Spider vs Insect ID Key
Count the legs on the bug you found, note whether it has wings or antennae, and get its likely arthropod group with the reasoning behind the call.
Likely group
Eight legs and two body regions (cephalothorax and abdomen), with no antennae and no wings, mark this as an arachnid — a spider, tick, mite, or harvestman.
The three-way split
Nearly every bug found indoors falls into one of three groups. Insects(Insecta) have 6 legs and three body regions — head, thorax, abdomen — and usually a pair of antennae; wings, where present, only ever occur here. Arachnids(Arachnida) — spiders, ticks, mites, harvestmen — have 8 legs and just two body regions, a cephalothorax and an abdomen, with no antennae and no wings. Myriapods— centipedes and millipedes — have 10 or more legs running the length of a long, segmented body.
Leg count alone resolves the great majority of household finds. The exceptions worth knowing are harvestmen (8-legged arachnids that aren’t spiders) and immature insects that haven’t grown their full leg or wing complement yet.
Frequently Asked Questions
Why does leg count matter more than anything else?
It's the most visible, most reliable field mark for sorting household arthropods into their major groups without a magnifying glass. Insects have 6 legs, arachnids have 8, and myriapods (centipedes, millipedes) have 10 or more running the length of a segmented body.
What about a daddy-long-legs — is that a spider?
It depends which "daddy-long-legs" you mean. Cellar spiders (Pholcidae) are true 8-legged spiders. Harvestmen (Opiliones) are also 8-legged arachnids but aren't spiders at all — they have a single fused body segment instead of two, and no venom or silk glands.
I count 8 legs but it also has visible antennae — what's going on?
True arachnids never have antennae — what looks like one is usually a leg-like pedipalp near the front, which arachnids use for sensing and (in males) mating, not a true antenna. If you can clearly confirm 6 legs plus antennae, it's an insect instead.
Identification aid only. This key sorts broad arthropod groups from visible field marks; it does not cover bite or sting treatment, and is not a substitute for a specialist ID on anything you’re unsure about.