In everyday talk, yes: plenty of people call any small crawling creature a bug, and nobody will correct you for saying it about a spider.1Wikipedia. Hemiptera A scientist would disagree. A spider is neither a bug nor an insect. It belongs to a separate branch of the same large animal family, with a different number of legs and body sections.2San Diego Zoo Wildlife Alliance. Spiders So the spider in your bathtub is a closer relative of ticks and mites than of ants and flies.
What the Word Bug Really Means
Insect scientists keep the word bug for one particular group of insects, often called true bugs.3Wikipedia. Hemiptera True bugs are separate from ants and bees, from butterflies and moths, and from flies and mosquitoes.4Ask A Biologist. True Bugs Beetles fall outside the true bugs too.5Wikipedia. Hemiptera
True bugs include familiar creatures like stink bugs, bed bugs, aphids, and cicadas. Most have mouthparts shaped like a straw for piercing and sucking. A ladybug is a beetle, not a true bug, despite its name.
The body of a true bug is built for drinking. A beak-like mouthpart pierces a plant or an animal and draws fluid up. Beetles chew with jaws instead, which is one reason they sit outside the group.
Everyday English stretches the word much further. In some kinds of English, every land creature with jointed legs, spiders and centipedes included, gets called a bug.6Wikipedia. Hemiptera
With a child, “it’s a creepy-crawly, but not an insect” covers both meanings. In a quiz or a school setting, the expected answer is that spiders are arachnids.
Where Spiders Fit in the Animal Family Tree
Insects and spiders both belong to the arthropods, the animals with jointed legs.7San Diego Zoo Wildlife Alliance. Spiders Yet they sit in two separate classes: insects and arachnids.8San Diego Zoo Wildlife Alliance. Spiders An arachnid is a member of the class that includes spiders, scorpions, ticks, and mites. Crabs, centipedes, and millipedes are arthropods too, each in its own group.9ThoughtCo. What Are Arachnids? Crustaceans like crabs usually have ten limbs, another way arthropods differ.10Ask Dr. Universe. Why do all insects have six legs? What they all share is limbs with joints.11Ask Dr. Universe. Why do all insects have six legs? Calling a spider an insect is a bit like calling a cat a dog because both are mammals. Ticks, mites, scorpions, and harvestmen (the long-legged “daddy longlegs”) are all arachnids.
The branch leading to spiders and the branch leading to insects both appear to have started around 535 million years ago.12PubMed Central. A Cambrian–Ordovician Terrestrialization of Arachnids That makes them separate lines for over half a billion years.
How to Tell a Spider From an Insect at a Glance
- Legs: spiders and other arachnids have eight, insects have six.13Ask Dr. Universe. Why do all insects have six legs?
- Body sections: insects have three (head, thorax, abdomen), spiders have two.14University of California Riverside. W.E. CH. 4
- Antennae: insects have them, arachnids don’t.15U.S. National Park Service. Insects, Spiders, Centipedes, Millipedes – Catoctin Mountain Park
- Wings: many adult insects have them, arachnids never do.16U.S. National Park Service. Insects, Spiders, Centipedes, Millipedes – Catoctin Mountain Park
A spider’s head and chest are fused into one front section.17Wikipedia. Spider anatomy Behind it sits the abdomen, usually soft, rounded, and without visible segments.18University of California Riverside. W.E. CH. 4
Spiders have no antennae, but they still sense their surroundings. They taste and smell through sense organs on their legs and on the pedipalps, the short leg-like feelers by the mouth.19Smithsonian Education. Are Spiders Bugs? The Key Differences Explained They also sense vibration, a kind of hearing, through hairs and tiny slits over much of the body.20Smithsonian Education. Are Spiders Bugs? The Key Differences Explained A spider’s legs do much of the work an insect’s antennae do.
