Arthropods are invertebrate animals that have an exoskeleton.
Arthropods are invertebrate animals that form the most diverse phylum in the animal kingdom. These animals have their bodies covered by an exoskeleton known as cuticle and formed by a linear series of ostensible segments, with appendages of articulated parts. Arachnids , insects , and crustaceans are arthropods.
Specialists estimate that there are more than one million species of arthropods, which represents about 80% of all known animal species. Most arthropods are insects, and many of them are adapted to life in the air.
Despite the great variety of arthropods, several common characteristics can be mentioned, such as the presence of a chitinous exoskeleton and articulated appendages , or the body made up of repetitive segments (a phenomenon called metamerism ).
Reproduction of arthropods
In addition to all these characteristics mentioned, it is also important to establish that another of the main hallmarks of arthropods is their reproduction system. In this case we can emphasize that the female is the one in charge of laying eggs, once fertilization by the male has occurred.
In this case, the result of this process can be of two types. Thus, from that egg an individual similar to its parents can be born directly or from it the birth of a larva can occur that, little by little, will be transformed in a process known as metamorphosis until giving rise to the aforementioned being.
Among the arthropods we find spiders.
The exoskeleton
The exoskeleton is made up of different layers. The superficial layer, called epicuticle , is very thin, is composed of proteins and lipids, and has a waterproofing function. The procuticle is the thickest layer of the cuticle and can be divided into exocuticle (the most rigid part) and endocuticle (flexible).
It should be noted that, throughout its growth, the arthropod sheds its exoskeleton through a process of ecdysis .
Classification of arthropods according to their legs
There are various classifications that exist to determine the different types of arthropods that exist today. However, the most common thing is to make groups of these living beings based on the number of legs they have. In this way, we would find four large sets:
Arthropods with six legs. Insects would be found within this group.
Arthropods with eight legs. Those known as chelicerates are those that give shape to it. As an example of these we would have spiders, scorpions or even those known as horseshoe crabs. The main characteristic that defines them and differentiates them from other arthropods is that they do not have antennas.
Arthropods with ten legs. Crustaceans are those included in this set, that is, crabs, shrimp or lobsters.
Arthropods with more than twelve legs. In their case, the members of this group are the myriapods, that is, living beings like centipedes.
the eyes
Finally, we can highlight the particularity of the eyes of arthropods.
These eyes can be simple , with a simple retina and a transparent cornea that covers them, or compound , made up of various elements (the ommatidia) that are located radially and can point in different directions.