Development is an essential factor in life. It begins with a fertilized egg and ends with puberty. The postembryonic period is characterized by direct and indirect development. Direct development is a biological process in which a multicellular organism grows and enlarges, complicating its organization. This phenomenon is typical for humans, fish, birds and mammals.
Indirect development is a process in which an embryo develops into a mature individual with the involvement of a larval stage, which is accompanied by metamorphosis. This phenomenon is observed, for example, in most invertebrates and amphibians.
Features of the post-embryonic period
Periods of postembryonic development are accompanied by changes in morphological features, habits and habitat. For direct development, a characteristic feature is that after birth, the embryo is a reduced copy of an adult organism, itdiffers only in size and the absence of some characteristics that are acquired only over time. An example would be the development of man, animals and some reptiles. Indirect development is characteristic of invertebrates, mollusks and amphibians. In this case, the embryo has significant differences compared to the adult animal. As an example, an ordinary butterfly is suitable. Only after going through several stages of development, the small larva will be transformed beyond recognition.
Development periods
Periods of post-embryonic development include juvenile stage, maturity and aging.
The juvenile period covers the time from birth to puberty. This stage is accompanied by adaptation to the new environment. It is worth noting that many animals and reptiles, which are characterized by a direct path of postembryonic development, develop in approximately the same way. The only difference is the time frame. This period ends with puberty
- The period of maturity, called the reproductive stage, is characterized by stunting. The body undergoes self-renewal of certain structures and their gradual wear.
- The aging period is accompanied by a slowdown in recovery processes. As a rule, there is a decrease in body weight. If there was no violent intervention, then natural death occurs when vital systems cease to function as a result of the slowdown of all processes.
Indirect development: examples and steps
Let's look at how life is born in a new creature. Direct and indirect development are terms that describe various animal life processes that begin with a fertilized egg. During postembryonic development, organ systems are finally formed, growth, puberty, and subsequent procreation are observed. Then aging occurs, and in the absence of external interventions, natural death occurs.
- Immediately after birth, a whole series of transformations begins. At this time, a small organism differs from an adult both externally and internally.
- The second stage is the transformation into a completely new body. Metamorphosis is a postembryonic change in the shape of the body with alternation of several stages.
- The third stage is the final stage, which ends with puberty and procreation.
Characteristic of indirect development
Indirect development is characteristic of multicellular organisms. A larva emerges from a laid egg, which externally and internally does not resemble an adult. In structure, this is a simpler creature, as a rule, having a smaller size. In its appearance, it can be remotely similar to its distant ancestors. An example would be the larva of an amphibian like a frog.
Outwardly, the tadpole is very similar to a small fish. Thanks to the presence of special larval organs, he can lead a completely different life thanmature individuals. They do not even have rudimentary sexual differences, so it is not possible to determine the sex of the larva. For a certain number of animal species, this stage of development takes up most of their lives.
Radical metamorphoses
With indirect development, the newborn animal differs greatly from the mature form in a number of anatomical features. The embryo hatches from the egg as a larva that undergoes a radical metamorphosis before reaching its adult stage. Indirect development is characteristic of animals that lay numerous eggs. These are some echinoderms, amphibians and insects (butterflies, dragonflies, frogs, and so on). The larvae of these creatures often occupy a completely different ecological space than an adult animal. They feed, grow and at some point transform into an adult animal. These global metamorphoses are accompanied by numerous physiological changes.
Pros and cons of direct development
The advantage of direct development is that much less energy and vital ingredients are needed for growth, since there are no global changes in the body. The disadvantage is that the development of the embryo requires large reserves of nutrients in the eggs or gestation in the womb.
The negative point is also that between young and adult animals there may be competition within the species, as their habitat and food sourcesmatch.
Pros and cons of indirect development
Due to the fact that organisms with an indirect type of development live in different ecological niches, competitive relations between larvae and adults, as a rule, do not arise. The advantage is also that the larvae of sedentary creatures help the species to expand its habitat. Among the minuses, it is worth pointing out that the indirect development of animals into adults often lasts a long period of time. For high-quality transformations, a large amount of nutrients and energy is needed.
Types of indirect development
The following types of indirect development are distinguished: with complete and partial metamorphosis. With complete transformation, indirect development is characteristic of insects (butterflies, beetles, some Hymenoptera). The larvae that are born begin to eat, grow, after which they become motionless cocoons. In this state, all organs of the body disintegrate, and the resulting cellular material and accumulated nutrients become the basis for the formation of completely different organs characteristic of an adult organism.
With partial metamorphosis, indirect postembryonic development is characteristic of all fish and amphibian species, certain types of worms, mollusks and insects. The main difference from the complete transformation is the absence of the cocoon stage.
The biological role of the larval stage
The larval stage is a period of active growth and supply of nutrients. Appearance, as a rule, is very different from the adult form. There are unique structures and organs that a matured individual does not have. Their diet can also differ significantly. The larvae are often adapted to the environment. For example, tadpoles live almost exclusively in water, but can also live on land, like adult frogs. Some adult species are immobile while their larvae move and use this ability to disperse and expand their habitat.
