Meaning of Arthropods
According to DigoPaul, the arthropods are invertebrate animals that form the most diverse phylum of the animal kingdom. These animals have their bodies covered by an exoskeleton known as a cuticle and formed a linear series of ostensible segments, with appendages of articulated pieces. The arachnids, the insects and crustaceans are arthropods.
Specialists calculate that there are more than a million species of arthropods, which represents about 80% of all known animal species. Most arthropods are insects, and many of them are adapted for life in the air.
Despite the great variety of arthropods, several characteristics in common can be mentioned, such as the presence of a chitinous exoskeleton and articulated appendages, or the body formed by repetitive segments (a phenomenon that is called metameric).
In addition to all these characteristics, it is 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 their parents can hatch directly or from that egg a larva can be produced that, little by little, will be transformed into a process known as metamorphosis until giving rise to said being.
The exoskeleton is made up of different layers. The surface layer, which is called the 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 stiffest part) and endocuticle (flexible).
It should be noted that, throughout its growth, the arthropod changes its exoskeleton through an ecdysis process.
There are various classifications that exist to determine the different types of arthropods that exist today. However, the most common is to make groups of these living beings based on the number of legs they have. In this way, we would find four great sets:
Arthropods with six legs. Within this group would be insects.
Arthropods with eight legs. Those known as chelicerates are those that shape it. As an example of these we would have spiders, scorpions or even those known as horseshoe crabs. The main characteristic that defines and differentiates them from the rest of 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 his case, the members of this group are the myriapods, that is, living beings like centipedes.
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 made up of various elements (the ommatidia) that are located radially and can point in different directions.
