Development is an integral factor in life. It begins with a fertilized egg and ends with puberty. For the postembryonic period, direct and indirect development is characteristic. Direct development is a biological process in which a multicellular organism grows and grows, complicating its organization. This phenomenon is characteristic of humans, fish, birds and mammals.
Indirect development is a process in which the embryo develops into a mature individual with the involvement of the larval stage, which is accompanied by metamorphosis. This phenomenon is observed, for example, in most invertebrates and amphibians.
Features of the postembryonic 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 the adult organism, it differs only in size and in the absence of some characteristics that are acquired only with time. An example would be the development of humans, animals, and some reptiles. Indirect development is characteristic of invertebrates, mollusks and amphibians. In this case, the embryo has significant differences compared with an adult animal. As an example, an ordinary butterfly is suitable. Only after several stages of development have been completed , the little larva will be transformed beyond recognition.
Development periods
Periods of postembryonic 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 approximately the same. The only difference is the time frame. This period ends with puberty.
- The maturity period, called the reproductive stage, is characterized by stunted growth. In the body, self-renewal of certain structures takes place 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 interference, then natural death occurs when vital systems, as a result of the slowdown of all processes, cease to function.
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 the various processes of animal life, which begins with a fertilized egg. During post-embryonic development, organ systems are finally formed, growth, puberty, followed by 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, the small organism differs from the adult both externally and internally.
- The second stage is the transformation into a completely new body. Metamorphosis is a postembryonic change in body shape with the alternation of several stages.
- The third stage is the final stage, which ends with puberty and procreation.
Indirect development characteristic
Indirect development is characteristic of multicellular organisms. A larva emerges from the laid egg, which is externally and internally unlike an adult. By structure, this is a simpler creature, usually having a smaller size. With its appearance, it can be remotely similar to its distant ancestors. An example would be the larva of such an amphibian as a frog.
Outwardly, the tadpole is very similar to a small fish. Due to the presence of special larval organs, it can lead a completely different life than sexually mature individuals. They do not even have primordial sex differences, so it is not possible to determine the sex of the larva. In a certain number of animal species, this stage of development occupies most of their lives.
Radical metamorphoses
With indirect development, a newborn animal is very different from a mature form in a number of anatomical features. The embryo hatches from the egg in the form of a larva, which undergoes radical metamorphoses before it reaches 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 growth requires much less energy and vital ingredients, since no global changes in the body occur. The disadvantage is that for the development of the embryo, large reserves of nutrients in the eggs or gestation in the womb are needed.
The negative point is that between young and adult animals there may be competition within the species, as their habitat and food sources coincide.
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 that the larvae of sedentary creatures help the species expand its habitat. Among the minuses, it is worth pointing out that the indirect development of animals into an adult often lasts a long period of time. High-quality transformations require a large amount of nutrients and energy.
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). Larvae that are born begin to eat, grow, after which they become motionless cocoons. In this state, all organs of the body decay, and the formed cellular material and accumulated nutrients become the basis for the formation of completely different organs characteristic of the adult body.
With partial metamorphosis, indirect postembryonic development is characteristic of all species of fish and amphibians, certain types of worms, mollusks and insects. The main difference from complete conversion is the absence of a 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 an adult does not have. Their diet may also vary significantly. Larvae are often adapted to the environment. For example, tadpoles live almost exclusively in water, but can live on land, like adult frogs. Some species in the adult stage are motionless, while their larvae move and use this ability to settle and expand their habitat.