If there is a group with an indefinite number of species, it is the arthropods, as they are the most diverse animals on the planet. Estimates include anywhere from 2 to more than 10 million species, which undoubtedly shows their diversity. These have conquered all existing media, so they are, from the adaptive point of view, very efficient.
On the other hand, this group is of great importance for the ecological relationships of all ecosystems on a planetary level, so its presence is fundamental in them. Given its relevance, in this article on our site we present information about the arthropod animals, what they are, characteristics, classification and examples
What are arthropods?
The word arthropod comes from the Greek árthron, meaning “joint”, and podo, meaning “foot”. Arthropods correspond to various groups of invertebrates. They are segmented animals that have a articulated exoskeleton made of a substance called chitin. In addition, they have various even numbered appendages on the body segments.
Due to the presence of chitin, many arthropods have a certain rigidity, so they are unable to grow like other animals do. Therefore, they usually go through several molts as the outer skeleton outgrows the growing body. They discard it and form a new one.
Arthropods are the most diverse group of multicellular animals on the planet, with sizes ranging from a few millimeters to over a meter in marine species. Their forms and their adaptations are quite variable depending on the group, but they share certain common features.
Classification of arthropod animals
Traditionally, arthropods have been classified into four subphyla, which are:
- Trilobites: exclusively marine, they are extinct animals. They were made up of three lobes, which were the head, thorax and abdomen, and their appendages were biramous (two branches).
- Chelicerates: The first appendages of these animals were modified into chelicerae (mouthparts). They have a pair of pedipalps (second pair of appendages) and four pairs of legs. They lack antennae and jaws and usually have a cephalothorax and an unsegmented abdomen.
- Uniramid: they are characterized by the presence of single-axis appendages, a pair of antennae and mandibles, in addition to numerous body segments.
- Crustaceans: These are primarily aquatic, usually have a dorsal shell, and the myrrhamous appendages have been modified to serve various functions. They have two pairs of antennae and one pair of mandibles.
Characteristics of arthropods
Let's learn about the main characteristics of arthropods:
- They have bilateral symmetry.
- The body is divided into tagmata, which can form head and trunk, head, thorax and abdomen, or cephalothorax and abdomen.
- Generally, appendages have been specialized to fulfill particular functions.
- The exoskeleton is a cuticular structure made up of proteins, lipids and chitin, which is an organic-type polymer.
- They have a complex muscular system, associated with the exoskeleton.
- As for the digestive system, it is complete. The first appendages have been modified into mouthparts and, depending on the group, feed in a particular way.
- The circulatory system is open, with the presence of a fluid called hemolymph, which is analogous to blood.
- The way you breathe varies. In some cases it is done through the surface of the body, while in others it is through gills, tracheas or book lungs.
- They have excretory glands.
- Some species are poisonous.
- The nervous system includes very well developed sensory structures.
- They usually have separate sexes and internal fertilization. In some cases they are oviparous and, in others, ovoviviparous. There are also species that present parthenogenesis.
- It is very common to develop metamorphosis.
Types of arthropods
As we have mentioned, arthropods are the most diverse animals on the planet. They have conquered the aquatic, terrestrial, and aerial environments, including the depths of the earth, caves, the interior of trees, and endless spaces. Let's learn about the types of arthropods.
- Arachnids: they have four pairs of legs, the abdomen may or may not be segmented, they are mainly oviparous and lack a real metamorphosis.
- Pycnogonids: generally quite small, about 3-4 mm, although some reach up to 5 cm. They have four to six pairs of long legs, with long proboscises and simple eyes.
- Merostomates: they are aquatic chelicerates with cephalothorax and abdomen, compound eyes and appendages with gills.
- Branchiopods: These are crustaceans that may or may not have a shell, compound eyes, and rather reduced antennae.
- Remipedes: these are blind crustaceans, lacking a shell, with antennules and antennae of a single axis, with all the appendages very Similar.
- Cephalocarids: shellless benthic crustaceans. Uniramous antennae and biramous antennae.
- Maxillopods: These are crustaceans that may or may not have a shell and lack the typical appendages of the abdomen.
- Ostracods: microscopic crustaceans protected by shells. They have no more than two thoracic appendages.
- Malacostracea: all segments have appendages and usually have biramous antennules. Some have a shell, which covers the head and part of the thorax or even all of it.
- Diplopoda: they have short antennae and legs with simple eyes. The number of metamers is variable.
- Chilopods: the body is flattened dorsoventrally and the metameres are variable, but in each one they have a pair of legs and long antennae.
- Pauropods: Tiny, eyeless, cylindrical animals with 9-10 pairs of legs.
- Symphylos: threadlike body, long antennae and no eyes. The body has between 15 and 22 segments and 10 to 12 pairs of legs.
- Insects: body divided into head, thorax and abdomen. Presence of a pair of antennae and mouthparts adapted, depending on the group, to different ways of feeding. They have one or two pairs of wings and three pairs of jointed legs. They metamorphose gradually or abruptly.
Examples of arthropod animals
These are some of the best-known arthropod animals:
- Scorpions.
- Spiders.
- Mites.
- Ticks.
- Sea spiders.
- Horseshoe crabs.
- Water fleas.
- Copepods.
- Lobsters.
- Cigalas.
- Millipede.
- Centipede.
- Ants.
- Butterflies.
- Dragonflies.
- Grasshopper.
- Stick insects.
- Flies.
- Mosquitoes.
- Cockroaches.
