organisms are also characterized by complex life cycles with
striking metamorphic changes. For parasites the different stages may be
specialized for dispersal, host-seeking, attachment and penetration, feeding,
and reproduction.
ARMAND KURIS
Bibliography: Barnes, R. D., Invertebrate Zoology, 3d ed. (1974); Daly, H.
V., et al., Introduction to Insect Biology and Diversity (1978); Noble, E. R.
and G. A., Parasitology, 4th ed. (1976).