The eastern tiger swallowtail (Papilio glaucus) is one of the most recognizable butterflies in North America, but its caterpillar is something most people have never noticed. That’s by design. The larva spends its early days looking like something you’d find on the underside of a leaf after a bird flew overhead, and its later days staring back at you with a pair of giant fake eyes.
There are five stages of caterpillar growth, called instars, and the change from first to fifth is dramatic. What starts as a small, brown, bird-dropping mimic eventually becomes a plump green caterpillar with yellow and black eyespots large enough to make a predator think twice. The whole process takes about three to four weeks before the caterpillar pupates and eventually becomes the familiar yellow-and-black butterfly.
If you’ve found one on a cherry, tulip poplar, or wild black cherry tree and weren’t sure what you were looking at, this guide covers each stage in detail, what to expect, and how to identify them at any point in development.
Key Takeaways
- Eastern tiger swallowtail caterpillars pass through five instars over roughly three to four weeks, with appearance changing dramatically between early and late stages.
- Early instars (1 through 3) are brown and white, mimicking bird droppings to avoid predator detection on leaf surfaces.
- Later instars (4 and 5) are bright green with large yellow and black false eyespots near the head, a pattern that makes the caterpillar look like a small snake to potential predators.
- All instars can deploy the osmeterium, a forked orange defensive gland behind the head that releases a pungent odor when the caterpillar is threatened.
Early Instars: The Bird Droppings Disguise
When an eastern tiger swallowtail egg hatches, the first-instar caterpillar is tiny, around 3 to 5 mm. It is dark brown to black with a white or cream patch across the middle of the body. The texture looks slightly rough or shiny depending on light conditions. The resemblance to a bird dropping is very convincing at this size, which is exactly the point.
This camouflage works because predators such as birds and wasps are searching for food that looks like food. Something that looks like waste on a leaf gets ignored. The caterpillar doesn’t need to hide under the leaf or stay still in a crack. It can rest right on the upper surface of a leaf and go unnoticed in plain sight.
The second instar looks similar but larger, typically 6 to 10 mm. The brown and white patterning is still present and still effective. The caterpillar feeds on the leaf it hatched on and nearby foliage, chewing notches from the edges. Both first and second instars are fragile. They’re easily dislodged by rain or wind and are vulnerable to parasitoid wasps that specifically target young caterpillars.
By the third instar, around 12 to 18 mm, the caterpillar begins to look slightly more substantial. The bird-dropping disguise is still in play, but the pattern starts to become less crisp. The caterpillar is also starting to produce a silk mat on leaves, giving it a slightly more secure surface to rest on. This is still the hiding-in-plain-sight phase of development.
Each time the caterpillar completes an instar, it sheds its skin (a process called molting) and enters the next stage. The head capsule, which doesn’t stretch, is shed each time as well, and you can sometimes find the old head capsule near a resting caterpillar after a molt.
Later Instars: Green with Eyespots
The fourth instar is where the eastern tiger swallowtail caterpillar makes its most dramatic visual shift. The bird-dropping disguise is abandoned entirely. The caterpillar becomes bright green with a yellow and black eyespot on each side, positioned just behind the head on the thorax. The spots are ringed with yellow and have a black pupil, giving them a convincing resemblance to eyes.
From above, a resting fourth-instar caterpillar looks somewhat like the head of a small snake. The green body widens toward the front, and those eyespots stare outward from either side. This is an example of mimicry aimed at predators, particularly birds, that are hardwired to avoid snakes.
The fifth instar is the largest and final caterpillar stage. At full size, fifth-instar caterpillars reach about 50 to 55 mm. The green is vibrant, and the eyespots are even more pronounced. There is also a pale yellow stripe that runs along the upper sides of the body, and the overall appearance is smooth and plump. This is the stage most people encounter and photograph, simply because it’s the most visible and striking.
Fifth-instar caterpillars eat heavily. They can consume several leaves a day and will move to different branches as they deplete foliage nearby. If you find one wandering on the ground, it may be in the pre-pupation phase and looking for a place to form a chrysalis, not necessarily sick or lost.
For comparison with other swallowtail caterpillar appearances and behaviors, the swallowtail species identification guide covers the key differences between eastern tiger, spicebush, black, and giant swallowtails, including their larvae.
The Osmeterium Defense
Every instar of the eastern tiger swallowtail caterpillar has an osmeterium. This is a forked, fleshy gland tucked into a groove just behind the head. Under normal conditions it’s invisible. When the caterpillar is touched, grabbed, or disturbed, it everts the gland rapidly, pushing it out like a small orange or yellow-orange tongue.
The osmeterium releases chemicals that smell strongly unpleasant to predators. The scent is often described as sharp, citrus-like, or acrid, depending on who’s describing it. For humans handling the caterpillar, it’s noticeable but not harmful. For a small bird or insect predator that gets a face full of it, the effect is more deterring.
The gland retracts quickly once the perceived threat is gone. A caterpillar that extends its osmeterium will pull it back in within seconds if you stop disturbing it. It can deploy the gland multiple times in succession if the threat continues.
Early instars have a smaller, less pigmented osmeterium than later ones. By the fifth instar, the gland is bright orange and quite visible when everted. If you want to observe it without stressing the caterpillar excessively, a very brief, gentle touch to the front end of the body is usually enough to trigger a quick deployment and retraction.
