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Why are insects classified as arthropods?

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Illustration showing a variety of adult insects, each labeled with its common name and order, including walkingstick, earwig, stonefly, tree cricket, dragonfly, female scale, cat flea, thrips, sucking louse, thysanuran, ant, scarab beetle, stinkbug, mosquito, and cabbage butterfly, highlighting their diverse shapes and sizes. — insect An illustration of insect diversity.

Insects are classified as arthropods because they share the fundamental characteristics that define the phylum Arthropoda, the largest group in the animal kingdom.1

The primary features that categorize insects within this phylum include:

Exoskeleton: Like all arthropods, insects possess a rigid external skeleton composed of chitin bound to protein. This structure provides support, protection, and a surface for muscle attachment.12
Segmented Bodies: Insects exhibit a segmented body plan, which is a hallmark of arthropods.31
Jointed Appendages: The name "arthropod" is derived from the Greek for "jointed feet." Insects possess paired, jointed appendages, which are adapted for various functions, including locomotion.12

While insects are distinct in their specific anatomy—such as having a body divided into a head, a three-segmented thorax, and an abdomen—they are grouped under the subphylum Hexapoda within the phylum Arthropoda because they fundamentally adhere to the structural blueprint of this diverse group.314