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.
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.
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.
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.