By Linda Ray
As we finish putting our gardens to bed and start thinking about the winter months ahead, I notice how quiet it has been, caterpillar-wise. Even the wooly bears and tussocks have disappeared, and the late summer caterpillars, such as swallowtails (Pterourus glaucus and Papilio polyxenes austerius), who recently were eating the parsley and dill, have started their winter journey and the final transformation into their adult selves. Butterflies are holometabolous, which means they go through the four stages of development: egg, caterpillar, pupa, and adult.
The onset of shorter, cooler days signals the need for butterfly caterpillars to prepare for the harsh realities of the coming winter. What are the strategies for winter survival? The primary goals are to stay protected from predators, keep from freezing through various means, and meanwhile transition from a pupa into the adult butterfly when they emerge in the spring. The evolved survival strategy is to form a chrysalis. A chrysalis is created when the caterpillar sheds its skin and attaches itself to a leaf, twig, or plant stem fairly close to the ground. Underneath the shed skin is a hardened layer of skin made mostly of chitin (a tough substance that organisms like insects produce to form an exoskeleton). The hard case of a chrysalis protects the overwintering pupa and offers camouflage from predators.
To clarify what a chrysalis is, we can say what it is not. It’s not just a mushy mess of pupal caterpillar goo in a protective shell. Inside the protective shell, there are the beginnings of what will become the future adult butterfly: tracheal tubes for breathing, neural tissue, an immature digestive tract, and imaginal disks that will later form legs, antennae, and wings. But still it seems pretty vulnerable hanging from its perch and open to wind, snow and sleet. How on earth does it survive?
When overwintering butterfly caterpillars enter the pupal phase of their development, their bodies produce a hormone that causes them to go into diapause. Diapause is a biological state where the insect’s metabolic activities decrease. They stop eating and moving around as a way to wait out the stress of cold temperatures and to save energy. There is a cessation of development. They are in what we might think of as suspended animation. To keep from freezing, a swallowtail pupa produces a substance called glycol which acts like antifreeze and lowers the freezing point of their body fluids. This prevents ice from forming in the cells that would otherwise lead to tissue death. Some butterflies, such as the Mourning Cloak (Nymphalis antiopa), overwinter as adults, snuggling down into leaf litter and utilizing glycol and other cyroprotectants to keep from freezing.
The spring returns, the earth warms, and the days lengthen. There is a mad rush in the chrysalis to complete the metamorphosis to coincide with host plant emergence. The metabolic suppression of diapause is reversed, and development surges until the adult ecloses (emerges from the chrysalis). These amazing survival adaptations took millennia to evolve. The fluctuating temperature caused by climate change may be the next challenge for these beautiful insects.
Sources:
Https://doi.org/10.1139/z2012-011 Canadian Journal of Zoology
Wilson, H. 2024. Scaled Jewels: The Biology of Butterflies. Eagle Hill Institute, Stueben, Maine
Photo Credits:
Black swallowtail chrysalis, Sdetwiler, This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.
Black Swallowtail Butterfly, EzerKenegdo67, This file is licensed under the Creative Commons Attribution 4.0 International license.