Insects throughout their lives experience a deep restructuring of the external and internal structure of the body. In other words, metamorphosis common to all animals. It’s just that it proceeds with characteristic and very specific features. The postembryonic development of an insect usually goes through three successive stages: an egg, a larva, and an adult. In many species, there is an additional stage: triungulin, pupa, etc.
Depending on how many stages of development an insect goes through, it can be assigned to one of two large groups: with incomplete or complete transformation. Thus, the method of metamorphosis should be regarded as one of the main classification criteria.
From the article you will learn the important features of the stages of development of insects with complete and incomplete transformation, with hypermetamorphosis.
Imago - what is it?
The word "imago" is of Latin origin and literally translated as "image". In zoology, this term refers to the adult (final) stage of development of animals with a complex life cycle (insects and some arthropods). The phase is final. With its onset, the animal acquires the ability to reproduce and, as a rule, to resettle. Adults are not susceptible to molting. At this stage, the growth of insects and arthropods ceases.
Depending on what kind of metamorphosis is characteristic of each individual species, the stage of imago can begin in different ways.
Incomplete Evolution
Incomplete transformation is characterized by three stages of development of insects: an egg, a larva, and an adult. A characteristic feature of this type of metamorphosis is the similarity of the appearance of the larva with the imago, like the latter, they have mouth organs, complex eyes, pronounced rudimentary wings. Moreover, a similar lifestyle is often observed. Incomplete transformation insect larvae are called nymphs.
This type of metamorphosis is observed in Orthoptera, Cockroach, and Damselfly.
Complete Transformation
When fully transformed, insects and arthropod animals go from 4 to 5 stages of development: an egg, a larva, a pupa, and an adult. Sometimes it is possible to determine the stage of pre-pupae. In this case, the larva is practically not similar to adult shoes, to understand this, it is enough to recall the butterfly caterpillar. It is devoid of eyes of a complex structure and the external beginnings of the wings and, as a rule, has a different type of oral apparatus.
After the true larva finishes its growth, it stops feeding, becomes motionless and turns into a chrysalis. Then an imago hatches from it. True larvae are usually divided into three groups: caterpillar-shaped, vermiform (beetles, dipterans, etc.), campode-like (ground beetles, swimmers, etc.).
This type of metamorphosis is characteristic of all Lepidoptera (butterflies), Coleopterans, Diptera.
Hypermetamorphosis development
The most difficult way to develop insects is hypermetamorphosis. It is characteristic of some beetles (micromaltus, crows, etc.), retina, bugs, buzzing, and a number of hymenopterans.
With hypermetamorphosis, the larval stage of insect development proceeds in several stages. In the structure and lifestyle of larvae of different ages, strong differences are observed. At the first stage, they do not feed, but at the same time they actively move and settle. An older larva usually lives in a specific habitat. She not only eats, but also grows. Sometimes, in order to move from one active form to another, restructuring of the body is required. In these cases, the larva becomes immobile and stops feeding. The so-called "false pupa" is formed. In the future, as, for example, in beetles from the family Naryvnikov, it turns into a real chrysalis, from which the imago of the beetle develops.
Features of the structure of adults
As mentioned above, the final stage of development of insects and some arthropods is the imago. What is it, it is easy to understand, referring to the original language. In some sources, the term is translated as "true appearance" and this is true. Throughout their life, at this stage, adults practically do not change, their development process stops. Shedding is absent in most species. However, all rules have exceptions. So, the lower orders (bristles, tails, etc.), even in the adult stage, periodically drop the cuticle.
The main differences between adult insects and young individuals are as follows. Their wings and genitals develop, the antennae consist of a constant number of segments, a certain type of oral apparatus is formed (often different from that of the larvae). The imago is maximally adapted to the environment, and this is expressed in adaptive mechanisms and many structural features: features of the respiratory system, body shape, development of sensory organs, additional protective mechanisms, etc.
For the vast majority of arthropod animals, insects, this phase is the peak of development. The only exceptions are some ticks. Everything happens for them exactly the opposite. Parasitic species moving inside the body cavities and host tissues lose the morphological features inherent in the imago. This process is called de-magnetization.
Age-related discoloration
Hatching from the pupa and thereby passing the last molt in the larval stage, the insect leaves its shell. Often the initial color is light in tone (not white, but paler). During a certain period (from several hours to 2-3 weeks), pigmented substances accumulate and oxidize, in contact with atmospheric air, as a result of which the cuticle darkens.
Certain species, for example, the imago of butterflies (the order Lepidoptera) appear with “their own” formed color.
A change in color may be associated with certain changes in the body. Thus, the desert locust initially has a yellow-green color. During the reproduction period, sexual enzymes accumulate in the body and as a result of the integument acquire a pink hue (pictured above).
Imago life span
The duration of the definitive developmental stage can vary greatly depending on the species. The shortest life of butterflies from the family of the Mothworms is limited to literally a few minutes. Species whose adult stage lasts no more than 2-3 days (including mayflies, chironomids) do not feed and have reduced oral organs.
Long-livers can be called social insects, such as ants and termites. Their uterus and queens live up to 20 and 50 years respectively. The species of cicada Magicicada septendecim is characterized by a 17-year life cycle, which is also quite a lot by the standards of insects.
There are also very paradoxical cases in nature. For example, a deer-beetle familiar to everyone (pictured above) in the imago stage spends only a few weeks, and then dies, while its larva lives up to five years.
Quite often, the habitat and the ecological niche occupied by the larva and adults of the same species differ. For example, mosquitoes. The larva has a typical diet for a predator (plankton, bacteria) and lives in water. Adults are flying insects. Females feed on the blood of vertebrates, while males feed on sap and plant secretions.
The main function of the adult stage
So, the final stage of insect life is the imago stage, what it is, you already learned above. Having understood the essence of the definition, having learned the main structural features, it is easy to guess what is the main function of this phase. In the adult state, absolutely all organisms, reproducing, maintain the abundance of the species. If among insects the number of oviparous females exceeds the number of males, then they are fertile. For example, the queen bee can lay up to 1,500 eggs per season.
The adult stage is important in terms of the distribution of species. This process is facilitated by the presence in adults of developed wings and limbs.
Touching upon the general biological significance of the imaginal stage, it should be pointed out that imago is such a peculiar manifestation of evolutionary processes associated with the development of land by arthropod animals and their subsequent strengthening in occupied ecological niches.