HEXAPODS: CHARACTERISTICS, CLASSIFICATION AND DISTRIBUTION - BIOLOGY - 2025

The hexápodos (Hexapoda) are formed by bodies arthropods subphylum having six legs, unirrámeos appendages and a body divided into three segments: head, thorax and abdomen. This regionalization is known as tagmosis and the regions are “tagmas”. It is divided into two classes: Entognatha and Insecta.

Hexapods belong to the phylum Arthropoda, the most diverse group within the animal kingdom. Currently, more than 1,100,100 species of arthropods have been reported, and presumably there are many more that have not been identified so far.

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characteristics

The members of the first class are characterized by having the bases of the parts of the mouth encapsulated within the head. Within this class there are three orders: Protura, Diplura and Collembola. The first two are very small organisms and lack eyes. Collembola, in contrast, are more abundant and well-known.

The second class is Insecta. It is characterized by having an overwhelming diversity, with representatives that have managed to colonize virtually all environments.

They are distinguished from the previous class mainly by the presence of the mouthparts outside the capsule of the head and most have wings.

Taxonomy

The hexapods within the arthropods

Arthropods are protostomized animals that belong to the phylum Arthropoda and the clade Panarthropoda. Their bodies are segmented, they have jointed appendages and a cuticle made of chitin. This extensive animal group is divided into five subphyla: Trilobita, Myriapoda, Chelicerata, Crustacea, and Hexapoda.

Historically, arthropods were divided into unirrámeos and birrámeos, depending on the branching of the appendages.

However, current phylogenies that have been reconstructed using molecular evidence do not support the monophyly of these groups. Thus, the current view does not support that the unirrámeos appendages have been inherited from a single common ancestor that possessed this structure.

Are spiders and scorpions insects?

It is common that some arthropods popularly known to man are mistaken for insects. For example, spiders, scorpions, centipedes, and millipedes are mistaken for insects, when they actually belong to other groups of arthropods.

Spiders can easily be distinguished from insects, as they have four pairs of legs instead of three, as well as no antennae.

By looking for these characteristics, which are easily observable, we will be able to discern whether or not an arthropod is an insect, thus avoiding confusion.

Relationships with arthropod subphyla

The relationships between the subphyla that make up arthropods are controversial.

One hypothesis proposes the union of the subphyla taking into account the presence of mandibles. Following this line of ideas, Hexapoda would be found next to myriapods and crustaceans. However, the homology of the jaws has been the subject of debate, as the structures differ considerably between the groups.

It is assumed that the Trilobita subphylum was the first to separate. Furthermore, the sister taxon of Hexapoda is considered to be crustaceans.

Different sources of evidence, molecular and taxonomic, support that these groups are related. Thanks to this arrangement, hexapods and crustaceans are usually grouped in the clade Pancrustacea.

Current classification of hexapods

The classification of hexapods usually varies depending on the reference consulted. Some tend to combine some groups, while others divide them. However, the classification that we will develop below is widely accepted.

According to Hickman (2007), there are two classes of hexapods and these are divided into their respective orders as follows:

-I. Entognatha class

The most important characteristic of this class of hexapod is the presence of an oral appliance that retracts into the head. They do not have wings.

Order Protura

This order contains about 500 species of tiny animals with subterranean habits - so they lack pigmentation - that inhabit the uppermost layer of the soil.

Although they are not very notable organisms due to their small size, they are ubiquitous in soils, particularly in forests. In many cases, they can reach significant densities.

Diplura Order

Dipluros are organisms that are closely related to true insects. They comprise about 800 species and, like the previous order, they are small, without any pigmentation and inhabitants of the soil. Some of them are found as cave dwellers.

The name of the group refers to the presence of two tails or long extensions, located at the end of the individual's body.

Order Collembola

The Collembola order has a worldwide distribution, occupying all continents - including Antarctica. In some classifications, collembola fall within the group of insects.

The abundance of these organisms is extraordinary. In fact, according to some authors, they could be considered the most numerous animals on planet earth.

They are characterized by an appendix called a furcula. They have two antennae and six or fewer abdominal segments. In the thorax there is a pair of legs for each of the segments.

