Periods of postembryonic development
Postembryonic development of animals is subdivided into three periods: juvenile, maturity and aging.
The juvenile period is characterized by the continuation of organogenesis that began in embryonic life and an increase in body size. By the beginning of this period, all organs are developed to such an extent that a young animal can exist and develop in the environment. The nervous, circulatory and excretory systems perform their functions.
With the release of the organism from the embryonic membranes, the respiratory organs, the digestive system and the sensory organs begin to function. During the juvenile period, the species and individual characteristics of the organism are finally formed and the individual reaches the sizes characteristic of the species.
Later than other organs, the reproductive system develops. When its formation ends, the second stage of postembryonic development begins.
Reproduction takes place during the period of maturity. The duration of this period is different for different animal species. In some species, it lasts only a few days, in others, many years.
The aging period is characterized by a slowdown in metabolism and organ degradation. Aging leads to natural death.
Direct and indirect development
The juvenile period is characterized by direct or indirect development.
With direct development, an individual is born, similar to an adult, but much smaller in size. Its further development is reduced mainly to growth and puberty.
Direct development is characteristic of animals with ovipositor and intrauterine types of ontogenesis: mammals, birds, reptiles, some invertebrates (small-bristle worms, spiders, etc.).
With indirect development, the organism (larva) born into the world differs in structure and way of life from adults. In order for the larva to become an adult, a restructuring of its body is required – transformation, or metamorphosis.
Metamorphosis is a rapid change that occurs during the transition from the larval stage to the adult form.
This is a postembryonic maturation process that is characteristic of many groups of animals: most invertebrates (flatworms, insects, bivalve molluscs, etc.), as well as fish and amphibians.
Example: the frog larva (tadpole) does not look like an adult amphibian, but looks like a fish (no limbs, gill respiration, lateral line, etc.). The organs of adult amphibians develop gradually.
Indirect development of insects occurs with complete transformation (with complete metamorphosis) and with incomplete transformation (with incomplete metamorphosis).
During development with complete transformation, a larva appears from an egg, which feeds, grows, and then turns into a pupa. Inside the immobile pupa, a complete restructuring of all organs takes place. An adult insect (imago) emerges from the pupa.
Complete transformation is characteristic of Lepidoptera (butterflies), Coleoptera (beetles), Diptera (flies and mosquitoes), Hymenoptera (bees, wasps, bumblebees), etc.
Incomplete transformation occurs when the pupal stage is absent. In the process of molting, the larva gradually turns into an imago. Incomplete transformation is typical for cockroaches, hemiptera (bugs), orthoptera, dragonflies.
Comparison of direct and indirect development
Benefits of direct development of organisms:
- the development of an organism into an adult (juvenile period) usually takes place in a shorter period of time;
- there is no significant restructuring of the body, and therefore less energy and nutrients are required.
Disadvantages of direct development of organisms:
- for the implementation of embryonic development, a large amount of nutrients are required in the oocytes or intrauterine development of the offspring;
- overpopulation intensifies intraspecific competition between young and mature individuals, since they need the same vital resources.
Benefits of indirect development of organisms:
- in many animal species, larvae and adults occupy different ecological niches – this reduces intraspecific competition;
- in sedentary or attached animals, the larvae contribute to the dispersal of the species, the expansion of its range.
Disadvantages of indirect development of organisms:
- development into an adult usually takes a long period of time;
- metamorphosis requires a lot of food and energy.