Spiders, ticks, pill bugs, centipedes, and scorpions are all routinely called “bugs,” yet none of them are insects. The English word “bug” has become a catch-all for any small, leggy creature, but biology draws sharp lines between these groups based on body plan, number of legs, and evolutionary ancestry. Some of these non-insect “bugs” are more closely related to shrimp than to beetles, while others sit on entirely separate branches of the animal kingdom. Knowing which is which turns out to matter for more than just trivia.
What Actually Makes Something an Insect
An insect’s body follows a consistent blueprint. It has three clearly distinct body regions: a head bearing antennae and mouthparts, a thorax carrying exactly three pairs of legs (and often wings), and an abdomen with no walking limbs. The insect head is built from six fused segments, the thorax from three, and the abdomen typically from nine to eleven segments, though some groups have fewer.
1PubMed Central. The development and evolution of arthropod tagmataInsects also breathe through a network of internal tubes called tracheae that pipe air directly to their tissues, rather than relying on lungs or gills. When any of these features are missing or different, you’re looking at a creature outside the insect class. A spider has eight legs and only two body regions. A pill bug breathes through gill-like structures on its underside. A centipede has far more than six legs. Each deviation points to a fundamentally different evolutionary lineage.
Confusingly, “True Bugs” Are Insects
Before looking at non-insects, it helps to clear up a common source of confusion. In entomology, the word “bug” has a precise meaning: it refers specifically to the order Hemiptera, the so-called true bugs. This group includes stink bugs, bed bugs, aphids, and cicadas. True bugs are very much insects, with the standard six legs, three body regions, and antennae. They are, in fact, one of the largest insect orders, with roughly 90,000 described species.
2ScienceDirect. Medical and Veterinary Entomology (Third Edition) – Chapter 8 – True Bugs (Hemiptera)What sets true bugs apart from other insects is their piercing-sucking mouthparts and, in many species, forewings that are partly thickened and partly membranous. So when an entomologist says “bug,” they mean a specific kind of insect. When your neighbor says “bug,” they mean anything small that moves and startles them. Nearly every creature discussed in the rest of this article falls into the neighbor’s category but not the entomologist’s.
Arachnids Are the Most Commonly Confused Group
Spiders, scorpions, ticks, and mites are arachnids, and they are probably the creatures most often mistaken for insects. The fastest way to tell them apart is to count legs: arachnids have eight. They also lack antennae entirely and have only two main body sections rather than the insect’s three. Their front pair of appendages are chelicerae, pincer- or fang-like structures used for feeding or defense, rather than the mandibles found in insects.
Arachnids are diverse in their own right. Spiders have specialized silk-producing organs. Scorpions have a venomous stinger on a segmented tail. Ticks are external parasites that feed on blood. Despite their obvious differences from one another, all share that fundamental eight-legged, antenna-free body plan. A survey of locomotor appendages across all ten living arachnid orders found broad morphological diversity but confirmed a shared underlying skeletal and muscular architecture, underscoring how all these forms trace back to a common chelicerate ancestor.
3Oxford Academic. Morphology of locomotor appendages in Arachnida: evolutionary trends and phylogenetic implicationsMajor phylogenetic analyses place arachnids together with horseshoe crabs and sea spiders in the chelicerate branch of the arthropod tree, well separated from insects and crustaceans.
4Annual Review of Entomology. Reevaluating the Arthropod Tree of LifePill Bugs Are Crustaceans, Not Insects
Pill bugs, also called roly-polies or woodlice, are among the most surprising entries on this list. They look vaguely insect-like with their segmented armor, but they are actually isopod crustaceans, more closely related to crabs and lobsters than to any beetle or ant. They have seven pairs of legs (fourteen total), two pairs of antennae (one pair is tiny and easy to miss), and they still rely on modified gill-like structures called pleopods for gas exchange, a leftover from their marine ancestry.
Because they breathe through those gill-derived structures, pill bugs are vulnerable to drying out. Their famous curling-into-a-ball behavior, called conglobation, serves partly as a moisture defense. In laboratory tests, conglobated pill bugs lost water about 35% more slowly than when they were uncurled, and this reduction held across a wide range of humidity levels.
5Journal of Insect Science. Conglobation in the pill bug, Armadillidium vulgare, as a water conservation mechanismPill bugs are one of the few crustacean groups that made the full transition to life on land. They still need damp environments, which is why you find them under logs, rocks, and leaf litter rather than out in the open. Their presence in a garden is a sign of moist, organic-rich soil, and they play a helpful role by breaking down decaying plant material.
