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All Lives Transform



Metamorphosis



Metamorphosis:
Changing Form in Life-Stages

egg

caterpillar

chrysalis

adult
transformations of the monarch butterfly

There are many kinds of changes as organisms move through the life-stages of their existence. Some of those changes are very visible. Some are internal. Many insects and amphibians go through extreme changes of form as they develop from larvae into adults. Humans change a lot too, but not nearly so much as butterflies. 

Complete Metamorphosis of a Typical Fly

1. A fly begins life as a fertilized egg.

 


 

 

2. Larva: The egg hatches into a tiny worm-like maggot which looks nothing like a fly. The head of the maggot is enlarged 100 times. The maggot eats and grows large. Then it grows a kind of leathery case around itself and becomes a pupa. Inside what used to be the maggot, all tissues break down and virtually liquefy, then begin to form an adult fly inside the protective casing.

pupa

 

adult


 

4. Adult. When the fly is fully formed inside the pupa it emerges into its new adult life. It comes out of its casing soft and crumpled with a white skin. It spends a few hours swallowing air to expand and dry its body and wings into their final form, and as its color darkens, it flies off to make its living in an entirely new way in an entirely new habitat.

Butterflies and moths are the prime examples of complete metamorphosis in the media and in elementary school, because the change from larva (caterpillar) to adult is a transformation from a bizarre appearance to a beautiful appearance, and because we enjoy the whole idea of a beautiful creature making its living by drinking the nectar of flowers.

Incomplete Insect Metamorphosis

Some insects do not go through this complete change; in some, such as grasshoppers, the babies look like tiny adults (much more so than humans) , and as they grow they must shed their exoskeletons over and over to make room. These successive stages of growth are called instars.

nymph 1st instar nymph 3rd instar adult grasshopper,
with wings

Aquatic Insect Development

Many insects that live in water live a long time, months or even years, as larvae, but only a day or a few days as adults, just long enough to mate and lay eggs. The mayflies do not even eat as adults; the adult stage exists for reproduction.

One insect that lives in streams and the shallows of lakes is the caddisfly larva. These larvae build amazing tube-houses to live in, for protection through camouflage. 


adult caddisfly

case made of sand grains (enlarged)

case made of grasses case of twigs with larva

Caddisfly larvae may live underwater for years. All over Earth they build protective cases or tube-houses which they carry around with them on the bottoms of streams and ponds. Some houses are made of tiny pebbles, some of snailshells, some of small twigs. All are protective camouflage against predators.

Amphibian Metamorphosis

Most frogs, toads and salamanders begin life as eggs floating in water, each egg encased in protective jelly.

The eggs hatch into tiny pollywogs, or tadpoles.

pollywogs bullfrog tadpole two hind legs are out
tadpoles crowded in a shrinking pool froglets have four legs before they absorb the tail new froglets are tiny
     
salamander tadpoles have exterior gills, visible in this tadpole just about to hatch recently discovered tree frog that becomes a tiny frog inside the egg toadlet on fingertips

Internally, tadpoles change from  vegetarian to carnivore. Their intestine as a tadpole is long and coiled like a spring. It becomes short, like all carnivore guts. In frogs and toads, the tadpole's mouth changes from an algae-browsing sucker mouth into a jawed mouth that opens to shoot out a long tongue that can capture food on the fly. Its gills turn into lungs that breathe air. The eyes enlarge and migrate toward the top of the head.

Crustacean Metamorphosis

Crustaceans are mostly marine animals such as lobsters, shrimps and crabs.Some tiny crustaceans, like copepods and daphnia, live in freshwater ponds, while some, such as pillbugs, live on land. The marine species include most of the zooplankton. As some crustaceans metamorph, they gain more legs with each molt. Many early crustacean larva are transparent.


Slipper Lobster larva
Photo © Peter Parks

 

Slipper Lobster juvenile
Photo © Peter Parks
Slipper Lobster adult
Photo © Roger Steene

Metamorphosis Over Generations

One of the most ancient kinds of animals on Earth are called polyps. Polyp animals (cnidarians) are a kind of tube, with one end stuck to something solid, and the other end open and surrounded by a flowerlike circle of tentacles which have stingers. The tentacles grab passing prey and stuff it in the mouth. The polyp, to be a success in life, must be stuck to something solid where the prey is plentiful. So we find polyps in shallow ocean waters and shallow freshwater wetlands, where life is incredibly profuse. 

freshwater hydra, with budding offspring
colonial marine polys

Coral is one kind of polyp. Colonies of coral polyps slowly build coral reefs out of stoney shells they create. Another kind of polyp is the sea anemone, which you may have seen in a tide-pool.

A long time ago, polyp animals invented metamorphosis. Their transformations are even stranger than the insects'. 

A polyp sometimes reproduces by budding (not sexually; one parent). It grows another polyp right out of its side, tentacles and all. When the new polyp is ready, it releases from its parent, turns upside down in the water so its tentacles hang down and floats away.

medusa stage of cnidarian
medusa of another type
Planula stage of cnidarian that swims to an anchor location where it grows into a polyp.

Why Metamorphosis?
Why Transform in Life-Stages?

When something happens in Nature, there is a reason, and that reason is connected to a species' survival and flourishing--food, reproduction, shelter.What we observe in nature usually exists because it solves a problem and gives an organism an advantage. Transforming in life-stages has great advantages:

1. Food for young

For a species to survive, its young must feed, whether they stay with parents or not. Many food sources for insect larvae and tadpoles are temporary and seasonal. Some flies lay their eggs in dead animals, which guarantees food for the maggots if they eat quickly enough and pupate. Dead animals don't last long. Monarch caterpillars lay their eggs on young milkweed plants in early summer. The caterpillar (larva) and chrysalis (pupa) stages both have to complete before frost comes in the autumn. Frog ponds are often temporary, disappearing by early summer.
     The advantage of temporary places to live and eat is that there is less competition for food than in permanent habitats.

2. Dispersal of the species

     For polyps, metamorphosis is partly a way to disperse the species around the oceans. Dispersal is a way of making sure that each generation born has a good chance of getting enough food to thrive and reproduce. In the corals' fixed stage of life, the little animals create their own real estate (the coral reef) which many other creatures then find attractive and move into, providing the coral polyps with an excellent supply of food.

Wings give insects excellent dispersal.

3. Sequence of niches creates opportunities

(A niche is both where something lives and how it eats.) If an animal has to spend its whole life in one niche, its chances of survival are reduced, because the conditions of life--food, water, shelter, weather--are always changing. If an animal transforms into an adult that can travel, it can go find other niches when it must, so it can survive and its offspring can survive. More options means more flexibility.

 

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Explore Further in Transformation

Transformation over many lifetimes
Transformation within the mind
Transformation of Communities
Death The Final Transformation
Return to Transformation Index

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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