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HOW BUTTERFLIES DEFEND THEMSELVES
text by Walter L. Meagher

fotos by Wayne Colony

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Butterflies seem defenseless, too vulnerable for their own good, giving short lives to ornament gardens.  But if they elude the camera, if they don’t hold still long enough for us to read their name tags, they must be able to elude a bird.

The erratic pattern of flight is a defensive strategy.  How can a bird, even one as maneuverable as a fighter plane, move fast enough to catch a weaving wandering butterfly?  Consider the Gray Hairstreak (left), seen in El Charco.  Small, yes; but also its flight pattern is fast and erratic.


Speed, on the other hand, is not usually a factor in butterfly defense.  Skippers (Hesperiidae) are the fastest of all butterflies, clocking 30 mph. They are faster than other butterflies because attached to their short stout bodies are wings stronger than those of other butterflies.

Larger butterflies, swallowtails (PAPILIONIDAE), for example ... look for the super-elegant Two-tailed Tiger Swallowtail (right), glide short distances and fly as slow as 8 mph.

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mariposa    A second strategy is camouflage.  Some butterflies are simply not colorful, let alone dazzling. Perhaps an extreme of the kind is the Funereal Duskywing (left). It too is a Skipper, and so it is small and chunky; but what we are observing is that it is the color of burnt wood, and might be invisible on pine tree bark. Indeed, it has a moth-like appearance; but then butterflies and moths are in the same order of insects, Lepidoptera. 

An interesting example of camouflage, from the English butterfly fauna, is the Comma (Polygonia c-album). When the wings are folded, the butterfly is as dark as the Funereal Duskywing; but it has this added feature: the wing edges are ragged - the whole insect has been likened to a ‘fallen leaf’ (right) .  Find me if you can, it says. 

Color is an attractant, and male butterflies, like male birds, are often colorful, both for the same reason.  But is not visibility a handicap?  There is good evidence to the contrary.  Bright and varied colors actually help in defense.  Here’s how.

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Each color is laid down on a scale, the scales overlap, like roof tiles, and lie on the supporting wing beneath them.  The powder that rubs off when one touches a wing are scales; there may be 100s or 1000s of scales on one wing.  But how may a butterfly befuddle an avian predator with powdery wing scales?

The answer is light: the color of the scales on the upper side of the wings changes with the angle of light.  A bird wants a steady fix on its prey but, ‘As the butterflies move their wings up and down during flight, they seem to disappear.  This coming into and going out of focus is confusing to predators.’  (Christina Brodie, U.K.  See: http://www.microscopy-uk/mag/indexmag.html

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This phenomenon - color tints shifting according to the angle of light - is especially true of blue.  Many butterflies, seen in El Charco, are blue.  Let me mention one small - Reakirt's  Blue - and one large - the Pipevine Swallowtail (left).


Eyespots are so distinctive of butterflies (and moths), and add so much to their interest and charm, that they suggest, to me, the beauty spot on the high cheek bone of an elegantly coiffeured lady at the Court of Versailles.  Well, that is farfetched; but eyespots are farfetched.  If every species had them we would be overwhelmed with beauty. 

Their defensive function, we suppose, is twofold. First to convince a predator that eyespots are eyes. If unconvinced, the bird bites the butterfly; it will then have snatched a piece of unnutritious wing. The eyespots were a decoy.  By biting the wing, the spot saved the vital organs - head, thorax and abdomen remain unmolested. 

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When the dauntless Spanish adventurer Hernando Cortes marched into battle against the Tlacalans - Indians he had to defeat and win as allies before he could march on the Aztec capital - they assembled before him in a great array. ealthier Indians, great lords of the land, came into battle with fantastic headpieces representing wild animals, and frequently displaying a formidable array of teeth. (cf.: History of the Conquest of Mexico, William H. Prescott, The Modern Library, N.Y., 2001, p. 315) 

A butterfly with eyespots (and these are usually on the under side of the hind wings) can suddenly open and hold the hind wings aloft.  A bird has to think, Wow!  I thought you were a little guy. What is this?  I will have none of this. And off he goes. This is Flash & Startle Defense. It must work; it has been preserved over thousands of years. (See: Martin Stevens, 205, The role of the eyespot as an anti-predator mechanism.  Biol. Rev. 80 (4): 573-588.)

If a bird eats a butterfly that tastes bad, the bird will probably spit it out.  Having found the butterfly to be unpalatable, the bird doesn’t bite another butterfly of the same species. The Monarch butterfly is the most famous example of the defensive use of toxicity.

But let us first mention Cabbage Whites.  They are members of the same family, the Pieridae, as the sulphurs but no numerous in the conserve.  They could become more so if a great proportion of wild and cultivated plants belong to the Cabbage Family.  I mention the Whites because of the principle: toxicity as a deterrent to predators.

Their particular potency lies in sequestering a mustard oil (glucosinsolate) in the larval stage of growth.  Mustard oil glycosides probably evolved to deter herbivory; it is known, for example, to be an active chemical agent against herbivores, fungi, viral and bacterial pathogens, nematodes, even other plants.  If you don’t like the taste of Brussels-sprouts or the scent of boiled cabbage, these glucosides may be the reason.

But let's return to the Monarch, an occasional visitor to El Charco. 

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Toxicity is not a widespread defense strategy of butterflies, but, in the case of the monarch butterfly (Danaus plexippus), it is famous.  Famous too is the migration of the Monarch from Canada to the mountains of Michoacan in Mexico. Female Monarchs lay their eggs on milkweed plants, commonly Asclepias syriaca, but on other species of Asclepias in other parts of North America as wellLarvae hatch in two weeks, feed on the host plant selected by the mother, ingesting a cardiac glucoside which makes the adult butterfly unpalatable to most birds. On most occasions when a bird bites a Monarch, it is sorry, and promises not to do it again. 

In summary, butterflies avoid predation in several ways: by erratic flight patterns, by camouflage, by confusing changes in wing colors, by eyespots as decoys, and as flash & startle defense, and, in a few cases, by storing toxins to make their bodies unpalatable.  If butterfly populations fall, I doubt that it is due to excessive predation by birds.

16 June 2008