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
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