We explain what arthropods are and how this group of invertebrates is classified. Also, its main characteristics and examples.
What are arthropods?
An evolutionary group (or phylum) of invertebrate animals with complex organization, provided with an external skeleton, segmented bodies and articulated legs (hence its name: from the Greek arthron“articulation” and poús“foot”). They 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 body, united through joints that allow precise and fast movements. From there, different forms of articulated appendages also emerged, such as antennae, pincers, chelicerae, etc.
The phylum of the arthropods appeared on 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 many arthropods have.
At the moment There are approximately more than 1,200,000 species of arthropods which represents 80% of the known animal species. Incidentally, of the classes of arthropods, insects are the most numerous.
See more: Oviparous animals
Classification of arthropods
Arthropods are classified into four large groups or classes:
- Arachnids. Equipped with chelicerae, they lack wings and antennae, and have four pairs of legs. Its body is divided into cephalothorax and abdomen.
- Insects. The most varied and numerous of all arthropods, they have a pair of antennae, three pairs of legs and two pairs of wings (functional or not). Adapted to almost any physical environment, they range from detritophages, parasites, herbivores to predators.
- Crustaceans. They are mostly aquatic, with a presence in the seas, or also in humid terrestrial environments. They always present a nauplius larva stage, characteristic of their evolution as a class.
- Myriapods. Equipped with multiple legs and long bodies, as well as jaws (chelicerae), they are similar to insects in many ways, but they are eminently terrestrial and are usually poisonous.
Characteristics of arthropods
A typical feature of arthropods is that their body is successively segmented in a way similar to that of annelids. Besides, have well differentiated sections which usually respect the following order: head, thorax and abdomen, apart from their extremities or appendages.
Their bodies are also protected from their rivals or predators by an articulated skeleton made up of chitin (a carbohydrate), which is outside the body (exoskeleton) and covers it.
This poses a problem for the animal when it comes to growing, so the skeleton must be changed in several successive stages throughout its life. In this way, the development of a new skeleton is allowed, adapted to the larger dimensions of the animal. This process is called ecdysis either change.
On the other hand, arthropods reproduce sexually and their sexes are usually well differentiated. When fertilized, the female deposits eggs from which the offspring will emerge, whose development can be direct or indirect, depending on the species:
- Direct development When the egg hatches, an individual identical to the adult emerges, only smaller in size. With time and nutrition, it will grow to maturity.
- Indirect From the egg a larva will emerge that is very different from the adult individual, which will grow and go through different stages of profound change, called metamorphosisuntil changing its shape to that of an adult or imago.
Examples of arthropods
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. Centipedes, millipedes, pauropods, symphylls and woodlice.
