Yellow Swallowtail Butterfly: Identification, Facts, Pictures

Common Yellow Swallowtail (Papilio machaon)

The Common Yellow Swallowtail is a species of butterflies that are known for their large size and pronounced ornamental patterns. Very easy to be recognized, the species is spread across many parts of the world in 37 recognized subspecies.

Scientific Classification

  • Family:Papilionidae
  • Genus: Papilio
  • Common names:Old World Swallowtail, Swallowtail, Schwalbenschwanz
  • Scientific Name: Papilio machaon

Description and Identification


Caterpillars emerge from the eggs after around a week from hatching. During this time, they are black with white bands. By July, when ready to pupate, the caterpillars are fully grown, turning into bright green and ornamented with thin bands in black along with orange spots.

The chrysalis may be either pale brown or light green with a darker brown stripe and remains attached to the stem of a reed. In this state, they hibernate throughout the winter, until the arrival of the spring in the following year.

Adult Butterfly

Sexual Dimorphism: Not present

Color and Appearance: When the wings are open, the wings of both the males and the (larger) females display a dark gray base with elongated off-white to cream marks at the center of the primary wings, and even white spots arranged parallel to the border of the wings. The side of each of the hindwings close to the characteristic ‘tail’ displays a bright reddish-orange eyespot with a black border around it along the lower border that touches the inner edge of the pair of hindwings. There is also three to four cyan to light blue spray marks just above the two tails. When the wings are closed, the ventral side shows a faint or blurry mirror image of the dorsal side.

Average wingspan: Typically 9 to 10 cm

Flight pattern: Strong flaps

Eggs are light brown in color laid singly on the upper leaves of Hog’s Fennel, also known as Milk Parsley.

Swallowtail butterfly

Where to see a swallowtail

Swallowtail © Terry Whittaker/2020VISION

The swallowtail butterfly

Meet the largest and one of the most localised butterflies in the UK — the swallowtail. Adults emerge from their over-winter chrysalises, drying their wings in the safety of the reedbeds before fluttering into view; bright and spectacular. This butterfly’s swallow-like tails mimic antennae, which together with two red and blue ‘false eyes’ confuse predators into thinking it is a two-headed butterfly. Even as caterpillars, they are impressive beasties with bulging horns to frighten predators, and two horn-like orange scent glands that, when threatened, emerge from the back of its head, producing a smell that has been likened to pineapple.

Find swallowtail butterflies

The British race of the swallowtail butterfly is now limited to the Norfolk Broads, choosing sites with a vigorous growth of milk parsley, where it lays its eggs on the tallest plants. Visit one of these special places on a windless day between late May and mid-July, and with a bit of luck you will spot one of these rare British insects.

Norfolk Wildlife Trust

  • Alderfen Broad
  • Barton Broad
  • Cockshoot Broad
  • Hickling Broad (marsh harriers, bearded tits and common crane could all add to your day’s highlights)
  • Ranworth Broad (you might even spot a swallowtail before leaving the car park!)
  • Upton Broad and Marshes

What to look out for

Do some research before you visit, and find out how to identify milk parsley, the host plant where the females lay their eggs singly on the leaves that will provide the caterpillars’ food supply. Check any plants you see, as the impressive green and black stripy caterpillar may be hidden in the leaves. The adult butterflies feed on all species of flowers but prefer yellow and purple ones so can often be found on red campion and yellow iris.

Pick a still, warm day, and take your binoculars: swallowtails are very fast flying butterflies, and don’t often sit still.

If you can’t get to these places

On rare occasions, migrant swallowtails arrive here from the continent, and in 2014 they were spotted hatching in Sussex. These continental arrivals prefer downland, where they lay their eggs on wild carrot. If you keep your eyes peeled along the south coast, you could be in luck and catch a glimpse of a rare European wanderer.

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Eastern Tiger Swallowtail (Papilio glaucus)

(aka Tiger Swallowtail)
This unique species of swallowtail is a quick and strong flier, gliding when able. The males are a bright yellow, while the females can exhibit two different color forms; yellow and black and black and blue. The darker form is more common in the southern states. The caterpillar is just as remarkable, it resembles a small snake with eyespots. This butterfly particularly enjoys pink, purple, and red flowers, and is a wonderful visitor to any garden.

Family: Swallowtail (Papilionidae)

Subfamily: Swallowtail (Papilioninae)

Average Wingspan: 3″ — 6″

Habitat: Fields, parks, suburbs

Plants That Attract This Butterfly

  • Nectar Plants (What is this?)
    Butterfly Bush, Dianthus Family, Petunia, Mexican Sunflowers, Chaste Tree
  • Host Plants (What is this?)
    Sweet Bay, Willow
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* May not be available for purchase at your local nursery.