Centipedes and Millipedes
Centipedes and millipedes belong to a group called myriapods, meaning “many feet.” Neither is an insect. Both have elongated bodies made up of many similar segments, a design radically different from the three-region insect body plan. But centipedes and millipedes differ from each other in important ways, too.
Centipedes are predators. They have one pair of legs per body segment and move fast. Their most distinctive feature is a pair of modified front legs called forcipules that function as venomous fangs. Research on the material composition of these structures found that they evolved from what were originally walking legs, transforming into rigid, piercing tools capable of injecting venom. The tip of each forcipule is a fusion of what would have been the foot and claw segments in a normal leg.
6PubMed Central. Material composition and mechanical properties of the venom-injecting forcipules in centipedesCentipedes also have surprisingly sophisticated sensory equipment. The common house centipede, for instance, has specialized sensors at the base of its antennae that appear to detect both humidity and vibrations simultaneously, a dual function not seen in insect humidity receptors, which work by a different mechanism.
7PubMed Central. The antennal scape organ of Scutigera coleoptrata (Myriapoda) and a new type of arthropod tip-pore sensilla integrating scolopidial componentsMillipedes, by contrast, are slow-moving detritivores. They have two pairs of legs per visible body segment, which gives them that distinctive rippling gait. Rather than venom, they rely on chemical defense. When threatened, many millipede species secrete irritating or toxic compounds from glands along their bodies. Analysis of the defensive secretions from one European species found 44 different compounds, overwhelmingly dominated by quinones, the same class of chemicals responsible for the pungent smell of some beetle sprays. In males, quinones made up about 95% of the secretion.
8PLOS ONE. Chemical Defence in a Millipede: Evaluation and Characterization of Antimicrobial Activity of the Defensive Secretion from Pachyiulus hungaricus (Karsch, 1881) (Diplopoda, Julida, Julidae)Springtails and Other Near-Insects
Springtails are tiny creatures found in soil, leaf litter, and even on the surface of puddles. For decades, they were classified as insects. They have six legs and a head-thorax-abdomen layout that, at first glance, looks insect-like. But closer study revealed that their mouthparts are tucked inside a pouch in the head rather than being externally visible, placing them in a group called Entognatha (“hidden jaws”) rather than Insecta. Their abdomens also have fewer segments than typical insects.
1PubMed Central. The development and evolution of arthropod tagmataSpringtails get their name from a forked, tail-like appendage on the underside of the abdomen called a furca. When released, the furca snaps against the ground and launches the animal into the air. Research into the mechanics of this jump found that the energy is stored in elastic structures within the furca’s basal segments, with zones potentially rich in resilin, a rubber-like protein also found in insect wing hinges and flea legs.
9PubMed Central. On springtails (Hexapoda: Collembola): a morphofunctional study of the jumping apparatusOther entognathans include proturans and diplurans, both soil-dwellers that rarely come to public attention. Together with insects, these groups form the Hexapoda, the six-legged arthropods. So springtails are six-legged relatives of insects, not insects themselves, a distinction that sounds pedantic until you realize it reflects hundreds of millions of years of separate evolutionary history.
Horseshoe Crabs and Sea Spiders
Not all commonly misidentified “bugs” live on land. Horseshoe crabs, despite their name, are not crustaceans. They are chelicerates, more closely related to spiders and scorpions than to any crab. They have a distinctive horseshoe-shaped shell, a long tail spine, and five pairs of walking legs arranged around a central mouth, a body plan that has remained largely unchanged for hundreds of millions of years.
Sea spiders, or pycnogonids, are another marine group within the chelicerate lineage. They look alien even by arthropod standards: spindly, long-legged creatures with bodies so small that some of their organs extend into their legs. Despite the name, they are not spiders. Molecular studies of the oxygen-carrying protein hemocyanin found that sea spiders possess a simpler version of the molecule compared to horseshoe crabs and land arachnids, consistent with sea spiders occupying a basal position in the chelicerate tree, meaning they branched off early.
10PubMed Central. The diversity and evolution of chelicerate hemocyaninsBoth horseshoe crabs and sea spiders share features with arachnids that they do not share with insects. Sea spiders, for example, have median eyes whose visual pathway is organized similarly to the median eye pathway in horseshoe crabs, with two pairs of median eyes that develop sequentially and connect to the same brain region.
11PubMed Central. The visual pathway in sea spiders (Pycnogonida) displays a simple serial layout with similarities to the median eye pathway in horseshoe crabsVelvet Worms and Water Bears
Velvet worms (onychophorans) and water bears (tardigrades) are sometimes encountered in discussions about “weird bugs,” though they are not arthropods at all. They belong to a broader group called Panarthropoda, which includes all arthropods plus these two lineages. Velvet worms are soft-bodied, caterpillar-like animals with stubby, unjointed legs and a pair of slime-squirting glands they use to trap prey. Tardigrades are microscopic, barrel-shaped animals with four pairs of stubby legs, famous for surviving extreme desiccation, radiation, and even the vacuum of space.
