Whoa Whoa Whoa, How Does a Black Swallowtail Caterpillar Grow?

One time, in the fennel…Black Swallowtail Caterpillar

There were many many many caterpillars. The time was July. And also August. And September. AND also NOW! That is to say, the very very end of October. Yes, the time change and the first frost are nearly upon us. And yes the Monarch caterpillars seem to all actually be grown up and going/gone now. But the Black Swallowtail larvae are with us in force!! No, this blog has not yet lived up to the promise of moving on to other topics. Not yet!

Because on a recent lovely day at Wild Peace Farm, under the blue sky and the puffy clouds and surrounded by bird calls and the most delicious so fresh it’s still alive food, the Farmer, or I, who can remember, spotted a bright green Black Swallowtail caterpillar. Of course at everycreepingthing this is a familiar creature. I have even posted a video of one making a chrysalis in my bedroom. But I did not know that they were as abundant this time of year as the Woolly Bear. The Woolly Bear actually overwinters as a caterpillar– but I am guessing that like their cousin the Spicebush Swallowtail, these Black Swallowtails will be spending the cold dark days to come inside their comfy chrysali. So when do they start making it? Will they survive this weekend’s predicted frost? And also, in addition, what are all these different instars they’ve got going on?

3 instars of the Black Swallowtail caterpillar

A caterpillar is an insect after all, and it is an invertebrate- instead of bones inside it has an exoskeleton on the outside… its “skin” is like its skeleton. Since those “bones” are hard, they can’t stretch much to accommodate all the food a very hungry caterpillar turns into body mass. Instead, a caterpillar sheds its skin a number of times on the road to butterfly adulthood. The Monarch does this 5 times total, going through 5 different instars before it pupates- but each one has the same basic coloring and is readily identifiable as a Monarch if you have been paying attention.

The Swallowtails are playing the game a bit differently. There are 6 Swallowtail species that I know of in Maryland. All that I have seen or read about spend at least part of their caterpillar lives a bright color, and part of their lives blackish and speckled and maybe looking like birdpoop. In the Black Swallowtail’s case there are a number of black phases, although they have those bright red dots that mostly take away from the obvious camouflage argument. So what is going on here? I did some Googling and I cannot find a satisfactory answer. I was left with one obvious option: I took the smallest caterpillar I could find home with me, in the hopes of recording her growth and seeing the stages that unfold. I will keep you posted… (That is a pun, hon.)

Different Black Swallowtail caterpillar instars

Here she is, my smallest friend, alongside the largest caterpillar that the fennel had to offer. Those green stripes on big brother most definitely belong to the last instar… His next stop is a butterfly (/chrysalis yay!)! But what comes in between?!?!  Stay tuned.

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