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What is an Insect? A weird way to breathe, Body in 3 parts, Metramorphosis

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What is an Insect?

In the adult stage, the body of insects is divided into three parts: the head, thorax and abdomen. It has three pairs of legs and often wings. Keeping in mind these characteristics peculiar to insects, they can easily be distinguished from other arthropods.

However, it should be noted that the world of insects is full of exceptions: some species do not have wings, others do not have eyes … In addition, an insect in the larval state can be very different from the adult.

A weird way to breathe

Insects don’t have noses and lungs to breathe. The air enters through a series of small holes located on each side of the body: the “stigmas”. From the stigmas, the air is transported to the smallest recesses of the body by a network of rigid tubes called tracheae.

Free circulation

The heart of insects is shaped like a long tube pierced with tiny openings. The blood of insects is called hemolymph. This fluid is often transparent, yellow, or greenish, and it circulates freely throughout the body. Insects do not have veins.

A body in three parts

The basic model of insects in adulthood is simple: a body divided into three parts (the head, thorax and abdomen), three pairs of legs and two pairs of wings. Insects have adopted different shapes, colors and a host of adaptations, but their bodies are always, or almost always, built from these common elements.

The head

The head of an adult insect has two large compound eyes formed from several facets. These correspond to so many small organs of sight that allow the insect to detect the slightest movements. A single eye sometimes has thousands of facets. Several species of insects have in addition, at the top of the head, two or three simple eyes called “ocelli”. Ocelli are usually the only type of eyes present in larvae.

The head also carries a pair of articulated antennae, of shape and size very variable according to the species. They allow insects to perceive, among other things, smells and vibrations. The mouthparts are in most cases located on the ventral side of the head.

The thorax

The thorax is in a way the motor center of the insect. It is made up of three segments. Each wears a pair of legs. The second and third segments also bear the wings, which vary in shape, size and texture from one group, or order of insects, to another. Most insects have four wings. However, others, such as silverfish, fleas and lice, do not. Flies, on the other hand, only have two.

The abdomen

The abdomen contains the digestive system, the heart, part of the respiratory system, and the reproductive organs. Like other parts of the body, it is covered with the exoskeleton, which protects it against impact and desiccation.

From egg to adult

The lifespan of insects is very variable. Some, like cicadas, complete their cycle in several years. Others, like Drosophila, or fruit fly, only take a few days. During this more or less long period which elapses between the laying of an egg and the moment, for the individual which comes out of this egg, to reproduce in turn, the insect undergoes a series of sometimes spectacular transformations.


Insects have an outer skeleton, or exoskeleton, which resembles a shell or armor. This protective envelope has advantages, but also certain disadvantages, in particular with regard to growth. Due to its high rigidity, the exoskeleton has little stretch and quickly becomes too small. The insect must therefore replace it periodically, as it grows. This phenomenon of skin change is called molting.

When ready to molt, the insect comes to rest in a suitable place. A new exoskeleton has already started to form under the old one. When the time comes, the old integument splits at the level of the thorax and, in a few minutes, the insect emerges from this envelope which has become too small. It emerges, covered with its new cuticle which hardens in contact with the air. The insect resumes its activities a few hours later and leaves behind its old exoskeleton, or “exuvia”.

The number of molts in the life of an insect usually ranges from four to eight, but it can go up to ten or twelve in dragonflies and even up to nearly thirty in mayflies. With some exceptions, as with silverfish, insects stop growing once they reach adulthood. They are therefore no longer forced to molt.

The vast majority of insects change their appearance during their growth. This transformation is called metamorphosis. Metamorphosis is classified as incomplete or complete depending on the extent of the changes that occur during development.

The absence of metamorphosis

Only a few groups of insects do not undergo real metamorphosis. They are said to be “ametabolic”. In these wingless (wingless) insects, the nymphs resemble adults except for their size, glands and genitals. They also undergo a number of molts to grow taller. Silverfish are part of the ametabolic insects.

The incomplete metamorphosis

The life cycle of insects with incomplete metamorphosis (or hemimetabolous insects) takes place in three main stages: the egg, the pupa and the adult. As soon as it comes out of the egg, the nymph often already looks like an adult. Some authors give the name of “larva” to the nymph of hemimetabolous insects. The term naiad is also sometimes found in the literature, which applies to the aquatic nymph of certain insects, such as dragonflies and mayflies.

Usually, the pupa is as mobile as the adult, except that it cannot fly. She can live in the same environments as an adult or in very different environments. Its diet may or may not correspond to that of the adult. After having undergone a certain number of molts, she metamorphoses directly into an adult, also called an imago .

Life stages of the locust

In most hemimetabolous insects, such as locusts, grasshoppers, bed bugs, praying mantises and earwigs, very young nymphs do not have wings. But after one or more moults, wing blanks appear, then gradually lengthen from one moult to another. The last moult constitutes the passage to adulthood. The wings then become functional and the insect acquires all of its sexual characteristics.

The scenario is much the same for dragonflies and mayflies, but this time the differences between nymphs and adults are more marked. Nymphs live in an aquatic environment and breathe through gills, while adults are aerial and breathe through tracheae.

We observe in mayflies a unique phenomenon in the whole world of insects. Once its aquatic life is over, the nymph gains the surface of the water and transforms into an individual who, although winged, is not yet quite an adult. It is called subimago , or sometimes pre-adult. The last moult and emergence of the adult will occur a few hours later.

The complete metamorphosis

Flies, butterflies, caddisflies, bees, ants, beetles, in short, the vast majority of insects undergo a complete metamorphosis (more than 85% of species). The differences between larvae and adults are sometimes so pronounced that it becomes difficult for someone unfamiliar with insects to know what will become of a larva once it reaches adulthood.

The life cycle of fully metamorphosed insects, or holometabolous, has four distinct stages: egg, larva (caterpillar), pupa (chrysalis) and adult.

The egg and larval life

After mating, the female insect usually lays its eggs in a place where the larvae can easily find their food. Depending on the species, the eggs are laid individually or in groups, and hatching occurs after a few days, months or even years.

Some species lay only a few eggs, others several hundred or even thousands. In some insects, such as some species of mosquitoes, females are able to lay several eggs in a matter of weeks. In others, such as social insects, females tend to the eggs, clean them, protect them, and feed the young larvae in the nest for a period of time.

Larval life

As in insects with incomplete metamorphosis, larval life is essentially a stage of growth. Between each moult, the insect spends almost all of its time feeding. Some insects only feed during this period.

The larvae of completely metamorphosed insects are often worm-shaped with or without legs. More or less mobile, they undergo a certain number of moults before transforming, not into adults, but into nymphs.

Pupal and adult life

Pupation is a stage in the life cycle that is peculiar to fully metamorphosed insects. This is the stage where the important internal transformations take place that will allow the insect to become an adult. The wing blanks appear at this point. In the nymph stage, the insect does not feed and is usually or almost motionless.

Adult life

After a more or less long period in the pupal stage, the last moult occurs, which marks the transition to adulthood. The adult insect is usually winged and very mobile. It is able to reproduce and can thus ensure the continuity of the species.

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