Stages of insect development: incomplete and complete transformation

Postembryonic development, otherwise known as post-embryonic, in all living organisms of our planet, can be divided into two forms: direct and indirect. The first species is found in reptiles, birds, mammals. Born or left from the egg cubs are a small copy of an adult. Another form of development occurs in fish, amphibians and arthropods. In this article on specific examples we will consider the stages of development of insects.

The biological role of metamorphosis

The development of various species of animals with incomplete and complete transformation (metamorphosis) provides the most optimal conditions for the life of juvenile and adult forms and reduces competition for the forage base between them. This allows the use of alternative food resources, and also facilitates the settlement of a species occupying different habitats (air, land, water or underground). The metamorphosis of insects is one of the reasons for the enormous number of species of these organisms that currently live on the Earth (more than one million). They occupy almost all existing ecological niches. Insects represent a class of arthropod type. Stages of development of insects represent the following phases: egg (embryonic development), larva, pupa, imago (postembryonic development).

The egg stage is the first and mandatory phase of the life cycle of insects. It has several shells. The first is called the chorion (performs a protective and mechanical function). In some species it is complicated by layers of wax or chitin and is permeated with pores. The second coat, yolk or serous, is in direct contact with the developing embryo. Its food is provided by the yolk. The shape, color of the chorion and the size of the eggs of various insects are varied. Thus, in grasshoppers the length of the egg reaches 11 mm, and for spider mites - only 0.14 mm. Most insects lay eggs, although there are viviparous forms, such as the Madagascar cockroach. The larva leaves the egg, which is the next stage in the development of insects.

Complete transformation

It is characteristic for species of the subclass winged insects. Before becoming an adult individual - imago, the organism, coming out of the egg, experiences two completely dissimilar life stages: larvae and pupae. Insects, for which a complete metamorphosis is characteristic, are called holometabolic. These include the groups of Lepidoptera, Diptera, Coleoptera, etc.

Features of the larval stage

They are, first of all, in the anatomical structure of the body. Most larvae lack a reproductive system. Also different is the oral apparatus, and hence the type of nutrition. Consider the larval stages of development of insects with incomplete transformation.

One of the most ancient organisms - a dragonfly, lays eggs in the water of standing water. After 20 days, and in some species in 2-9 months appears Pronima (proverb), which lives only a few seconds, then it molts, and a naiad is formed - a real larva of a dragonfly. It has small dimensions (1.5 mm), and the life cycle, depending on the type of insect, can last from several days to one to three years. The larva actively hunts in the water and has trachea for breathing, so it often rises to the surface.

Continuing to shed and grow, it crawls out of the water onto the stems of aquatic plants and transforms into an adult insect - the dragonfly, which has soft wings and covers of the body. She does not move for a while. The chitinous layer covering the insect hardens. The dragonfly becomes capable of flying. Summing up, let's say the following: the stage of the larva in dragonflies of various species provides an expansion of the habitat of these insects. Note that the sexually mature insect of the dragonfly and its larva have similarities in the way they feed (both are predators), as well as respiration (tracheal organs). The difference lies in their habitat: the adults live in the air, and the naiads live in the water.

Insect larvae with complete transformation

In representatives of the order of lepidopterans, for example, in butterflies, they are called caterpillars and are very different from the adult. The larva emerges from the egg, gnawing through its shells and immediately begins to eat the leaves and other parts of the plant with its powerful jaws - mandibles. Her body is vermiform, has a head, three thoracic and ten abdominal segments. The integument is provided with hairs-setae. Butterflies are insects, for which a complete transformation is characteristic. On the lower lip of the larva there is a pair of gland secreting secret. Frozen in the air, it forms a thread, which the larva uses to form a cocoon. Staying in it, the larva turns into a pupa. She can live from several weeks to one or three years, and a baby doll lives up to 10 years. In her body formed glycerin and betaine - natural antifreeze.

Larvae of butterflies - insects that have a complete transformation, often moult. The last molt in them ends in pupation. In some species of insects, larvae have separate names. For example, beetle saw beetles are a false-caterpillar, in pollen-eaters and darkling beetles - a false wire, nymphs call larvae of dragonflies, and their pre-larvae are called nymphs.

What is a chrysalis

This is the phase of the life cycle of insects, which leads to the development of a sexually mature specimen - imago. The stage of the pupa is characterized by the fact that the body does not feed and can not move. In addition to silk, for the construction of a cocoon and its strengthening, animals often use sand particles or shells. In free pupae, the antennae, legs and wings of the future adult are free and pressed to the trunk. Covered pupae are typical for many species of butterflies, ladybirds, some Diptera.

Imago

The last stage of development of insects is characterized by the formation of the reproductive (sexual system), as well as all external signs inherent in this species. Like the larva, the imago performs the function of settling insects in various areas. In addition, adult individuals are responsible for reproduction and have genitals. In males they are called testes, and in females they are called ovaries. There are also accessory glands secreting secret and copulatory organs for mating.

In this article, we examined the stages of development of insects that have an indirect development with incomplete and complete transformation.