The osmeterium is a shared feature across all Papilio swallowtails. It’s one of the traits that defines this group of butterflies as a genus and distinguishes them from other butterfly families. Research published through institutions like the Ohio State University Department of Entomology and similar programs has examined the chemical composition of osmeterium secretions across different swallowtail species, finding that the compounds vary between species and may serve multiple signaling purposes beyond just predator deterrence.
What They Eat
Eastern tiger swallowtail caterpillars are not as host-plant specialized as some other swallowtails. They feed on a wide range of deciduous trees and shrubs, which is part of why the species is so widespread and common.
The most commonly used host plants include wild black cherry (Prunus serotina), tulip poplar (Liriodendron tulipifera), sweet bay magnolia (Magnolia virginiana), basswood (Tilia americana), and white ash (Fraxinus americana). In gardens and suburban areas, ornamental cherry, apple, and magnolia trees are also used regularly. Females are opportunistic and will choose from a broad list based on what’s locally available.
Early instars tend to feed near the midrib of a leaf, where the tissue is tender. Larger caterpillars eat from the leaf edge and can take off substantial portions of a leaf in a single feeding session. They rest on leaves between feeding bouts, often in a characteristic slightly curved posture.
Host plant selection matters most at the egg-laying stage when the female chooses where to deposit eggs. Once the caterpillar hatches, it typically stays on the host plant it was laid on, though it will move to nearby branches or adjacent plants if food runs out. The eastern tiger swallowtail host plants guide covers the full list of accepted plants and which ones perform best for supporting larvae through all five instars.
It’s also worth noting that caterpillars at different instars can vary in their acceptance of certain plant species. A first-instar caterpillar hatched on tulip poplar will usually continue feeding on that tree, even if wild black cherry is available nearby. The feeding preference tends to follow the host plant the female chose originally, though this isn’t absolute across all individuals.
Pre-Pupation and Chrysalis
When the fifth-instar caterpillar finishes feeding, its behavior changes noticeably. It stops eating, empties its gut (you may notice greenish frass at this stage), and begins moving with purpose. This is the wandering phase. The caterpillar leaves the host plant and searches for a sturdy stem, twig, or other vertical surface where it can pupate.
Once it finds the right spot, the caterpillar spins a silk pad and anchors its rear end to it, then spins a silk girdle around its middle to hold the body upright. Within a day or two, the caterpillar skin splits for the final time and the chrysalis beneath is revealed.
The eastern tiger swallowtail chrysalis is typically brown or gray-green with angled lines and ridges that help it blend in against bark or woody stems. It looks convincingly like a broken stick or a seed pod. The chrysalis is attached in a head-up orientation, held by the silk girdle and cremaster at the tail end.
Summer chrysalises produce adults in about 10 to 20 days, depending on temperature. Chrysalises formed in late summer enter diapause and overwinter, not emerging until spring. In warmer parts of the range, two or even three generations can complete per season. In the northern part of the range, one or two generations are more typical.
The full caterpillar-to-butterfly journey, including what happens inside the chrysalis, is covered in detail in the caterpillar host plants guide, which also includes tips on setting up your garden to support multiple stages of the butterfly life cycle. The Butterflies and Moths of North America species page for Papilio glaucus includes verified occurrence records and observer notes that are useful for tracking local populations and seasonal timing.
Frequently Asked Questions
How many stages does an eastern tiger swallowtail caterpillar go through?
Eastern tiger swallowtail caterpillars go through five instars, or growth stages. Each instar ends with a molt, where the caterpillar sheds its old skin to reveal a larger body underneath. The full caterpillar phase lasts about three to four weeks total. The first three instars use a bird-dropping disguise, while the fourth and fifth instars switch to a green body with large false eyespots near the head.
What does an eastern tiger swallowtail caterpillar look like?
It depends heavily on the instar. Early instars (1 through 3) are brown and white with a pattern that mimics bird droppings. They are small, typically under 20 mm, and easy to overlook on a leaf surface. Later instars (4 and 5) are bright green with a swollen thorax and large yellow and black eyespots on either side near the head. Fifth-instar caterpillars grow to about 50 to 55 mm and are the most commonly photographed stage because of their striking appearance.
What trees do eastern tiger swallowtail caterpillars eat?
They feed on a wide range of deciduous trees. Wild black cherry, tulip poplar, sweet bay magnolia, basswood, and white ash are among the most commonly used host plants. In suburban and garden settings, ornamental cherry, apple, and magnolia trees are often used as well. The species is less host-plant specific than many swallowtails, which is one reason it’s so adaptable and widely distributed across eastern North America.
What is the orange thing coming out of the caterpillar’s head?
That’s the osmeterium, a forked defensive gland that all Papilio swallowtail caterpillars have. It sits in a groove just behind the head and is normally invisible. When the caterpillar is disturbed or feels threatened, it pushes the gland outward rapidly. The osmeterium releases a pungent chemical smell that deters predators. It retracts within seconds once the threat passes. It’s harmless to humans but can be startling the first time you see it, especially on a large fifth-instar caterpillar where the gland is bright orange and quite prominent.
How long does it take an eastern tiger swallowtail caterpillar to become a butterfly?
The caterpillar stage itself takes about three to four weeks across five instars. After that, the caterpillar forms a chrysalis, which takes another 10 to 20 days during warm summer months. So from hatching to adult emergence, the process runs roughly five to seven weeks in summer. Chrysalises formed in late summer can overwinter in diapause, with the adult not emerging until the following spring, which extends that timeline to several months. The egg stage before hatching adds about four to ten days on top of that.
