An obligatory condition of indirect development is the presence of metamorphoses (from Greek ldquo;metamorphosisrdquo; - ldquo;conversionrdquo;)

Metamorphosis [Larval stage]

An obligatory condition of indirect development is the presence of metamorphoses (from Greek “metamorphosis” — “conversion”) — the deep body transformations that occur during postembryonic period of life. In animals, this process affects not just parts of the body, but the whole body (diagram 89). In the development process the larval organs disappear, and their place is taken by organs, inherent for adult animals.

Larval development

Larval development occurs in most groups of animals (sponges, arthropods, worms, mollusks, bony fish, amphibians, etc.), and it is not due to the fact, what environment — water and land — live specimens of these species in the adult state.

An important advantage of the larval development is the fact that often the larvae and adults live in different environments (in the water occurs the development of winged insects: dragonflies, mayflies, mosquitoes, flies, and many others). It is believed that this way eliminates competition between themselves and individuals of one species for food resources.

Diagram 89. Frog metamorphosis — is a typical example of indirect development

The larvae may so differ from their adult forms that, at first, they were often described as the representatives of different genera, orders, classes, and even the phyla of the Animalia kingdom (diagram 90). Since then, many larvae retained their own names: naiad in Greek mythology — the nymph of forest creeks — dragonfly larvae, ammocoete — lamprey larvae that lives in holes, dug in the mud; trochophores (from Greek “trohos” — “wheel” and “foros” — “bearing”) — free-swimming larvae of some groups of mollusks and annelids; nauplii (from Greek “nauplius” — “animal with a shell”) — early larvae of many crustaceans; Leptocephalus (from Latin “shortheaded”) — larva of the eel, which is characterized by a completely transparent body and others.

Reasons of metamorphosis

It is believed that this is due to deficiency of nutrients that are lacking in the egg to complete development of the embryo. As a result, the cubs are born underdeveloped, in fact, while still embryos. This explanation is confirmed by comparing the size of the eggs of vertebrates with the larval stages (bony fish and amphibians) with the eggs of reptiles and birds, which have no such stages. Reptiles and birds female eggs are huge and rich of egg yolk, which is enough for the full development of the embryo. In fish and amphibians eggs (caviar) are small, there are not much of reserved recourses. Giant eggs are peculiar to another non-larval animal — cartilaginous fishes. The larvae are not found in viviparous animals (some sharks, sea snakes, mammals and some other). Obviously, mother's body of these species has sufficient resources for their offspring to complete embryonic development in utero. Taken from http://worldofschool.org

Diagram 90. A variety of larvae: a — dragonflies; b- butterflies; c — polychaete worm; d — lobster

Metamorphosis in plants

The plants also have metamorphosis. It is not only different modifications of its parts under the influence of the environment, for example, the death of the terrestrial part of the shoot in a hot or cold period or a temporary transformation of plants in bulb or rhizome, but also the metamorphosis of parts with the change of form and its functions. Thus, in the ontogeny of certain species, their shoots may be transformed in leaves, or thorns. The character of the plants metamorphosis is determined by the environmental conditions. Furthermore, it does not affect, as in animals, the whole body. That is why the postembryonic development of the plant, even though it has metamorphoses, it hasn’t larval stages.

Questions:
  • What is the larva?

  • Name some animals that go through metamorphosis?

  • Why so many animals must undergo larval stage in their development?