Black Swallowtail - Papilio polyxenes |
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Black Swallowtails are a fairly common but very pretty butterfly with a wingspan of about a 2.5 to 4 inches wide. They are easy to attract to host plants to lay eggs, and the caterpillars are easy to raise also. 
An adult male Black Swallowtail Black Swallowtail Host Plants
Black Swallowtwails use plants of the carrot family, including:
carrots, as host plants.
Some of the more common plants
grown for Black Swallowtails are:
Fennel - Foeniculum vulgare, Black Swallowtail Life Cycle
A Black Swallowtail Butterfly Egg.
Black Swallowtail butterfly eggs are a very pale, almost white color. They are spherical (ball like)
and the female will
lay them on both the undersides and tops of the plant leaves. It takes around 3-5 days for the
eggs to
hatch. Longer if the weather is cooler. When the eggs are close to hatching they will turn
to a darker
color. The dark color is the actual caterpillar growing inside. The caterpillars chew their way out
of the egg. After they hatch they eat what is left of the egg.
When they first hatch the Black Swallowtwail caterpillars are only around 2mm long, and are
black with a white band around
their middle. Unfortunately, my pictures of first instar Black Swallowtails are not clear enough to
make out all these details.
Black Swallowtail caterpillars change their appearance quite a bit as they grow. Second and third instar caterpillars look a lot alike, both are black with red/orange spikes, with the white band around their middle. Size range for 2nd instar is about 4.5 to 8 mm, for 3rd instar - 8mm to 13mm (1.3cm).
Fourth instar caterpillar look very different from the 2nd and 3rd instar. It has black and white/ pale greenish bands, or stripes all along its body and yellow spots. On early fourth instar, the middle white band can still be seen, but it fades away as the caterpillar grows and stretches its skin. Their size ranges from around 1.3 cm to 2.5 or so cm.
This is a picture of a 5th instar caterpillar right after it molted. Notice the dead skin laying behind it, and its face/head capsule is still light colored. 
This is the same caterpillar as the previous picture, only viewed from the front. You can see how its face/head capsule is much lighter here better than in the last picture. 
Caterpillars usually rest for a little while after molting while the pigments in their head capsules forms. Then they eat their dead skin they just molted, as this one is doing. The usual color pattern of the head capsule is visible in this picture too.
Swallowtail caterpillars have an orange/red fleshy forked organ called an osmetrium that they stick out when distubed to scare off predators. Osmetria give off a bad smell when stuck out - from two chemicals released from it - isobutyric and 2-methyl-butyric.
Once they find a place to pupate, they will stay in that spot for a while without moving very much. Sometimes for as long as 24 hrs. Eventually they spin silk thread to attach themselves to the object they decided to pupate on.
After another 24 hours or so (depending upon the temperature) they molt one last time. After this molt they are no longer a caterpillar/larva, but a pupa that has formed a chrysalis. Black Swallowtail chrysalids can be any one of three colors. The most common is a dark brown color, that makes it look like part of a stick. Less common are a green color and a light brown.
After pupating for around 9-11 days, longer if its cooler out, or if it pupates over the
winter, the adult butterfly emerges from its chysalis.
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Typical dark brown colored 
Adult Black Swallowtail from the side with wings closed.
A group a freshly emerge adults. Notice the way they are hanging kind of upside
down. All butterflies do this right after emerging from the chrysalis to help get fluids down
into their wings so they can fly.
This is a 5th instar that has more black than usual. I've seen both the regular and
the 'black form' caterpillars sometimes colored this way by their 5th instar. Notice that the
yellow spots are now nearly perfectly round and completely separated from the green/white bands.
