Caterpillar Identifier
Photograph the caterpillar and get a plain answer: what it is, and whether it stings. Fuzzy does not mean friendly.
Drop a photo of the caterpillar, or use your camera.
Know That Bug never stores the photo. Our in-process copy is discarded after the answer; we keep the answer and a one-way hash, never the image.
The ones that sting
Caterpillars are the one group where the dangerous species look softer than the safe ones. A puss caterpillar looks like a scrap of cotton wool and delivers one of the most painful stings of any insect in North America — children pick them up precisely because they look strokeable.
The ones that don’t
How to photograph a caterpillar
Caterpillars are easier to photograph than spiders — they are slow — but the detail that matters is small.
- Side-on, filling the frame. The arrangement of tufts and spines along the body is what identifies it, and a top-down shot flattens exactly that.
- Get the surface texture in focus. Even bristles versus clustered spines is the whole safety question.
- Photograph the plant it is on. Foodplant narrows the answer enormously — hickory tussocks are on hickory and walnut, io moths on willow and cherry.
- Don’t pick it up to get a better angle. That is how most stings happen.
The rule that actually works
Even, uniform bristles are usually fine — that is a woolly bear and it will walk across your hand. What you are looking for is structure: tufts standing proud of the coat, fleshy horns, or spines arranged in clusters. Those are the stinging apparatus, and they belong to the saddleback and the io moth.
The exception is the puss caterpillar, which hides its spines completely under what looks like plain fur. That is why it is the one worth learning by sight.
What to do if you have been stung
Some species sting on contact through venomous spines or irritating hairs. The bite and sting guide gives the US-focused escalation signs and Poison Control contact.
We identify the animal; we do not diagnose a reaction or prescribe treatment. Call emergency services for difficulty breathing or swelling of the face or throat, and seek medical advice for severe pain, spreading swelling, or a sting to a young child.
Common questions
Are all fuzzy caterpillars dangerous?
No — and that is the problem, because it means fuzziness is not a usable signal. A woolly bear is entirely harmless. A puss caterpillar looks softer and is the worst sting in North America. Look for structure in the coat rather than judging by texture.
Do stinging caterpillars bite?
No. Nothing here bites. They sting passively, on contact, through hollow spines or irritating hairs — which is why brushing against one is enough and why you can be stung without ever seeing it.
What will it turn into?
Usually a moth, and usually one far less remarkable than the caterpillar. The saddleback becomes a plain brown moth; the woolly bear becomes the Isabella tiger moth; the io moth is the exception and becomes genuinely spectacular.
Is it safe to move one off a path?
Use a stick or a leaf, never a bare hand, unless you have positively identified it as harmless. Thin gloves are not always enough — puss caterpillar spines can pass through light fabric.
When we can’t tell you
Caterpillars change appearance dramatically as they grow, and an early instar can look nothing like the animal in a field guide. Where a photograph doesn’t settle whether something is a stinging species, we say so rather than guessing — and on this group we err firmly towards caution, because the cost of being wrong falls on somebody who has already touched it.
We publish what we get right and wrong with real numbers.
Read next
Read the bite and sting guide for safety escalation, and accuracy and limits for how we handle a photograph that cannot rule out a stinging species.