Note: Always check a plant’s Hardiness Zone to make sure it will grow in your area. (What Is My Hardiness Zone?)

Butterfly Flight Range Map

Photo Credits:
Male (open wing) — Zina Flippen
Male (closed wing) — Fred Miller
Dark Female (open wing) — Big Stock Photo
Female (closed wing) — Bud Hensley
Light Female (open wing) — Bud Hensley
Caterpillar -Fred Miller

Master Gardener Program

Division of Extension

Black swallowtail, Papilio polyxenes

Black swallow tail is a common butterfly throughout eastern North America.

The black swallowtail – also called the eastern black swallowtail or American swallowtail, and a variety of other colloquial names such as parsley worm – is a common butterfly found throughout much of North America. Papilio polyxenes is one of many species in the largest genus in the butterfly family Papilionidae (swallowtails). It ranges from southern Canada to northern South America, but is most common east of the Rocky Mountains. There are several subspecies that occur in Mexico, Central America and South America. It has been designated as the state butterfly of Oklahoma.

The male black swallowtail has more noticable yellow and less blue on the wings.

Adult black swallowtails are usually found in open areas, such as fields, meadows, parks, wetlands, prairies and sunny backyards. Females tend to be larger than the males, with a wingspan of 3¼ to 4¼ inches. The wings are black with yellow, blue, orange and red markings. On the upper surface there are two rows of yellow spots along the edges, with a powdery iridescent blue area between the two rows and a

The female black swallowtail has more blue and less yellow on the wings.

red eyespot (red circle with a black bulls-eye) near the margin of each hind wing. The yellow spots are typically large and bright and the blue not very prominent on males, while females have smaller and lighter colored yellow spots but a prominent blue area (although some males have markings similar to females) – this difference is called sexual dimorphism.

Both male and female black swallowtails have distinctive markings on the undersides of the wings and the characteristic tail.

The underside of the wings has two rows of pale yellow spots on the edges of the front wings and bands of orange spots separated by pale blue on the hind wings. Both sexes have the characteristic narrow lobe on the hindwings, called the tail.

Although black swallowtail is fairly easy to identify, it is most likely to be confused with the pipevine swallowtail (Battus philenor), which is black with an iridescent blue-green sheen and only faint white spots on the upper surface and has only a single row of spots below, and the spicebush swallowtail (Papilio troilus), an uncommon stray into Wisconsin, which has white spots on the upper surface and the yellow spots on the underside are interrupted in the middle by a blue area. The underside of the wings mimics that of the poisonous and distasteful pipevine swallowtail which can fool vertebrate predators into avoiding eating them (Batesian mimicry). Since they spend a lot of time roosting with the wings closed, this maximizes the protective effects (the upper side of the wings of males doesn’t resemble that of the pipevine swallowtail.

Male butterflies emerge before the females, and maintain and defend territories, where they perch and patrol for receptive female butterflies. Both sexes obtain nectar from a variety of flowers, including milkweed, thistles, purple coneflower, zinnias, and Verbena bonariensis. They will also visit moist ground to obtain salts.

Parsley is a common host of the caterpillars in backyards.

Larval host plants include a variety of species in the carrot family (Apiaceae). Cultivated dill, parsley, fennel, celery, caraway, and carrot are common food sources in backyard gardens, where it could be considered a pest. Usually they are not numerous enough to present a real problem. But for gardeners who do not want their plants eaten, handpicking would eliminate the problem in most situations. Insecticides are rarely justified, but if needed, foliar insecticides and the bacterial insecticide BT (Bacillus thuringiensis)

When numerous black swallowtail caterpillars could be considered a pest in gardens as they can consume entire plants (on dill in this photo).

provide effective control. Black swallowtail will sometimes also utilize plants in the citrus family (Rutaceae), including common rue, Ruta graveolens. Wild plants that are utilized as larval food include introduced species such as Queen Anne’s lace (wild carrot, Daucus carota), poison hemlock (Conium maculatum), and wild parsnip (Pastinaca sativa) and native species including spotted water hemlock (Cicuta maculata) and golden alexander (Zizia aurea).

Female butterflies lay eggs on the larval host plants (L). The yellow eggs (R) turn dark right before hatching (insert).

Females lay pale yellow, spherical eggs singly on host plants, usually on new foliage but occasionally on flowers. The eggs darken as the caterpillar develops inside. The eggs hatch in 3 to 9 days, with the caterpillar chewing its way out of the egg and then consuming the eggshell. The first instar larvae are spiny and mostly black with a whitish saddle, mimicking bird droppings. In the second and third instars the spines are reddish or orange.

The early instar caterpillars are black and spiny with a white saddle (L), resembling bird droppings (R).

