What Animals Have 6 Legs? Insects and Other Creatures

Insects are the animals with six legs, and they make up the vast majority of six-legged creatures on Earth. But insects are not the only ones. They belong to a larger group called Hexapoda, which also includes a handful of lesser-known relatives that share the same basic body plan of three pairs of legs attached to a three-segmented middle body, or thorax. With well over a million described species and estimates of several million more awaiting discovery, the six-legged animals dominate animal life on land in ways that can be hard to appreciate until you look at the numbers.

The Six-Legged Club Beyond Insects

When most people think of six-legged animals, beetles and butterflies come to mind. Technically, though, the group Hexapoda contains four classes, and only one of them is Insecta. The other three are far less familiar: Collembola (springtails), Protura (coneheads), and Diplura (two-pronged bristletails). All four share that defining trait of six thoracic legs, but they differ in some important ways, particularly in how their mouthparts sit relative to the head capsule. Insects have exposed, external mouthparts, while the other three groups have mouthparts that are partially recessed inside the head, which is why they are sometimes grouped together under the label Entognatha.

Springtails are by far the most commonly encountered of these non-insect hexapods. They live in soil, leaf litter, and decaying wood all over the world, sometimes in enormous densities. Most are tiny, just a millimeter or two long, and they get their name from a forked, spring-loaded appendage on the underside of the abdomen that can launch them into the air when disturbed. Protura are even smaller, usually under two millimeters, and they have no eyes or antennae. Instead, they hold their elongated front legs forward and apparently use them as sensory organs, a role their missing antennae would otherwise fill. Diplura are slightly larger and live in similar soil habitats, with a pair of prominent tail-like appendages at the rear of the body.

How these four groups relate to each other evolutionarily has been debated for decades, and the picture is still being refined. Molecular studies have consistently supported Protura as the earliest branch to split off from the rest of Hexapoda, with Collembola and Diplura more closely related to each other or to insects than to Protura.¹ An earlier phylogenetic analysis similarly found that Protura is the sister lineage to all other hexapods, while also confirming that major insect groupings like Holometabola (the insects with complete metamorphosis, such as beetles and flies) are solidly supported as natural groups.²

Terrestrial Crustaceans With Six Legs

One of the more surprising findings in animal evolution over the past few decades is that hexapods are not most closely related to millipedes and centipedes, as biologists long assumed. Instead, molecular and anatomical evidence points to crustaceans as the closest relatives of six-legged animals. Features of the eyes and nervous system support this relationship, as does a large and consistent body of genetic data.³ One phylogenetic analysis of nuclear genes from dozens of arthropod species found that hexapods are most closely related to fairy shrimp, water fleas, and an obscure group of cave-dwelling crustaceans called Remipedia, making hexapods essentially terrestrial crustaceans.⁴

This means crustaceans as traditionally defined are not a neat, self-contained group. Instead, hexapods sit inside the crustacean family tree, making “Crustacea” paraphyletic unless hexapods are included. It is a bit like discovering that birds are dinosaurs: the popular understanding of one group turns out to be nested within another. The broader grouping that includes both crustaceans and hexapods is now often called Pancrustacea or Tetraconata.

The fossil record of early hexapods is patchy but stretches back at least to the Early Devonian, roughly 400 million years ago. Fragmentary remains of springtails and an enigmatic insect called Rhyniognatha have been found in rocks from that period.⁵ A remarkable fossil from the Lower Devonian Hunsrück Slates of Germany, Devonohexapodus, shows a clear three-segmented thorax with three pairs of legs, suggesting that the six-legged body plan may have originated in marine environments before hexapods colonized land.⁶

Why the Tripod Gait Works

Having six legs is not just about the number. It is about what that number enables. The signature locomotion pattern of six-legged animals is the tripod gait, where three legs move together as a unit (front and back on one side, middle on the other) while the remaining three stay planted. This means an insect always has a stable triangle of support on the ground, even while the other half of its legs swing forward. You can think of it as constantly leaning on a tripod while repositioning the next one.

Research using computer models and hexapod robots has shown that the tripod gait is not simply the default way to use six legs. It emerges as the best strategy specifically when an animal needs to climb on surfaces using leg adhesion. On flat surfaces without adhesion, the models actually predict that a two-legged coordination pattern would be faster. When researchers covered the sticky pads on the feet of fruit flies, the animals shifted to a more bipod-like leg coordination, consistent with this prediction.⁷ So the tripod gait appears to be an adaptation strongly shaped by the need to climb and cling, which is something insects do constantly in their natural environments.

