What's All the Flap About?
Butterflies need their hind wings not to stay aloft but to evade predators
It’s not just poetic alliteration that makes the pat phrase “a butterfly fluttered by” so appropriate. The insects, although not always that speedy, often take a flight path that involves so many erratic dips and turns that they almost look out of control. But it’s not because they can’t do any better: Such unpredictable flight is how butterflies evade birds and other predators. However, most butterflies are brightly colored, which would seem to counter their evasiveness by making them easier to spot and track. “The question always bothered me,” says Thomas Eisner, a biologist at Cornell University. “Why are butterflies flaunting their visibility?” As Eisner and Benjamin Jantzen, a doctoral candidate now at Carnegie Mellon University, report in the October 28 issue of the Proceedings of the National Academy of Sciences, a butterfly’s ability to evade and its blatant pigmentation may go hand in hand.
The first step was to find out what physical feature of butterflies allows them to move so erratically. It’s been known for about a century that the front wings in butterflies are the ones driven by the insect’s muscles; the hind wings are passively coupled to the front ones. Eisner decided to investigate just what the back wings were doing by trimming them away bit by bit. To his surprise, he found that if he removed the entire hind wing, the insects had no problem flying. Indeed, when Eisner went on to test an extensive list of butterfly and moth species, he found that without exception they were all capable of sustained flight with only their front wings. “It is pretty startling that they’re that overendowed with lifting surface,” says Jantzen.
Eisner was careful not to use any rare species in the study. For each species, he used only one or two individuals. “I have one of the monarch butterflies in my home who has lived four months with me,” Eisner says. In the wild the insects often run into obstacles and cause their own wing rips and tears. “Wing breakage is very common in natural circumstances,” Eisner says. “I’ve found butterflies that have literally one wing completely gone, and they can fly.”
To determine just what the hind wings contribute to the flight, Jantzen and Eisner set up an enclosure with two video recorders. They used two pest species, the white cabbage butterfly and the gypsy moth. Jantzen wrote software that translated the video imagery into a three-dimensional trajectory of each insect’s flight, first intact and then without the hind wings. With half their wing area, the insects flew more slowly and could not turn as quickly, but they could still follow similar flight paths. In other words, they could zigzag as usual, but had to do it at a slower pace.
So although butterflies and moths don’t need their hind wings to stay aloft, the structures appear to be essential for evading predators. “Many other insects can go significantly faster than butterflies,” says Jantzen. “But by having these massive wings, they can manage these pretty enormous turning accelerations. Whether or not something is faster than you in a straight line, if you can take the corner more sharply, you don’t get eaten.” The hind wings therefore function somewhat like the paddle of a canoe: A larger paddle has more surface to push against the water, which makes for sharper turns.
And this result gets back to Eisner and Jantzen’s theory about why butterflies have such bright coloring. Nocturnal moths likely evolved evasiveness first to avoid predators such as bats. Butterflies evolved from moths into diurnal species. Moths have no reason to evolve colors for night life, but daytime butterflies are large and visible to birds. Since they are going to be spotted in any case, being showy about it isn’t a drawback, but is potentially a defensive maneuver—to advertise that they are difficult to catch.
“If a young, inexperienced bird sees a butterfly at a distance farther than any other insect is visible, it says wow, look at that big thing there. The bird goes after it and finds that it’s very difficult to catch. If the bird does catch it, it’s slippery, because the butterfly wing is covered in scales. And then the bird realizes that it’s all wrapper and no candy. If the bird tries again, with a different butterfly of a different color, but it’s the same experience, then a bird learns that it’s not worth chasing these colored things that are so easy to spot.”
Butterflies, in a way, are therefore getting double duty out of their investment in large hind wings. “Those giant wings are just so brightly colored, says Jantzen,” and those are exactly what make it hard to get hold of them.”—Fenella Saunders