Genomic and microRNA analyses support the idea that velvet worms are the closest living relatives of arthropods. One study found that velvet worms and arthropods share a specific microRNA not found in tardigrades, suggesting that velvet worms and arthropods are sister groups, while tardigrades branched off earlier.
12PubMed Central. MicroRNAs and phylogenomics resolve the relationships of Tardigrada and suggest that velvet worms are the sister group of ArthropodaYou’re unlikely to encounter a velvet worm unless you’re hiking through tropical or Southern Hemisphere forests, and you’d need a microscope to spot a tardigrade. But their existence highlights an important point: the “bug-like” body plan of small, multi-legged animals has evolved along several parallel tracks, and not all of them lead to Arthropoda, let alone to Insecta.
Why Getting the Identification Right Matters
Misidentifying these creatures is not just an academic issue. When the arthropod in question carries medical significance, confusion can have real consequences. A study examining what non-tick arthropods the general public submits as suspected ticks found that fear of the organism can compromise a person’s ability to identify it correctly.
13American Entomologist. I “Tick,” Therefore I Am: Which Arthropods Are Misidentified as Ticks by the General Public?Someone who finds a beetle on their arm and panics, believing it’s a tick, may seek unnecessary medical treatment or miss the actual tick that’s quietly feeding elsewhere. Conversely, someone who dismisses a tick as “just a beetle” may fail to check for tick-borne disease. The ability to distinguish arachnids from insects, and parasitic species from harmless ones, has practical health value.
Garden and household pest management also benefits from correct identification. Treating a centipede infestation the same way you’d treat an ant problem rarely works because the two animals have completely different biologies, diets, and habitat preferences. Centipedes eat other small arthropods and usually indicate the presence of prey species like cockroaches or silverfish. Eliminating the food source is often more effective than spraying insecticide at the centipedes themselves.
How All These Groups Ended Up on Land
One reason so many different “bugs” exist on land is that the transition from water to land happened independently in multiple arthropod lineages. Insects, arachnids, myriapods, and crustaceans like pill bugs all colonized terrestrial environments separately. A comparative genome study spanning 154 animal genomes across 21 phyla found that while each lineage followed its own path to land, the biological functions that emerged during terrestrialization were remarkably convergent, suggesting that the challenges of living on land push genomes toward similar solutions regardless of starting point.
14PubMed Central. Convergent genome evolution shaped the emergence of terrestrial animalsThe timing of these invasions is also being pushed back. Molecular dating and fossil evidence suggest that myriapods, early hexapods, and arachnids may have moved onto land as early as the Cambrian or Ordovician periods, potentially a hundred million years before the oldest well-known terrestrial ecosystems of the Silurian and Devonian.
15PubMed Central. Was There a Cambrian Explosion on Land? The Case of Arthropod TerrestrializationAll of these land-dwelling arthropods now contribute to soil ecosystems. Collectively, soil arthropods including mites, springtails, millipedes, and isopods play major roles in decomposing organic matter, cycling nutrients, and regulating microbial activity.
16PubMed Central. Soil Health and Arthropods: From Complex System to Worthwhile InvestigationA Quick Field Guide to Leg Counts
If you want a practical shortcut for telling these groups apart when you spot something small and skittering, the fastest method is to count legs. This is not foolproof at a glance, since legs can be hard to see on a fast-moving animal, but it narrows things down reliably:
- Six legs: Almost certainly a hexapod. If it has visible external mouthparts, it’s an insect. If it’s tiny and soil-dwelling, it could be a springtail or another entognathan.
- Eight legs: An arachnid. No antennae will be visible. Could be a spider, tick, mite, scorpion, or harvestman.
- Fourteen legs: Likely a terrestrial isopod like a pill bug or sowbug.
- Fifteen to 382 legs: A myriapod. One pair per segment means centipede (always an odd number of leg pairs); two pairs per visible segment means millipede.
Antennae provide another clue. Insects have one pair. Crustaceans like pill bugs have two pairs. Arachnids have none. Millipedes and centipedes each have one pair, but their many-segmented bodies make them hard to confuse with anything else once you get a good look. And if you’re holding something soft, stubby-legged, and vaguely worm-like that squirted slime at you, congratulations: you’ve found a velvet worm, and you should probably put it back under its log.