When Insects Break Into a Run

Not all insects stick rigidly to the tripod gait. At very high speeds, some switch to a fundamentally different mode of locomotion that includes brief moments when no legs touch the ground at all. Desert ants of the genus Cataglyphis, which sprint across scorching sand to minimize heat exposure, provide striking examples. One species, Cataglyphis fortis, transitions from walking to running at speeds above roughly 370 millimeters per second. At that point, the fraction of the stride each leg spends on the ground drops below half, and the ant enters aerial phases, essentially bouncing across the surface.⁸

Other Cataglyphis species use slightly different strategies to get fast. Smaller individuals of C. bicolor achieve some of the highest speeds recorded for ants, up to 80 body lengths per second, by lengthening their stride and incorporating aerial phases. Meanwhile, C. albicans relies more on extremely high stride frequencies, reaching up to 40 cycles per second, and tighter synchronization of leg movements.⁹ These are two solutions to the same problem: how to move very fast with six short legs over hot terrain.

Not All Six Legs Are Built the Same

One of the more remarkable things about insect legs is how diversified they have become across different lineages, even though they all derive from the same basic segmented structure. Insect legs have been modified for walking, running, digging, jumping, catching prey, and swimming, sometimes all within a single animal’s body.¹⁰

A grasshopper’s hind legs are massively enlarged for jumping, with thick femurs packed with muscle. A praying mantis has raptorial forelegs armed with spines for snatching prey. Dung beetles have front legs with tooth-like structures built for powerful burrowing through soil and dung.¹¹ Water beetles have fringed, oar-like hind legs. Mole crickets have broad, shovel-shaped front legs for tunneling. What unites all of these is the same underlying plan of segments (coxa, trochanter, femur, tibia, tarsus), modified in proportion and shape to suit radically different functions.

When Six Legs Become Four

Some insects have six legs in principle but fewer in practice. Butterflies in the family Nymphalidae, which includes monarchs, painted ladies, and morphos, are often called “brush-footed butterflies” because their front pair of legs is drastically reduced, tiny and brush-like, held up against the body and useless for walking. These butterflies walk and perch using only their middle and hind pairs. A systematic study of limb evolution across butterflies found that this reduction evolved independently in at least two families, Nymphalidae and Riodinidae, with the shrinkage most pronounced in the femur of the foreleg.¹² The reduction is linked to the same developmental gene pathways, specifically Hox genes, that control limb patterning across insects, but in these butterflies, the expression has been modified to suppress foreleg growth.

The reduced forelegs are not completely vestigial, though. In many nymphalids, they retain sensory bristles and appear to play a role in chemoreception, helping the butterfly taste or assess surfaces. So even when a leg shrinks past the point of being structurally useful for locomotion, it can be repurposed for other functions.

Legs That Taste the World

Speaking of tasting with feet: this is something many insects do, and it still surprises people. Butterflies and moths in the order Lepidoptera have gustatory receptors on their tarsi (the foot segments of their legs). These receptors are housed inside tiny hair-like structures called sensilla and can detect sugars, bitter compounds, amino acids, and various plant chemicals through direct contact.¹³ When a butterfly lands on a leaf, it is literally tasting the surface to determine whether it is a suitable food source or a good site to lay eggs. Flies do the same thing. Touch a sugar solution to a housefly’s foot and its proboscis will extend reflexively.

Legs are also covered in mechanosensory hairs that detect vibrations, air currents, and physical contact. Some insects use substrate-borne vibrations picked up through their legs to communicate. For many six-legged animals, legs are just as much sensory equipment as they are locomotory organs.

Walking on Walls and Water

Insects have evolved two distinct systems for dealing with surfaces that should be impossible to walk on. For vertical and inverted surfaces like walls and ceilings, many insects use a combination of claws for gripping rough surfaces and soft adhesive pads for sticking to smooth ones. The interplay between these two structures is key: on rough surfaces, the claws hook into irregularities; on smooth surfaces, the pads generate adhesive forces through close molecular contact.¹⁴ Some insects can generate adhesion forces greater than 100 times their body weight, yet release that grip almost instantaneously during each step.¹⁵

Water striders take a completely different approach. Rather than sticking to a surface, their legs exploit surface tension to stay on top of water. The legs are covered in dense arrays of microscopic hairs that trap air, creating a superhydrophobic surface that resists wetting. Measurements of water strider legs have found contact angles of almost 169 degrees, meaning water barely touches the leg at all, and a pull-off force of only about 2 dyn per leg.¹⁶ The insect essentially dimples the water surface without breaking through, distributing its weight across all six legs to stay afloat.

