Arthropods are a group of animals that have jointed appendages and no backbones. Mammals have also jointed legs but they aren’t alike. Arthropods belong to Kingdom Animalia and there are 5 subphyla under the main phylum Arthropoda. 84% of all known species of animals on earth are arthropods. It means the most successful group of animals on the earth is the phylum Arthropoda. It is divided into 5 major subphyla according to their morphological characteristics. They are Chelicerata, Myriapoda, Crustacea, Hexapoda, and Trilobita. The subphylum trilobite had only one genus called Trilobites. It had over 20000 species but they have been extinct about 250 million years ago. But still, now, the other 4 subphyla show great diversity.
There are over 800000 named species on earth. It describes how successful the phylum Arthropoda is. The success of an animal group is describing on few parameters. It should contain a large number of species, they should occupy many different kinds of habitats, they should feed on any kinds of food and finally, they should be good at defending themselves. From these perspectives, arthropods are very successful than other animals on earth. Behind this success, there are 5 main reasons.
The main thing is the exoskeleton. The exoskeleton is a thinner outer layer that adapts to protect internal soft body parts. The exoskeleton is one of the greatest inventions of nature. But in higher phyla such as mammals and birds don’t have an exoskeleton but an endoskeleton. Though exoskeleton helps helped arthropods to become the most successful group of animals on the earth, the reason to don’t having an exoskeleton for advanced animals is the exoskeleton works better in small animals that smaller than about 27 cm. but the endoskeleton works better in larger animals like birds, mammals and fish. This exoskeleton of arthropods is made up of chitin and various proteins as basic materials. There is a lot of functions from the exoskeleton to the success of arthropods.
Arthropods don’t have an internal skeleton thus, they don’t have a strong and denser structure to attach their muscles and appendages. So, the exoskeleton provides a larger surface to attach those muscles and segments. And also it helps with the o]movements of muscles. It gives arthropods support to move to well-conditioned habitats from harsh habitats in harsh time periods. It is a major reason behind see arthropods in various types of niches and habitats. They can be seen as terrestrial, freshwater and marine species. They’re originated from the ocean. Thus, the exoskeleton was a major reason to evolve terrestrial arthropods. Because it prevents water loss from the body. On the other hand, they are relatively smaller animals than other animal species. therefore, they have easily become prey to other larger animals. But the exoskeleton acts as a shield and helps them to prevent from being prey. It means the exoskeleton improves their self-defence. Because it is hard and quite unbreakable. And also, the exoskeleton prevents damages due to harmful radiation from pathogens.
The second major reason behind their success is having metamorphosis. Metamorphosis is a process that having different forms at different levels in the life cycle. In the subphylum Hexapoda, immature insects have a worm or grub-like life form and adults have a completely different form. It helps to decrease their competition for food. Because the immature form feeds on a completely different food source than the adults. As an example, butterflies show a complete metamorphosis (a complete life cycle). It has a larval stage, pupa stage, and an adult stage, in the larval stage they usually feed on leaves but the adult form mainly feeds on flowery nectar. It helps to reduce the competition among the same species and between other species.
The third reason for the success of arthropods is tagmatization. Tagmatization is a specialized group of multiple segments or metameres into a single functional morphological unit. Insects (subphylum Hexapoda), is the most successful group of animal in the phylum Arthropoda. They have 3 major segments in the body called the head, thorax, and abdomen. Their all body appendages that help to locomotion such as legs and wings are attached to the thorax. It is better to have all appendages in one single segment than to have appendages in separate segments. And also they have wings and legs and they help to move from niche to niche. it helps them explore well-conditioned environments.
As mentioned above, those features help arthropods to explore new environments and protect from predators, pathogens and natural harms. Having an exoskeleton, showing metamorphosis, and tagmatization are the main reasons behind the success of Arthropods on the earth.