-II. Class Insecta

Among the arthropods, the Insecta class takes the first position in terms of diversity and abundance of its members.

In fact, there are more species of insects than any other animal group combined. The experts who focus on the study of this group are called entomologists.

Thanks to this overwhelming diversity, it has been difficult to characterize the group, in ecological, medical and economic terms. They are divided into:

Subclass Apterygota

This subclass aims to group all organisms within insects that lack wings and live in humid areas. This type of classification is an artificial group, since it does not represent the evolutionary history of organisms. Therefore, it lacks taxonomic value.

Order Thysanura

The members of this order have a series of filaments that protrude in the terminal region and do not have wings. Currently there is a controversy related to the name of the order, some authors prefer to refer to these organisms as Zygentoma.

Subclass Pterygota

This group contains insects that have wings. In contrast to Apterygota, Pterygota is a monophyletic group.

Infraclass Paloptera

This group is characterized by not having the ability to fold its wings, which is an ancestral character.

Order Ephemeroptera

They have primitive characteristics, such as a tail of considerable length, and wings without the ability to fold. Juvenile forms are restricted to aquatic environments, particularly freshwater bodies. His name comes from his short adult life.

Odonata Order

It includes very common and well-known species such as dragonflies and damselflies. They are usually associated with aquatic environments.

Infraclass Neoptera

This grouping contains all insects with the ability to fold their wings when they are at rest - with the exception of butterflies.

Order Orthoptera

Includes crickets, grasshoppers, locusts, and others. Its most outstanding feature is the presence of a chewing mouthpiece and specialized legs for jumping.

Order Blattodea

Blatodeos include cockroaches and the like. The representatives of this order exceed 4,500 species.

Order Phasmatodea

There are more than 3,000 species of stick insects. These insects have very striking adaptive traits related to camouflage.

Order Mantodea

They include mantises and animals related to them. They are very diverse, with almost 2,400 species, distributed throughout the world, but particularly diverse in the tropics. Their coloration is green - and allows them to be confused with the undergrowth - and their front legs are specialized to capture their prey.

Order Mantophasmatodea

This group is considered a "mix" between stick insects and mantises. They are carnivores and do not have wings.

Order Dermaptera

They are popularly known by the name of scissors or scissors. Its name is due to the structure that is located at the end of the animal's body, which clearly resembles a scissors.

Order Coleoptera

Coleoptera are called beetles. They are the most diverse order in the animal kingdom, with more than 375,000 described species. They are characterized by having a chewing mouthpiece and a pair of wings in their morphology has been transformed into rigid areas that do not participate in flight, known as elytra.

Order Lepidoptera

They contain the diurnal and nocturnal butterflies; flying in most cases. They are characterized by having large wings and striking colors. Its larvae, the caterpillars, are well known. Currently, some 165,000 species have been described.

Order Hymenoptera

Hymenoptera include some 153,000 species of bumblebees, watchers, bees, and ants. They are characterized by the presence of two pairs of membranous wings.

There are other orders of species, such as Diptera, Embiidina, Psocoptera, Zoraptera, Phthiraptera, Thysanoptera, Hemiptera, Plecoptera, Isoptera, Strepsiptera, Mecoptera, Trichoptera, Siphonaptera.

Distribution

Insects are a group distributed in virtually any environment, with the exception of marine ecosystems. Few insects are considered truly marine. Some species only live on the surface of the sea, that is, their habitat is in the transition between water and sea.

They are extremely abundant in fresh water sources, in the soil of different ecosystems, in forests, in deserts, among others. Dry and aric environments do not pose any limitation, since its cuticle offers protection against possible drying events.

They are also found living on other animals, as parasites of these. Many of them have wings. The colonization of aerial environments by arthropods occurred long before the development of flight in groups of vertebrates.

References

1. Barnes, RD (1983). Invertebrate zoology. Interamerican.
2. Brusca, RC, & Brusca, GJ (2005). Invertebrates. McGraw-Hill.
3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
4. Irwin, MD, Stoner, JB, & Cobaugh, AM (Eds.). (2013). Zookeeping: an introduction to the science and technology. University of Chicago Press.
5. Marshall, AJ, & Williams, WD (1985). Zoology. Invertebrates (Vol. 1). I reversed.