Older larvae (fourth and fifth instars) are green with transverse bands of black with yellow spots, a color pattern that likely makes them hard to see while resting on the sun-dappled host plants. The caterpillars eventually grow up to 1½ to 2 inches long. The larval stage takes 10 to 30 days depending on temperature and type of host plant.

When the caterpillar molts from the 3rd to 4th instar it leaves the dark spiny form behind (L) and changes coloring (LC) to eventually become smooth and green with black markings (RC) that helps it blend in with the foliage (R).

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Swallowtail larvae have a eversible organ called the osmeterium for repelling predators.

All swallowtail larvae have an eversible horn-like organ behind the head known as the osmeterium (osmeteria, plural) that looks like a forked snake tongue. It is a bright yellow-orange color on the black swallowtail. When the caterpillar is disturbed it rears up and the organ is extended for a short period of time. When everted it it releases a chemical repellent with a foul smell to repel predators. It is harmless to humans, however.

Once the caterpillar matures it wanders away from the host plants to find a place to pupate. It positions itself in the typical swallowtail “head-up” position on something such as a plant stem, tree trunk, foundation wall or other location and spins a slender silken band around the thoracic area to support itself and is attaches at the hind end to a silk pad by a Velcro®-like cremaster. It then molts one last time into a naked chrysalis (not inside a cocoon like moths make) with short horn-like protrusions on the head.

The mature caterpillar spins a slended silken band around its upper portion (L), to attach to a support (LC) where it changes in the pupal stage (RC). The adult will eventually emerge from the naked chrysalis (R).

The pupa may be green or brown to blend in with its surroundings.

The color of the chrysalis is either greenish with yellow markings or mottled brown. This is determined genetically, not by the individual’s immediate surroundings, so that the majority of the pupae will blend in; overwintering pupae are always brown. This stage is very cryptic and not commonly seen in the garden. Those of the first brood will remain in the pupal state for 2 to 3 weeks, while later generations pupating in the fall will enter diapause and overwinter in the pupal stage. Adults emerge in the spring, generally eclosing in the morning. There are usually two generations in the upper Midwest, with adults generally flying from mid-May until late September. They are most numerous in early July until late August when the second brood adults are out. There is a third flight period in southern regions.

Encourage black swallowtails to visit by planting flowers.

This insect may be parasitized in the larval or pupal stages by flies in the families Phoridae and Tachinidae, and by wasps in the families Brachonidae and Ichneumonidae. Since some insect parasitoids find their hosts by the smell of volatile chemicals in insect frass, black swallowtail larvae use their mandibles to throw their fecal pellets off the plant so these natural enemies will be less likely to find them.

If you want to encourage black swallowtail to visit your garden, plant both flowers to provide nectar, provide some larval host plants, such as dill or parsley that you are willing to let them eat, and refrain from using insecticides in the garden.

– Susan Mahr, University of Wisconsin – Madison

Raise Black Swallowtail Butterflies Indoors

Raising a Black Swallowtail butterfly indoors is fun, easy, and educational. Black Swallowtail butterflies are great to raise indoors for several reasons including abundance, plentiful food source, attractive and interesting caterpillars, and of course beautiful butterflies.

The Black Swallowtail can be found anywhere East of the Continental Divide in America. The Anise Swallowtail is the Western version of this butterfly and is surprisingly different in appearance considering the caterpillars and chrysalises are nearly identical.

The host plants of the Black Swallowtail butterfly include such plants as carrots, parsley, dill, fennel, Queen Anne’s Lace and rue. The Anise Swallowtail caterpillar feeds on anise as well and reportedly citrus plants also. Many of these plants can be found relatively easily in garden centers and nurseries around the country. That makes it easy to find host plants to plant and to get more food for your caterpillars if you find yourself running low. However, there is a risk that the growers used pesticide on the plants which will kill the caterpillars. Ask before you buy and be sure to clean (rinse) the vegetation once you get it home. It’s always best to grow your own if you can but that is not always practical for everyone.

Although it is not generally recommended, I have always had success using parsley from the grocery store in a bind. This is a bit risky because, again, it is unknown how that parsley was grown. It may have pesticides on it. I have rinsed it thoroughly and it has worked for me. Personally, I feel that if it kills my caterpillars then I’m not so sure my family should be eating it either and I would definitely let the grocery store know about it!

Once you figure out what host plant does best in your area, be sure to plant a bunch of it in the spring/summer to attract egg-laying butterflies. Parsley has worked well in my area in the Southeast where as Dill has not been so great because it just seems to dry up too early before the end of summer. My favorite (and the butterflies favorite) is the rue plant as a host because it is an attractive and reliable perennial and is also a host plant for Giant Swallowtails.

Also, planting nectar plants will help to attract butterflies. Butterflies get their food from the nectar in flowers. Black Swallowtails are reported to especially like Milkweed, Phlox, Red clover and Thistle. In my yard I have seen them on Homestead Purple Verbena quite often and they love my Zinnias as well as the Milkweed family of plants.

