What Is the Orange Osmeterium on a Black Swallowtail Caterpillar?
Get too close to a black swallowtail caterpillar and it will rear up its head and push out a bright orange, forked organ from just behind its face. That organ is called the osmeterium, and it releases a strong smell that most people compare to rotten citrus. It is completely harmless to us. To the small predators the caterpillar actually worries about, it is a clear signal to back off.
We raise black swallowtails every year on dill and fennel a few feet from the back door in our North Texas container garden, so we have watched this happen up close many times. Here is what the osmeterium is, why it smells, and what it is really defending against.
What Is the Osmeterium?
The osmeterium is a fleshy, Y-shaped gland tucked in a fold right behind the caterpillar's head. When the caterpillar feels threatened, it arches its front end and everts the gland, pushing it out fast using internal fluid pressure. On a black swallowtail it is a vivid yellow-orange and looks a lot like a snake's forked tongue.
Every swallowtail caterpillar in the family Papilionidae has one, so this is not unique to black swallowtails. What makes each species different is the color and the exact chemical mix the gland releases. According to the University of Florida Extension, the black swallowtail's osmeterium is bright yellow-orange, and the caterpillar will rear up, extrude it, and try to smear its potential attacker with the chemical it carries.
Once the threat passes, the gland retracts back into the fold and the caterpillar goes right back to eating like nothing happened. In our garden, a gentle brush with a leaf is all it takes to trigger it, and seconds later it is head-down in the dill again.
Why Does It Smell?
The smell is the whole point. The osmeterium releases a blend of volatile compounds, including acids the caterpillar produces itself plus aromatic oils it pulls from its host plants. Since black swallowtails feed on the carrot and parsley family, dill, fennel, parsley, and carrot tops, those plant oils flavor the scent.
Descriptions of the smell vary a lot from person to person. Some people call it rotten citrus, others say rancid butter, and a few barely notice it at all. That range is normal. The odor is genuinely subjective, and it is stronger the closer you get and the more directly the caterpillar makes contact. It is unpleasant enough to register but does nothing to a human beyond a quick "what is that."
What the Osmeterium Actually Defends Against
This is the part that surprises people. The chemical spray is most effective against small invertebrate predators, and ants are the classic example. Research going back decades has shown the secretion reliably repels ants, and it can also deter other small hunters like spiders and praying mantises. When a caterpillar can smear the fluid directly onto an ant, the ant wants nothing to do with it.
Against birds, the chemistry matters less. Studies suggest the smell alone does not do much to a hungry bird. What can give a bird pause is the visual side of the display: the bright orange color works as a warning signal, and the forked, tongue-like shape mimics a snake for a split second when the caterpillar rears up. So the osmeterium plays two roles at once. It is a chemical weapon aimed at ants and small predators, and a visual bluff aimed at bigger ones.
The Texas A&M Field Guide to Common Texas Insects is a solid regional reference if you want to confirm the markings and life cycle on a black swallowtail you have found.
The Osmeterium Is a Backup, Not the First Line
The osmeterium only comes out when a caterpillar is already cornered. Most of the time, black swallowtails avoid trouble long before it gets to that point.
- Early instars mimic bird droppings. When they first hatch, the caterpillars are tiny, dark, and shiny with a white saddle, so they read as a fresh bird dropping that a bird would ignore.
- Older instars blend in. Once they get too plump to pass as a dropping, they molt into the bold green, black, and yellow banding that breaks up their shape in sun-dappled leaves.
- The osmeterium is the last resort. If camouflage fails and something gets too close, the orange gland flips out.
We walk through all of these stages, from egg to emergence, in our full guide on how to raise black swallowtail butterflies in a container garden. If you have found one of these caterpillars and want to know what happens next, that is the place to start.
Want to See One in Your Own Garden?
Attracting black swallowtails comes down to planting the right host plants and then leaving them alone. The females lay their eggs one at a time on carrot-family plants, and they are remarkably picky about it. We break down exactly how black swallowtails choose their host plants in a separate post, but the short version is that carrot-family herbs are what you want in the ground.
- Host plants for the caterpillars: dill, fennel, parsley, and carrot tops. Plant a little extra so there is enough to share once the caterpillars start eating. We seed a 25-gallon planter of dill heavily, almost like a tray of microgreens, so there is plenty for a full crew.
- Nectar flowers for the adults: zinnias are a favorite, and they double as a safe, sturdy perch for a newly emerged butterfly drying its wings.
You can grab dill, fennel, and parsley seed through True Leaf Market, which carries garden seed alongside microgreen seed. If you are short on space, these all grow well in containers. We cover why we lean on fabric grow bags for our backyard garden, and you can find the sizes we use plus garden feeding options on our garden fertilizers and grow bags list. For more on building a bug-friendly space in our part of the state, see our guide to pollinator-friendly gardens for North Texas.
The reason this works is simple: a garden full of host plants and blooms brings in the caterpillars, and a garden that is left unsprayed lets the whole life cycle play out on its own.
Most of our black swallowtail footage lives on Mandi's page, @mrsmandivaughn. You can watch the osmeterium in action in this reel, where a light brush with a leaf brings the orange gland right out.
Frequently Asked Questions
Is the osmeterium dangerous to humans?
No. The osmeterium is harmless to people. It smells strong up close, but it does not sting, bite, or cause any harm. The worst you will get is a scent on your fingers if the caterpillar touches you with it.
What does the osmeterium smell like?
Most people describe it as rotten citrus or rancid butter, though the smell is subjective and some barely notice it. The odor comes from a mix of acids the caterpillar produces and aromatic oils drawn from its host plants like dill and fennel. Some people even enjoy the smell.
Why is the osmeterium bright orange?
The bright orange color acts as a warning signal to predators, and the forked shape mimics a snake's tongue when the caterpillar rears up. Together they can make a bird or larger predator hesitate before attacking.
Can I touch a black swallowtail caterpillar?
You can, gently, but it is best to let them be. Handling stresses the caterpillar and can trigger the osmeterium unnecessarily. If you need to move one, guide it onto a leaf or stem rather than picking it up directly.
Do all caterpillars have an osmeterium?
No. The osmeterium is specific to swallowtail caterpillars in the family Papilionidae, which includes the black swallowtail. Other caterpillars have their own defenses, from spines to hairs to camouflage.
Related Blogs
- How to Raise Black Swallowtail Butterflies in a Container Garden
- How Black Swallowtails Choose Their Host Plants
- Pollinator-Friendly Gardens in Hunt County: Native Plants & Herbs for North Texas
- Fabric Grow Bags: Why We Use Them for Our Backyard Garden
- Holy Poly They're Taking Over! The Great Pill Bug Invasion of North Texas
- Worried About Brood XIX and XIII Cicadas Eating Your Garden? Grow Microgreens Indoors Instead
- Have a Few Empty Planting Pots Laying Around? Grow Microgreens in Them!
- Top 5 Gardening Trends to Watch in 2025: What's Growing This Year?
- How to Grow Your Own Food Indoors: The Best Way to Learn Microgreens
This post contains affiliate links. We may earn a small commission on qualifying purchases at no extra cost to you. We only link to products we use and recommend. See our full affiliate disclosure for details.
— On The Grow®, LLC
Happy growing!
