Arthropods | Definition, Anatomy and Classification

Arthropods | Definition, Anatomy and Classification

What is an Arthropod?

Arthropods are a large group of organisms, including insects, spiders, crustaceans, and many other animals. They have an exoskeleton outside of their bodies which protects them from predators and physical support for some parts. This article will discuss what arthropods are, the characteristics, types of arthropods, and examples.

Arthropods Definition

An arthropod is an invertebrate animal with a segmented body, jointed appendages, and a hard exoskeleton. They are the largest group of animals on Earth and include insects, spiders, crustaceans, and many other animals.

Arthropods Characteristics

They have several unique characteristics that distinguish them from other animals. They have a segmented body, meaning that their bodies are divided into sections called segments. Each segment has its muscles, nerves, and blood vessels. They also have jointed appendages, due to which limbs are attached to their body. Limbs are composed of multiple sections and can be used for many purposes such as feeding, defense, reproduction, and locomotion.

Parts of Arthropods body 

They have a head, thorax, and abdomen. Arthropod’s anterior end is the cephalothorax or prosoma, while its posterior end is called the abdomen or opisthosoma. The body of an arthropod consists of three major divisions: the Head, thorax, and abdomen. The head has a pair of antennae, eyes, and mouthparts. The thorax has six legs and two wings (if the arthropod is aerial). The abdomen typically contains the digestive and reproductive organs.

Exoskeleton of Arthropods 

A hard exoskeleton covered most of the body parts of an arthropod. The exoskeleton provides support for many organs, protects from predators, and protects against water loss. They have flexible cuticles that serve as both the exterior skeleton and skin. The exoskeleton is molted or shed as the arthropod grows.

Arthropod’s exoskeleton is composed of a rigid, flexible material called chitin. Chitins are long chains of polysaccharides that serve as the main structural component in the arthropod’s cuticle and cell walls. A thin layer of protein called the epicuticle covers the chitin. The epicuticle helps to protect the chitin from environmental factors and degradation.

Endoskeleton of Arthropods

Although they have an exoskeleton, they also have a rudimentary internal skeleton called the endoskeleton. The endoskeleton supports and helps maintain the body shape of unique structures such as muscle attachments. They lack a backbone, and the endoskeleton supports their body instead.

Arthropods have three different muscles:

– The outer muscle layer or epimysium is composed of thin muscles fibers.

– The middle muscle layer or mesoscutum has strong circular and longitudinal muscles.

– The innermost layer or hypodermis contains the thin oblique muscles responsible for moving arthropods’ appendages.

Types of Arthropods 

There are many types of arthropod species. Invertebrates such as mollusks, crustaceans, and others have exoskeletons, but they aren’t considered part of the group called “arthropods” because their body plans differ from those of the insects, spiders, and other arthropods.


The most well-known type of arthropod is the insect. Insects are the largest animals on Earth and include beetles, flies, bees, wasps, ants, and many others. Insects have a three-part body plan (head, thorax, and abdomen) and three pairs of jointed legs. They also have two pairs of wings (if they are aerial). Insects are the most diverse animals on Earth in terms of their size, shape, color, and lifestyle.


Spiders are terrestrial arthropods with eight legs, two body parts (cephalothorax and abdomen), a mouthpart called the fang for injecting venom into prey or enemies, and one pair of antennae sensory organs, and four pairs of walking legs. The difference between spiders and other arthropods is that the endoskeleton supports their body instead of an exoskeleton like in insects. Spider silk comprises proteins (polypeptides) called spidroins formed when a spider releases its spinneret glands and sericin to form silk fibers.


Another type of arthropod is crustaceans. Crustaceans include shrimp, crabs, lobsters, and others. They have a two-part body plan (cephalothorax and abdomen) and five pairs of legs. Crustaceans are mainly aquatic animals, but some species can live on land.


Mollusks are another type of invertebrate that is not considered an arthropod. Mollusks include clams, oysters, snails, and slugs. They have a soft body with a hard shell, lacking antennae and wings.

They are very diverse in their physical characteristics and lifestyles. Some arthropods are predators, which means they hunt and eat other animals. Others are herbivores, detrivores, or parasitic arthropods that feed on plants, dead organic material, or the bodies of living organisms, respectively.


They are marine arthropods with a hard exoskeleton, jointed legs for walking and swimming, compound eyes (similar to some modern insects), and antennae. Trilobites were more abundant during the Paleozoic Era but became extinct before humans evolved on Earth.


Chelicerate is another type of arthropod that is not considered an insect. They have a two-part body plan (cephalothorax and abdomen) and eight legs, chelicerate mouthparts such as fangs or pincers for capturing prey as spiders do, one pair of antennae that function as sensory organs, and a coelom that is present only in chelicerate.


Myriapods are arthropods that have a body composed of many segments (usually 30 or more). They include centipedes and millipedes. Centipedes have one pair of legs per segment, while millipedes generally have two legs per segment.



Hexapods are arthropods that have six legs. They include insects, spiders, and other terrestrial chelicerate, including scorpions.


Arachnid is a class of arthropods with four legs, an exoskeleton of chitin, and no antennae. They include spiders, scorpions, and ticks.

Many other types of arthropods don’t fit neatly into any one category. Some examples are horseshoe crabs, daddy-longlegs, water striders, and booklice.

Importance of Arthropods 

They are an essential part of the food web and play a vital role in the ecosystems in which they live. They are necessary for nutrient cycling and help break down dead organic matter. Arthropods are also a significant source of protein for many animals, including humans.

The diversity and abundance of arthropods are declining due to deforestation, pollution, and other factors. This decrease may cause more problems than just losing the arthropods themselves because their ability to break down dead organic matter is essential for the health of many ecosystems on Earth.

Facts About Arthropods

They are in the phylum Arthropoda. This means segmented bodies, an exoskeleton made of chitin, and jointed legs. They also lack antennae but may have sensory appendages such as claws or fangs for capturing prey.

They are found in nearly every habitat on Earth. They live underground, underwater, and even inside other organisms such as coral reefs! Arthropods have been around for hundreds of millions if not billions of years, and some species may live up to 30 years!

There are more than one million known species of arthropods. The largest group of arthropods is insects, including beetles, flies, butterflies and moths, bees and wasps, cicadas, dragonflies, and damselflies.