Arthropods (Arthropoda)

Phylum of animals

Food, Animal source foods

Consumption area(s): Earth

Introduction

Arthropods (Arthropoda) constitute a vast phylum of invertebrates within the protostomes, all equipped with a coelom. Their body is defined by multiple jointed appendages and a rigid cuticle, which forms a protective exoskeleton made of chitin. Collectively, they represent almost five-sixths of the known animal species.

With more than one million species already described—and estimates suggesting between five and ten million in total—arthropods demonstrate an exceptional flexibility and evolutionary adaptability of their structural design, allowing them to thrive in an extraordinary variety of habitats.

Description of arthropods (arthropoda)

All arthropods share a hardened cuticle that forms a lightweight yet sufficiently rigid exoskeleton, providing both protection and structural support. This exoskeleton is composed primarily of chitin, a substance secreted by the cells beneath it. As it contains no living cells, the exoskeleton cannot grow along with the organism; instead, arthropods increase in size through periodic molting. Rather than being a continuous layer, the exoskeleton is segmented into hardened plates called sclerites, connected by softer, flexible regions known as pleura, which allow movement.

A feature of arthropods is metamerism, in which the body is composed of repeated segments called metameres. In the most basal forms, segments are largely homonomous, showing little structural differentiation. Over evolutionary time, these segments became increasingly specialized, grouping into distinct body regions or tagmata—a condition termed heteronomous metamerism, typical of modern arthropods.

For instance, chelicerates display two main tagmata, the prosoma and opisthosoma, while primitive mandibulates (including crustaceans, myriapods, and hexapods) show a head and trunk division. In more derived mandibulates, the trunk differentiates further into thorax and abdomen.

Appendages in arthropods are jointed, which is the source of the phylum’s name. While not all appendages contain muscles, many are moved via tendons connected to muscles in the trunk. The most primitive appendages are legs, originally present on all segments, but in specialized forms they remain mainly on thoracic segments, while other appendages may be reduced or transformed for specialized functions.

Internally, the coelom is reduced and largely replaced by a hemocoel, a body cavity that surrounds the digestive tract and extends into other parts of the body. This cavity develops partly from embryonic coelomic pouches and partly from remnants of the blastocoel, forming a mixed cavity called a syncelom or mixocoel. The hemolymph filling this space also circulates through the open circulatory system, performing transport functions throughout the body.

Respiratory structures are highly efficient, supporting energetically demanding activities such as flight. Aquatic arthropods use external gills, while terrestrial forms rely on either tracheae, tiny branching tubes opening through spiracles (as in insects), or book lungs, layered structures folded inside the body to maximize gas exchange (as in arachnids).

Classification of arthropods (arthropoda)

Arthropods are classified into the following subphyla:

• Chelicerates (scorpions, spiders)
• Crustaceans (barnacles, crabs, lobsters, mantis shrimps, shrimps/prawns)
• Hexapods (insects)
• Myriapods (millipedes)