I was excited to find what I thought was a different species of swallowtail caterpillar on the dill plants in the back yard the other day — black with yellow spots.
But then a closer look revealed there were just two color morphs of the same Black Swallowtail caterpillar munching away on the dill plants: a few of the familiar green and white with yellow spots and black bands, along with more of the mostly black with yellow spots variety.
Green (summer version) and Black (fall version) of the Black Swallowtail caterpillar.
Why would a black form suddenly be more common than the green and white form of the caterpillar? As you might recall from an earlier post on this species (Munching the Dill), Black Swallowtail caterpillars vary quite a bit during their development, changing from black with yellow-spotted 1st and 2nd instars (molts) to the familiar green and white morph in later development.
The first two instars are black, gradually developing spots. The later instars are highly variable in color, with a white, green, brown, or black background.
It seems that caterpillars developing in the early fall weather with its decreasing day length and cooler temperatures have somehow turned off the color change that normally happens between 2nd and 3rd instar development in order to retain their black color. Searching for an answer to how and why this might be the case, I stumbled on a paper by Wade Hazel that addressed these very questions.
The how of the color difference between the two caterpillar morphs is explained by their exposure to short days during their first two to three instars of development: if the daylength is 12 hours or less, black swallowtail caterpillars stay black; if daylength is greater than 12 hours, the caterpillars adopt their typical green, white, and yellow coloration.
The answer to why this color change to the black morph occurs in the fall is explained by their ability to absorb more solar radiation, raising their body temperature a degree or two higher than the green morph in bright sunlight, and thus accelerating their development during the unpredictable fall weather.
Dark-colored caterpillars heat up faster and are warmer than paler-bodied ones, when exposed to bright sun. Data from Wade Hazel’s 2002 paper published in Evolution 56: 342-348.
A black caterpillar can reach the pupal stage 3 days faster than a green one based on these body temperature differences. Reaching the pupal stage is vitally important at this time in the fall because the pupa is the freeze resistant stage that can overwinter until the following spring.
Back Yard Biology 
Fascinating information, Sue, about a curious phenomenon. It’s amazing how species have adapted in order to survive.
Yes, it surely is, and that kind of stuff makes Backyard Biology fun.
Interesting information. Day length affects so many different things. It makes perfect sense that a caterpillar would stay black to attract more sunlight, but I’ve never really thought about it.
Glad you enjoyed it — I love solving a mystery.
I love finding out why.
So glad to help out when I can. I love to know “why” also!
Fascinating! Maryann
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I’m so glad you posted this! I found a dark one today and couldn’t ID it for sure. Seemed too black to be a swallowtail. Now I know why! It was in one of my raised beds where I have purposely let a bunch of dill seedlings grow. I hope it reaches the pupa stage before we get a hard freeze!
Fantastic! I guess that’s why I write this blog — hoping that others will find some use for all the trivia I come up with. Thanks for writing Aimee.
I collected 2 black swallowtail eggs from the same fennel plant after watching the female lay the eggs this spring. I hatched both of them in the same cup & left them there to grow. One ended up being typical green stripe & the other is black with yellow dots. I’ve grown hundreds of black swallowtails & never had a caterpillar stay black, even the fall ones that over-winter in chrysalises. I’m not sure why this occurred. Nature is always surprising!
How interesting! Good observations. Thanks for that information.
Very interesting, Sue! Perhaps you and Dawn can help me! I rescued some black swallowtail caterpillars last month that were destined for the garbage heap with the close-out host plants that hadn’t sold at a Wisconsin garden center. Ive read what I can to learn how to help them survive at this stage but can’t find much. These seem to be late bloomers… 1st and 2nd instar stages when I got them in October. I’ve lost a few : ( and only one of the remaining 4 seem to be doing okay. It’s just morphed into a third instar caterpillar two days ago- black! I’m exposing the cats to temps down to 40* as able, because I read they need to be conditioned to colder temps to over-winter. But I’m keeping them warmer during parts of the day to hopefully keep them eating and help them move into the pupa stage fast enough. Any recommendations on temperature, light and moisture would be very helpful! Just learning that this black version happens is really cool! Thank you!!
Wow — you sound like you have a pretty good handle on how to get them into dormancy. Sorry, I know enough to give you any good recommendations that you haven’t already though of. Best of luck with them.
Thank you for your reply, Sue. I love your blog and will continue to read it!
Wow, November in Wisconsin sounds like some chilly caterpillars! It sounds like your doing what you can to keep them going. As long as they have a food source & don’t get below freezing, they still stand a chance. I’ve found that these late bloomers tend to form a chrysalis while still smaller than average (& generally eclose into small but perfect butterflies in the spring). If they make it to that stage, just be sure & place the chrysalids in a sheltered but cold spot (covered porch, unheated garage, etc) otherwise they will make butterflies in February. Once in a chrysalis, they can withstand typical outdoor winter temps. There are always a few that just get too late a start & don’t make it. If they don’t make it, just know that you did the best for them. Best of luck!
Thank you so much for that additional information! Yes, we’ve had temps in the low 20’s at night and low-mid 30’s in the day a couple of times already. So, I’m doing the right thing by giving them some warmer time in the house to develop, while exposing them to cooler day or night temps around 40 to harden them off? They’re enjoying flat parsley now- the plants that were going to be pitched. I’ve also offered them dill but they have no interest. Any thoughts on how often to mist the enclosure while they’re still caterpillars?
Generally I don’t mist enclosures as humidity can cause more problems, but I do most of my butterfly raising in MO summer so lack of humidity is rarely an issue. If your house is particularly dry or if their food is drying out too quickly, a quick daily mist might help. Otherwise it sounds like you’re doing great.
Thank you both so very much!
Very interesting and informative! I learned something new today! Thanks for this post.
Thank you for solving this mystery for me. Thought I was just seeing things when I was checking on my caterpillars.