What are arthropods?

We explain what arthropods are and how this group of invertebrates is classified. In addition, its main features and examples.

1. What are arthropods?

It is known as arthropods to an evolutionary set (or  phylum ) of  invertebrate animals of complex organization,  provided with an external skeleton, segmented bodies and articulated legs (hence its name: from the Greek  arthron , “articulation” and  poús , “foot”) . These are the most numerous animals on the planet, adapted to any type of environment  that exists, that is, they are the animals with the greatest evolutionary success that exist.

One of the main characteristics of arthropods is the segmentation of their legs and their body, joined through joints that allow   precise and rapid movements . From there, different forms of articulated appendages arose, such as antennas, pincers, chelceres, etc.

The edge of arthropods  appeared on the earth about 570 million years ago , and due to their morphological relationship with annelids (worms and worms), it is assumed that they would have evolved from them. Strong evidence of this could be the larval stage (in the form of caterpillars and larvae) that have many arthropods.

There are currently more than 1,200,000 species of arthropods approximately , which represents 80% of the animal species known. Incidentally, of the arthropod classes, insects are the most numerous.

2. Classification of arthropods

Arthropods are classified into four large sets or classes:

• Arachnids . Endowed with chelceres, they lack wings and antennae, and have four pairs of legs. Your body is divided into cephalothorax and abdomen.
• Insects . The most varied and numerous of all arthropods have a pair of antennas, three pairs of legs and two pairs of wings (functional or not). Adapted to almost any physical medium, they range from detritophages, parasites, herbivores to predators.
• Crustaceans . They are mostly aquatic, with presence in the seas, or also in humid terrestrial environments. They always present a stage of nauplio larva, characteristic of their evolution as a class.
• Millipedes . Provided with multiple legs and long bodies, in addition to jaws (chelicers), they are similar to insects in many things, but they are eminently terrestrial and tend to be poisonous.

3. Characteristics of arthropods

A typical feature of arthropods is that their body is successively segmented in a manner similar to that of annelids. In addition, they have well differentiated sections that usually respect the following order: head, thorax and abdomen, apart from their limbs or appendages.

Their bodies are also protected from their rivals or predators by a skeleton articulated and formed by chitin (a carbohydrate), which is located outside the body (exoskeleton) and covers it.

This is a problem for the animal when it comes to growing, so that the skeleton must be changed in several successive stages throughout its life . This allows the development of a new skeleton, adapted to the larger dimensions of the animal. This process is called  ecdisis  or  molt .

On the other hand, arthropods reproduce sexually  and their sexes are usually well differentiated . When fertilized, the female lays eggs from which the offspring will emerge, whose development can be direct or indirect, depending on the species:

• D evelopment d irect . When the egg hatches, an identical individual to the adult emerges, only smaller. With time and nutrition , it will grow to maturity.
• Indirect . From the egg a larva will emerge very different from the adult individual, which will grow and go through different stages of profound change, called  metamorphosis , until its shape is changed to that of an adult or  imago .

4. Arthropod Examples

Some simple examples of arthropods are:

• Arachnids : spiders, scorpions, ticks, mites.
• Insects : cockroaches, praying mantises, butterflies, moths, beetles, bees, aphids, cicadas, fleas, flies, dragonflies, ants and a huge etcetera.
• Crustaceans : lobsters, crabs, shrimp, prawns, barnacles.
• Myriapods : centipede, millipede, pauropod, syphilic and moisture mealybugs.