American lady butterfly spots aren’t camouflage, but what they are remains a scintillating mystery
Unlike many other butterflies, when the weather turns cool American lady butterflies (Vanessa virginiensis) do not migrate south to warmer regions. They die out in our area and rely on their southern relatives to migrate back north in the spring to repopulate the territory. These beauties can travel up to 100 miles per day, and they are one of the earliest spring butterflies to arrive in Cambridge. You still might be able to see some of these butterflies in September if the weather is warm enough.
American lady butterflies have two large eyespots on each hindwing. Animals often evolve patterns to help camouflage themselves from predators, but eyespots make butterflies stand out. Why would American ladies want to stand out?
Scientists have three hypotheses about why eyespots might help butterflies. Some think butterfly eyespots scare away predators because the predator thinks there is a large, dangerous animal behind the eyes (the intimidation hypothesis). A second idea (the deflection hypothesis) is that the eyespots deflect predators: The predator attacks the eyespots on the edge of the wings rather than the butterfly’s head, giving the butterfly a better chance at survival. A third hypothesis is that the eyespots make the butterfly stand out, and standing out is often a signal that a creature is poisonous.
While it is not known which hypothesis is correct, having eyespots clearly aids butterfly survival. In one study, researchers compared butterflies with intact eyespots with ones that had the eyespots painted over. The butterflies with the intact eyespots survived much better; in fact, only one of 34 butterflies with intact eyespots was killed, whereas 13 of 20 butterflies with painted-over eyespots were killed. Additional studies have shown similar results.
How did this butterfly come to be called the American lady? The painted lady (found on every continent except Antarctica and South America) is a well-known European butterfly. Early naturalists saw a two-spotted version of the European butterfly in North America, which they called the American painted lady, subsequently shortened to American lady. American ladies resemble and are related to painted lady butterflies, but they are easily distinguished because they have on their hindwings two large eyespots rather than four small ones. American lady butterflies are more cold tolerant than painted ladies, so you will see more American ladies than painted ladies as the weather cools. Early season ladies are larger and more brightly colored than late-season ones.
Although adult American ladies prefer nectar from flowers, they will also feed on sap or rotting fruit, as they are not specialists like monarch butterflies. Male American ladies – is that an oxymoron? – sometimes sip water from damp soil or mud. Females fly low in search of ground-hugging plants that make good hosts for their caterpillars, such as pearly everlasting and pussytoes.
American lady females lay single green, pinhead-sized eggs on the upper leaves of host plants. The eggs are very small and blend in well with the green leaves, making them hard to see. The eggs hatch into caterpillars in three to five days. The caterpillars sew together bits of seed fluff or leaves to make nests on their host plants that they rest in when not eating. The nests are best seen in the early morning when sunlight reflects off the morning dew they collect.
These caterpillars form chrysalises that are different colors (and shiny or dark) depending on their surroundings. When an American lady caterpillar pupates near green leaves, it forms a greenish, shiny iridescent chrysalis. When it pupates near something dark, it forms a dark, dull chrysalis.
American ladies, like all butterflies, do not have lungs. They breathe through tiny holes on the sides of their bodies (spiracles). The holes open into a system of tubes (trachea), which carry the oxygen throughout their bodies. Butterflies are cold-blooded and require warmth to activate their flight muscles. Generally, butterflies will not fly when temperatures are below 55 or 60 degrees Fahrenheit.
How does climate change affect these butterflies? This subject has not been well studied, but it is known that as temperatures increase, some butterflies reproduce earlier. Unfortunately, their caterpillar host plants often do not mature earlier, so there may be a mismatch between when caterpillars hatch, needing to eat, and when host plants are available for munching. This mismatched timing can upset the entire food web – from caterpillars to birds to even larger predators.
Have you taken photos of our urban wild things? Send your images to Cambridge Day, and we may use them as part of a future feature. Include the photographer’s name and the general location where the photo was taken.
Jeanine Farley is an educational writer who has lived in the Boston area for more than 30 years. She enjoys taking photos of our urban wild things.