On a real creature, look for antennae first. They are easier to spot than counting legs on something that won’t hold still, and a pair is a strong sign of an insect, though ant-mimicking spiders can wave their front legs like antennae. Next look at the body: two sections points to an arachnid, three to an insect. Then count legs, from a photo if you can’t do it safely. Treat each clue as one check among several, since a single feature can mislead when an animal is damaged or oddly posed.
The leg count has a trap. The pedipalps can look like a short extra pair of legs, which makes a spider seem to have ten. Real walking legs are the long ones attached to the front body section, while the pedipalps are shorter and held near the face.
Some look-alikes confuse people. Harvestmen have eight legs but one oval body that looks unsegmented. Ticks also have eight legs and a single round body.
Eyes help too. Most spiders have several simple eyes, often eight, set on the front section. Insects usually have two large compound eyes made of many tiny units.
Differences on the Inside
Insects breathe through a network of tiny air tubes that carry air straight to their tissues.21UC Museum of Paleontology. The Arthropod Story Most spiders breathe at least partly with book lungs, gill-like structures where oxygen passes into the blood.22UC Museum of Paleontology. The Arthropod Story
Book lungs are named for stacked thin plates that resemble the pages of a book, and they sit inside the abdomen. Air enters through a slit in the underside of the abdomen and passes over the plates, and oxygen moves across them into the blood.
Spiders carry oxygen with a molecule built around copper rather than the iron in our blood.23Office for Science and Society. Snails, Spiders, and Octopi all have Blue Blood Our blood is red because of iron. Copper-based blood looks bluish when it holds oxygen and is closer to colorless when it does not, so spider blood turns bluish when oxygenated.
Silk is another split. In insects, silk glands usually work only at certain life stages, such as in caterpillars like silkworms.24PubMed. Evolution of arthropod silks Spiders keep their silk-making system for their whole life.25PubMed. Evolution of arthropod silks Their muscular spinnerets let them control the properties of the thread they make.26PubMed. Evolution of arthropod silks
Eating works differently as well. Spiders release digestive juices full of protein-breaking enzymes onto or into their prey.27PubMed Central. Characterisation of protein families in spider digestive fluids and their role in extra-oral digestion They digest food outside the body and then suck up the liquid. Many house and garden spiders are useful: they eat flies, mosquitoes, and other insects.
Why Some Spiders Look Like Ants
Some jumping spiders look like ants, and in one species this lowered their chance of being eaten compared with spiders that don’t.28PubMed Central. Imperfect ant mimicry contributes to local adaptation in a jumping spider Many predators avoid ants, which bite, sting, or taste bad. A predator that avoids ants may also avoid a spider that looks enough like one, so the disguise acts like a shield.
The copies are imperfect. Seen from above, ant-mimicking spiders copy an ant’s shape about as well as ant-mimicking insects do.29PubMed Central. Morphological ant mimics: constrained to imperfection? From the side, spiders are poorer mimics on average.30PubMed Central. Morphological ant mimics: constrained to imperfection? They are working against a two-part, eight-legged body, unlike insects already shaped like ants.31PubMed Central. Mimetic accuracy and co-evolution of mimetic traits in ant-mimicking species In spiders, color seems to make up for a shape that can’t match closely.32PubMed Central. Mimetic accuracy and co-evolution of mimetic traits in ant-mimicking species
The giveaways are easy to check. Ant-mimicking spiders often wave their front legs in the air like antennae, so they use two legs for show and walk on six. Their movement tends to be jerky with pauses, unlike the steady line a real ant follows. A side view shows the two-part body and eight legs that ants lack.
Why Spiders and Insects Bother Us
About a quarter of people are estimated to actively fear spiders and insects.33bugs.com. Why Are People So Afraid Of Insects And Spiders? Most house spiders are harmless to people. Nearly all spiders have venom for subduing prey, but for most species it has little effect on us, and the fangs of many small house spiders are too small or weak to break skin in any meaningful way. A few species can cause serious bites and need medical care, so knowing which ones live in your region is more useful than fearing every spider. Learning to identify spiders can make them feel less threatening. To remove one, a cup and a card work well for catching it and releasing it outdoors.