**Arthropods** are a group of animal species that have segmented bodies, a tough external exoskeleton that is used for protection and support, and joint appendages in the form of antennae, and legs. Examples of this species are insects, crabs, and spiders.

Members of the **arthropod** species are grouped and classified according to the number and appearance of their body segments. Moreover, they are also classified using their appendages, specifically the parts or structures of their mouth.

The three largest groups of arthropods differ in their structure and number of segments and functions.

1. **Crustaceans** have 2 pairs of antennae, 2-3 body sections, and a mandible.
2. **Chelicerates** (spiders and relatives) have chelicerae, 2 body sections, and 4 pairs of walking legs.
3. **Uniramians** (insects and relatives) have 1 jaw, 1 pair of antennae, and unbranched appendages.

**Spiders** are the largest group of arachnid species. They capture prey in many ways–some spiders spin webs, some fly to catch them. After the spider captured the prey, it injects venom via chelicerae and paralyzes the prey. Then, the spider injects digestive enzymes to break down the tissue and sucks the liquified tissue. It is then broken down into smaller molecules inside the spider’s stomach.

Horseshoe crabs aren’t considered as true crabs since their anatomical structure resembles closely to spiders. It differs from other chelicerates in terms of the number of appendages and other body structures. Normally, chelicerates have 5 sets of appendages (one pair of chelicerae and four pairs of legs), while horseshoe crabs 6 pairs of appendages (one pair of chelicerae and five pairs of walking legs). In addition, horseshoe crabs lack an antennae and a segmented body.

**Centipedes** have flat, segmented bodies that have 1 pair of legs attached to each segment. Centipedes inject venom, kill the prey, and eat it. On the other hand, **millipedes** have round, segmented bodies that have about 2-3 pairs of legs attached to each segment. Moreover, **millipedes** only eat the dead, decaying plants.
However, both species live in moist areas since their bodies are not water-proof.

Insects are closely related to centipedes since they both belong to subphylum Uniramia. Centipedes and insects both have antennae, unbranched appendages, and jaw-like mandibles.