Six-Legged Larvae in Eight-Legged Groups

Insects and their relatives are not the only animals that pass through a six-legged phase. Ticks and mites, which are arachnids and normally have eight legs as adults, hatch as six-legged larvae. A tick larva crawling on a blade of grass has just three pairs of legs; it gains its fourth pair only after feeding and molting into the nymph stage. Studies of mite embryology have shown that the rudimentary buds of the fourth pair of legs are visible in the embryo but do not fully develop, with expression of a key limb-patterning gene suppressed at that stage.¹⁷ So these animals are genetically equipped for eight legs from the start but actively delay the development of the last pair. This is a good example of how leg number can be a life-stage trait rather than a fixed species trait.

When Eight-Legged Animals Lose Legs

Spiders are firmly eight-legged animals, but leg loss is common in the wild, and how spiders cope with it sheds some light on what changes when you lose part of your locomotory apparatus. Cellar spiders that lost a leg showed reduced maximum speed on all surfaces, primarily because each stride cycle took longer, even though the spiders tried to maintain their normal stride length.¹⁸ Interestingly, a study on a different spider species found that limb loss had no detectable effect on running speed at all; instead, the spiders compensated by altering their gait, switching between a modified tripod pattern and other coordination schemes to maintain stability.¹⁹ Juvenile tarantulas proved even more resilient, achieving a stable gait immediately after losing a quarter of their legs and resuming their pre-loss speed within a single day.²⁰

These findings highlight something about the geometry of multi-legged locomotion. With eight legs, a spider has built-in redundancy; lose one and you still have more points of contact than many stable gaits require. The tripod gait that insects use needs only three legs on the ground at a time, so even a six-legged animal has a margin of safety. Whether this redundancy was an evolutionary pressure favoring higher leg counts or simply a fortunate consequence of arthropod body plans is an open question.

Extra Limbs and Developmental Chaos in Vertebrates

Among vertebrates, six legs almost never occur naturally. Mammals, birds, reptiles, and fish are locked into a four-limbed body plan that is deeply constrained by development, with changes in limb number tending to cause cascading problems in other body systems. But frogs occasionally turn up with extra legs, and the cause is surprisingly well understood. A parasitic flatworm called Ribeiroia infects developing tadpoles, and its cysts physically disrupt the limb buds during the critical growth window. The result is a range of limb deformities, including extra digits and even whole extra legs.²¹ Museum specimens and resurvey data have confirmed that Ribeiroia-associated malformations in frogs date back to at least the 1940s, and the parasite has been identified as the cause at multiple sites across North America.²² Researchers confirmed the mechanism by implanting inert resin beads into tadpole limb buds and producing similar extra-limb structures, showing that mechanical disruption alone, not a chemical produced by the parasite, is sufficient to trigger the abnormality.²³

These frogs are not truly six-legged animals in the way insects are. Their extra limbs are developmental accidents, not a stable body plan. The vertebrate limb-development system is deeply intertwined with the rest of the body during a critical embryonic phase, which is why genuine changes in limb number are vanishingly rare across mammals, birds, and reptiles. Organisms tend to evolve digit-like outgrowths rather than actual new limbs, and even reductions in limb elements, like the loss of toes in horses, proceed through a roundabout process of building and then destroying structures.

Six Legs in the Machine Shop

The engineering advantages of six-legged locomotion have not gone unnoticed by roboticists. Hexapod robots, modeled explicitly on insect body plans, are a well-established area of research. One early and influential design was based directly on the American cockroach, replicating its leg structure, leg placement, and even using actuators designed to mimic the way insect muscles work. The goal was to capture the remarkable ability of cockroaches to traverse rough, unpredictable terrain without losing stability.²⁴

The appeal of six legs for robots is the same reason it works for insects: inherent static stability. A tripod gait guarantees that the robot always has at least three ground contact points forming a triangle, so it does not need the sophisticated balancing systems that bipedal or quadrupedal robots require. This makes hexapod robots particularly useful for search-and-rescue scenarios, planetary exploration, and any application where the ground is uneven and unpredictable. The trade-off is mechanical complexity: more legs mean more joints, more actuators, and more control signals. But for traversing environments where falling over means mission failure, six legs remain hard to beat.

An Octopus Walking on Two Legs, Using Six for Disguise

In one of the stranger examples of six-limbed behavior in the animal kingdom, two species of octopus have been observed walking across the seafloor on just two of their eight arms, using a rolling gait, while wrapping the remaining six arms around their bodies to mimic coconuts or algae clumps drifting along the bottom.²⁵ The six non-walking arms are held in a fixed configuration for camouflage, not locomotion. These octopuses are not six-legged animals in any anatomical sense, but the behavior is a reminder that the functional use of limbs can be divided in creative ways when an animal has enough of them to spare.