Adapted from an Article by Naturalist and UCI Professor Peter Bryant that appeared in the June 2008 edition of Tracks
In Orange County we have about 70 species of butterfly. Many butterfly species are amazingly specific in their requirements for a larval foodplant, and the adults will lay their eggs only on the appropriate plants. In fact, finding the right foodplant is often the best way to locate these butterflies. For example, the Mourning Cloak will be found flying in riparian areas where its larval foodplant, willow, is found. This butterfly is also unusual in that the females lay their eggs in tight clusters, which makes them easier to find than for species that lay their eggs one at a time.
Scientists as well as collectors are fascinated by the spectacular color patterns of butterfly wings. We wonder not only how these amazingly diverse patterns form, but why? How the patterns form is a major unsolved problem in developmental biology. We know that they form well before we can see them – in structures called the imaginal discs of the caterpillar and the pupa. These imaginal discs wait until the animal goes through metamorphosis to develop the pigmentation that defines the adult pattern.
It is believed that the individual parts of the pattern— the stripes and spots that we call pattern elements – are positioned by the action of unknown substances that form concentration gradients in the imaginal discs. Cells in the wing tissue are able to sense the local concentration, and translate that information into instructions about which pattern elements to produce. The complex patterns may result from the operation of multiple gradients acting in coordination with each other.
Although the pattern may be generated from smooth concentration gradients, the final pattern is digital—the wing surface is covered by thousands of scales, arranged in regular parallel rows on the wing surface. Each scale is about a tenth of a millimeter in size, and is made by a small clone of cells. The scale is usually of only one color, caused by reflection of light from a specific pigment. Some pigments in butterfly wings are fluorescent, absorbing light at one wavelength and emitting it at another.
Some colors from butterfly wings are produced not by pigments but by nanotechnology. The surface of the scale is a submicroscopic structure that scatters some wavelengths of light but amplifies others, a phenomenon called iridescence, and this produces the brilliant blue flashing light from the morpho butterflies in tropical forests. The wings of some butterflies have remarkably similar structures to the high-efficiency light-emitting diodes (LEDs) that engineers invented and that we now see on all of our electronic devices.
But why do butterfly wings have such spectacular patterns? One possibility is that these patterns are needed for mate recognition. Perhaps butterflies need those patterns in order to recognize their own species, even from a distance, for courtship and mating. In some species the wing pattern may also advertise the gender of the individual – the males and females are sometimes very different from each other, as in our official state butterfly, the California Dogface, where the male but not the female carries a picture of a French Poodle on each forewing.
We have to remember that butterflies probably look very different to butterflies, because unlike us they can see in the ultraviolet part of the spectrum. Males of the orange sulfur butterfly and its relatives have strong reflectance in the ultraviolet, and females are attracted to the males with the brightest UV reflectance.
Another function of color patterns on animals is camouflage, but the bold patterns and colors of most of our butterflies make them more, rather than less conspicuous, so they are clearly not for camouflage. However, the pattern on the underside of the wing is often more muted than that on the upper surface and could function in camouflage when the butterfly is resting with its wings folded. Our three local ladies, the Painted Lady, the West Coast Lady and the Virginia Lady all show this upper-lower difference.
Species with extensive black color on the wings may be able to absorb heat and stay warm on cold days. Consistent with this idea some species, including our Orange Sulfur butterfly, have different seasonal forms – a darker form that emerges in the cooler spring season and a lighter form that emerges in the summer.
Some of the really bold patterns on butterfly wings may have evolved to warn predatory birds that the insect is poisonous, or at least distasteful, due to the accumulation in its body of chemicals that were taken up by the caterpillar feeding on poisonous plants. The monarch butterfly, where the caterpillar feeds on milkweed loaded with toxic glycosides, is the classic example of this. The warning pattern is so successful that a butterfly in another family (the Viceroy) has evolved a wing pattern to mimic the monarch, thus presumably gaining protection from predators even if it is not poisonous.
One likely function of wing patterns is to focus the attention of predators towards less critical regions of the body. Butterflies in two separate families – the Swallowtails and Hairstreaks—have taken this strategy to impressive extremes. For example, in the Tiger swallowtail butterfly the yellow and black stripes converge on the tail end of the insect, where it displays both false eyes and false antennae. Often you will find one of these butterflies with a piece missing from the back end; presumably taken by a bird that was taken in by the deception (you never find butterflies with a piece missing from the front end!). Some of the Hairstreak butterflies, of which we have several species, also have lines converging of the tail end where they have both false eyes and false antennae. Some of them enhance the deception by making the false antennae wiggle as they rub their wings together.
Investigating butterfly wings takes us into many directions—about engineering of photonic devices, about visual communication, about attraction, deterrence, mimicry and deception, about self-defense, and about cost-benefit analysis. They have been evolving these devices for many millions of years, and we still have much to learn from them.