Insects: general characteristics and classification
It is estimated that there could be thirty million species of insects in total, representing a very high percentage of all species on the planet. Undoubtedly, this shows that they have been able to adapt correctly to the most varied habitats. What are the characteristics that have allowed insects to diversify so much? In this article we will explain it.
General characteristics of insects
Insects are a type of invertebrate animal that belongs to the phylum of arthropods. They are named for their articulated legs, like crustaceans, arachnids or the extinct trilobites. The main differences between the different groups of arthropods are the number of segments into which the body is divided and the number of legs they have.
The body of insects is divided into three sections: head, thorax, and abdomen. Its entire body is covered by an exoskeleton made up of chitin , a hard and flexible substance. All insects have six legs when they are adults and a pair of sensory antennae located on the head. The legs are located on the thorax, and if the insect has wings, these are also located on the thorax.
Nervous, circulatory and respiratory systems of insects
The two most notable internal characteristics of insects are their nervous and respiratory systems. They have a brain and a dorsal nerve cord, as well as several nerve ganglia distributed throughout the body. That is why it is sometimes said that some insects have seven brains, but it is a wrong statement.
Their circulatory system is open, they do not have veins or arteries and the nutrients are distributed in the tissues by diffusion. On the other hand, they do not have lungs, but the exchange of gases is carried out through a series of tubes called tracheas, which exit to the outside through openings in the exoskeleton called spiracles, located in the thorax and abdomen.
Compound eyes and ocelli
A peculiar feature of insects is that they have compound eyes . Unlike simple eyes, each compound eye is made up of numerous independent cells, each equipped with an individual lens. Compound eyes have a number of advantages over simple eyes, such as better perception of rapid movements.
In addition, many insects have three small simple eyes (called ocelli ) on the dorsal part of the head, located between the compound eyes. Its function is unclear, but it could be related to maintaining stability during flight.
The ability of insects to fly
The ability to fly is perhaps the greatest evolutionary success of insects and what has allowed them to diversify to occupy a wide variety of ecological niches. Most adult insects have two pairs of wings located on the back of the thorax.
Diptera (like flies) have a single pair of wings, while other insects such as some aphids or ants do not have wings. Depending on whether or not insects have the ability to fold their wings on the body when perched on a surface, they are called Neoptera (more evolved, they can fold their wings) or Paleopteros (unable to fold their wings on the body, such as the dragonfly or ephemeral ). While all Neoptera come from a common ancestor, Paleoptera belong to different evolutionary groups.
Another characteristic of insects is that they undergo metamorphosis, that is, a transition between the juvenile form and the adult form throughout the life of the insect.
Insects can be classified into three categories depending on how this transformation is carried out.
Thus, the ametábolos insects are those whose juvenile form only differs from the adult form by being smaller and sexually immature. Amethabolic insects are primitive and wingless in their adult form.
On the other hand, hemimetabolic insects undergo a series of gradual changes, in which the wings appear progressively. Juvenile individuals in this group are called nymphs.
Finally, those insects that undergo a complete metamorphosis, in which the juvenile individual (called larva) does not resemble the adult, are called holometabolos.
These insects have an intermediate pupal stage between the juvenile individual and the adult. The pupa must remain immobile during metamorphosis, so some insects burrow (such as cicadas) or make protective cocoons (such as butterflies) to metamorphose. The case of butterflies illustrates well how little larvae resemble adult insects.
Social behaviors and other peculiarities
Some insects live in group societies in which individuals specialize in performing different functions. This is the case, to name a few examples, of termites, ants, wasps or bees. Different genetic regulations determine the role that each individual plays in the collective.
Normally a single individual, called a queen, is the one that lays eggs that allow the continuity of the colony. Those eggs that are not fertilized give rise to sterile individuals, while fertilized eggs give rise to new queens.
In order to live in a colony, insects need to communicate in some way with each other, either through the generation of chemical signals (in the case of apterous insects) or through the elaboration of visual signals during flight (in the case of bees and wasps). .
Some insects that do not live in colonies have orientation mechanisms that allow them to migrate over great distances. This is the case of the monarch butterfly ( Danaus plexippus ), which makes round trips between North and South America of up to 4,000 km.
Remarkably, the insect has several successive generations throughout the year, adult individuals living for a few weeks, but that generation that reaches maturity in autumn has a longevity of nine months and performs migration to survive the winter cold. This older generation is called the “Methuselah generation”.
Another group of insects with peculiar characteristics are the parasitoids. They carry out a part of their life cycle by parasitizing other insects. The adult individual places its fertilized eggs inside or on the surface of another insect, and the larvae begin to feed on the host when they hatch.
This can be a long process, always ending with the death of the host and the metamorphosis of the parasitoid as an adult. Some of these parasitoids feed exclusively on the eggs of other species. This is useful in biological pest control to eliminate some types of caterpillars or flies that affect arable plants.
Classification of insects
Insects can be classified into the following orders: siphonaptera (fleas, about 2,000 species), ftiraptera (lice, 5,000 species), odonates (dragonflies and damselflies, 6,000 species), dictyptera (cockroaches and termites, 10,000 species), orthoptera (grasshoppers and crickets, 20,000 species), Hemiptera (bed bugs and aphids, 80,000 species), Lepidoptera (butterflies and moths, 120,000 species), Hymenoptera (bees, wasps and ants, 150,000 species), Diptera (flies and mosquitoes, 160,000 species) and coleopterans (beetles, 375,000 species).
Of course, the number of species that are not yet known could be much higher, due to the small size of many insects and the difficulties in differentiating certain species that are similar. To put this in relation to other groups of animals, some 72,000 species of vertebrates are known, of which 33,000 are fish and only 5,500 are mammals. In other words, for every known species of mammal, about 70 species of beetles are known, and the actual number is even higher.