South America Cruises and the Blue Morpho Butterflies

The blue morpho butterfly of South America belongs to the family Nymphalidae, with 80 species in the genus Morpho. They are deceptively swift flyers common around streams and other sunlit areas as anyone knows who has seen them in the wild. On our Amazon cruise while walking in the rainforest or slowly making our way up a narrow stream by skiff, we see these magnificent butterflies at unexpected moments that never fail to produce expressions of admiration and awe. When resting, the underside of their wings has a cryptic coloration which allows them to blend into the forest background. When they suddenly fly off, the brilliant blue can be startling, and potential predators can be caught off-guard. It is thought they use this “flash and dazzle” strategy as an effective method of capture avoidance.

The entire life cycle of the Morpho butterfly from egg to death is approximately 137 days (4 ½ months) and the diet changes throughout each stage of its lifecycle. The larvae hatch from pale green, dewdrop-like eggs, and as a caterpillar will chew leaves of many varieties, but prefer to dine on plants in the pea family. The caterpillars have reddish-brown bodies with bright lime-green or yellow patches on their back. The hairs are irritating to human skin, and when disturbed they secrete a fluid that smells like rancid butter. The caterpillar will molt four to six times before entering the pupal stage. The chrysalis is jade-green and emits a deterring, ultrasonic sound when touched. When they become butterflies they can no longer chew but use a long, protruding mouthpart called a proboscis as a drinking straw to sip the juice of rotting fruit, the fluids of decomposing animals, tree sap, fungi, and wet mud.

The most outstanding and memorable aspect about one’s first sighting of a blue morpho butterfly in the forest is the shock of iridescent blue against the innumerable hues of green. The iridescence in the wings is produced by many transparent, strongly reflecting scales, not by pigments. The cuticle on the scales of the wings is composed of nano-sized, transparent, chitin-and-air layered structures. Rather than statically absorb and reflect certain light wavelengths as pigments and dyes do, these structures selectively cancel out certain colors through wavelength interference while reflecting others, depending on the exact structure and distance between diffracting layers.

In the last few years, the butterfly wing has inspired imitation structures that appear intensely blue from a wide range of angles and new techniques are being used in display devices (e.g., for cell phones) which can change colors rapidly, remain vibrant under low-light conditions, and require less energy than other electronic display methods. These new discoveries are also being used to replace traditional paints with environmentally-benign colorations which have unique and valuable properties thereby producing better colors and avoiding the toxic heavy-metals or manufacturing methods common to many pigments and dyes (e.g. for cars, iridescent fabrics, and cosmetics). Because the colors are pigment-free, they don't fade either. The lamellate structure of morpho wing scales has even been studied as a model in the development of anti-counterfeit technology used in currency.

Not to be overlooked are the recent studies on the black pigments found on some of these blue-and-black butterflies. This special blackness is almost twice that which could be achieved by pigment alone, and causes the bright blue to stand out even more. Black colors appear black because they absorb all frequencies of visible light. However, this is not perfect, and almost always some light scatters back. Recently it has been shown that the butterfly black is also caused by an optical light-trapping design. The scales are covered in tiny pits, about a micron across, that form a honeycomb-like array. These scales have a high refractive index, so they take advantage of total internal reflection. That is, the light enters the material, but whenever the light meets another part of the surface, instead of crossing, it is reflected back into the material. Since hardly any light can escape from the wings into the eyes, they appear very black indeed.

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