The answer to the question of what animal is defined by having six legs is found within the biological classification known as the subphylum Hexapoda. This group includes all true insects, which possess three pairs of jointed legs. Hexapoda represents more than half of all known living organisms on Earth. This six-legged body plan allows these invertebrates to inhabit nearly every terrestrial and freshwater environment worldwide.
The Hexapod Classification
The scientific name Hexapoda is derived from the Greek words hexa (“six”) and pous (“foot” or “leg”), directly indicating their defining characteristic. This subphylum belongs to the larger Phylum Arthropoda, which encompasses all invertebrates with jointed limbs and an exoskeleton, such as crustaceans and spiders. Hexapoda is primarily composed of the Class Insecta, the true insects, which are the most numerous and widespread organisms in the group. The remaining few groups, collectively called Entognatha, are small, wingless arthropods like springtails and proturans that share the six-legged structure but differ slightly in mouthpart arrangement.
Anatomy of Six-Legged Animals
The six legs are part of a highly organized body structure separated into three distinct regions: the head, the thorax, and the abdomen. All six legs are attached exclusively to the middle segment, the thorax. The thorax is composed of three smaller segments—the prothorax, mesothorax, and metathorax—and each bears a single pair of jointed legs. The legs are segmented into five main parts, ending in a tarsus, or foot, which provides grip and traction.
This specific leg arrangement enables the insect’s characteristic locomotion, often utilizing a highly stable “tripod gait.” During movement, the first and third legs on one side, along with the middle leg on the opposite side, move forward simultaneously. This alternating pattern ensures the animal always maintains three points of contact with the surface, providing remarkable stability even on uneven terrain. The head region, anterior to the thorax, houses a single pair of sensory antennae and usually a pair of large compound eyes. The posterior abdomen contains the majority of the digestive, excretory, and reproductive organs.
Major Groups of Insects
The six-legged body plan has diversified into a massive array of forms categorized into distinct orders. The order Coleoptera, or beetles, is the largest of all, containing over 400,000 described species. These insects exhibit a vast range of sizes and feeding habits. Their forewings are modified into hardened, protective covers called elytra. Another prominent order is Lepidoptera, which includes all butterflies and moths, characterized by their wings being covered in minute, overlapping scales.
The Hymenoptera order, encompassing ants, bees, and wasps, is known for complex social structures and is ecologically significant as pollinators and predators. They possess two pairs of membranous wings that are hooked together during flight to function as a single unit. Flies and mosquitoes belong to the order Diptera, meaning “two wings,” because they only possess a single pair of functional flying wings. Their hind wings have been reduced to small, club-shaped balancing organs called halteres, which assist in aerial maneuverability.
Clarifying Other Leg Counts
While the six-legged design is the hallmark of insects, many other terrestrial invertebrates are mistakenly grouped with them. The most common confusion involves the class Arachnida, which includes spiders, scorpions, and ticks, defined by having eight legs. Arachnids possess four pairs of legs attached to a fused head and thorax region, and they lack antennae. Furthermore, their bodies are divided into only two main segments, unlike the three segments seen in true insects.
Another major group of multi-legged arthropods is the Myriapoda, which means “ten thousand feet” and includes centipedes and millipedes. Centipedes (class Chilopoda) have elongated bodies with a single pair of legs on most body segments. Millipedes (class Diplopoda) have bodies where most segments appear to have two pairs of legs, resulting from two body segments fusing together. These distinct leg counts and body plans separate these groups from the six-legged classification of the Hexapoda.