Finding butterfly eggs of the Black Swallowtail is not too hard. In my experience Black Swallowtail eggs are usually found on the top of leaves. The eggs are tiny white/yellow spheres (there are several pictures in the Butterfly Egg Photos section of this website). You can bring the eggs inside on the leaves or wait until the caterpillars hatch out and grow a little. If the eggs are laid on a potted plant you may want to bring the whole pot inside. If the eggs are on a plant in the ground you can bring some cuttings in or wait.

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The caterpillar will change a lot in appearance as it grows. When it is young it is black with a white stripe in the center. Shortly before it pupates it will look like the pictures below. Even as an adult caterpillar they can vary in coloration. They are all striped but some have alot of white, others have much more green and some are very dark looking with a lot of black. They have all turned into healthy butterflies.

Don’t wait too long to bring them in or you will start losing caterpillars to their natural predators. Depending on how many caterpillars I already have indoors (and how much food I have) I sometimes only bring a few in and leave the rest outside to fend for themselves. Usually their numbers dwindle rather quickly. It is estimated that only 1 out of every 100 eggs laid will make it to a butterfly.

We have used both an open and enclosed caterpillar home for Black Swallowtails. However, the Black Swallowtail caterpillar loves to roam right before it gets ready to pupate so you may want to keep these caterpillars enclosed as they grow or just enclose them once they start to get big. Enclosed is much easier but I invite you to read our article about indoor caterpillar homes for more information including the plusses and minuses.

When the caterpillar reaches the size of about 1.5 inches (or about the size of the caterpillar pictured on my daughter’s hand) it will be ready to pupate. Also, these caterpillars make a dime-to-quarter size green mess as they purge undigested food shortly before pupating so if they are not enclosed you may want to put some newspaper under the caterpillars home.

Black Swallowtail Caterpillar Showing Osmeterium

Another interesting note about this caterpillar is the osmeterium (harmless to humans). It is probably the only possible negative (if it even is a negative) to raising any of the swallowtail caterpillars indoors, but it is such an oddity that the interest overcomes the negativeness. The osmeterium is shaped like a two prong fork that is right behind the caterpillars head. The osmeterium of the Black Swallowtail caterpillar is bright orange and pops up to let out an unpleasant smell when the caterpillar is touched or disturbed.

My children get a kick out of touching the caterpillar and watching the osmeterium pop out then disappear again. The smell has been described to be foul or like rotten cheese. It is certainly not pleasant but not intolerable either. I encourage them to only do it with the outdoor caterpillars and to wash their stinky hands when they are done!

One more note about the Black Swallowtail caterpillar: it is safe to humans to hold and touch the caterpillar. It is not always safe for the caterpillar, however. The smaller they are, the more delicate they are. Also, they will molt as they grow (see life cycle of a butterfly for more information). When they get ready to molt they will stop eating and stay still for around 24 hours then crawl out of their old skin. During this time they are delicate and should not be moved.

Black Swallowtail Caterpillar
Ready to Pupate

When the caterpillar gets ready to pupate it will scrunch up along a stick or stem and then spin its silk. It will attach itself to the stem/stick like a telephone lineman hangs on a pole with a belt. The caterpillar will use a belt of silk as well as attach its base to a patch of silk.

Brown form of a Black Swallowtail Butterfly Chrysalis

When it is ready, (about 24 hours after attaching) it will wriggle its outer skin off exposing the chrysalis beneath. The chrysalis will either be green or brown depending on the color of the surrounding area.

In 1-2 weeks the butterfly will emerge. This usually happens in the morning hours after the sun has come up but I’ve had just a few come out in the early afternoon also. The butterfly has to be able to hang with its wings down (see life cycle of a butterfly for more information) so if some sticks or leaves are close to the chrysalis then you may want to clear them before the butterfly comes out.

After several hours the butterfly will be ready to fly away. Often times when they get close to being ready and start fluttering we will carry them outside on a stick (or my kids love to have the butterfly hold onto their fingers). If they are not quite ready for flight we will put them on a stick in a pot or on a plant in the sun where they can finish.

Male Black Swallowtail

Black Swallowtail butterflies overwinter in the chrysalis. So, if your chrysalis does not form until late summer then it may not come out of the chrysalis until the following spring. To read more about this please visit How to Overwinter a Chrysalis.

There are many different ways to raise a Black Swallowtail butterfly. You have choices in type of indoor caterpillar housing, whether you start with an egg or caterpillar, how you feed them (potted plant or cuttings) and how/when you release them. This article shares our experiences with Black Swallowtail butterflies but many different methods will work. Once you give it a try you will find what works best for you. It is truly a spectacular experience and I hope you enjoy it as much as we do.

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