From a developmental perspective, the formation of a butterfly wing pattern is the result of a complex coordination of processes, timing and genetics. The mechanics that determine the ground scales (background), pigmentation, pattern element size, shape, position, and symmetry, ultimately determine the pattern. Experiments have revealed pattern determination is established and finalized within the first few days after the caterpillar enters its pupal stage. At this time, the wing views, the wing shape, the epidermal membrane, pattern elements and coloration are determined. At the root of the pattern development mechanism is the diffusion of a morphogenic substance through the epidermal layers. These diffusions, controlled by activating and inhibiting enzymes, result in gradients of the morphogenic substance. Reaction thresholds based on the concentration of the morphogen determine the contours of the actual pattern elements. Some pattern elements are formed from the source position of the morphogen while others are initiated by the absence of the morphogen (concentration sink). The exact pattern shapes are usually formed from the contours generated from the addition of multiple morphogen gradient sources or inhibitions. Controlled by genetic and environmental factors, the final pattern might best be described as a developmental freeze-frame at the beginning of pupal stage when the pattern finalized.
Whether you focus on the micro architecture of an individual scale cell, or prefer to sit back and enjoy the ethereal beauty of form and design in motion as a butterfly skirts the thermals, it is easy to see why this order of insects have become so revered.
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In a few cases, specific butterfly patterns are more readily associated with functional advantages. The dorsal patternation of butterflies function as gender signals, allowing mates to recognize one another. Advertising your unpalatable nature through bold aposematic (warning) coloration, successfully establishes a learned avoidance response from predators. Camouflage and cryptic coloration have the obvious advantage of rendering the butterfly harder to find. Eyespots (ocelli) flashed as an otherwise cryptic butterfly makes a hasty retreat, can confuse an attacker or at least help to focus the attack towards non-critical regions of the body. Melanization is a useful device employed by some butterflies and moths. Forms that have extra black (melanized) scales are better equipped to absorb heat from the sun and thus thermoregulate themselves to activity in cooler climates. Many other design and wing structure advantages have been studied but his sampling should give you an idea that many designs amount to considerably more than an aesthetically pleasing set of wings.
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Many adaptational and morphological observations can be made from the study of butterfly wing patterns including mimicry, polymorphism, polyphenism, and dimorphism. A couple of mimicry systems are observed in butterfly wing designs. Batesian mimicry is defined by a tasty butterfly resembling a well know distasteful species. In this case, a few butterflies gain the associative protection of mimicking the Monarch butterfly’s orange and black striped pattern. Mullerian mimicry is where a group of distasteful butterfly species have evolved to all look alike thus increasing the chances of recognition that a particular design and coloration scheme represents bad meal. Polymorphism the genetic code of a single species is capable of a couple of distinctly different looking adults. This characteristic is typically found in tropical butterflies. Polyphenism is defined by genetically identical caterpillars producing pattern variations in the adults due to environmental triggers such as the length of the day (season), temperature, or the relative availability of water. Dimorphism is where genetic differences between males and females result in differing color patterns for each gender. (continued…)

Characteristic patterns on a wing are the result of the wing shape, the mapping of the wing venation, and the actual shape size and coloration of pattern elements formed by the scale cells. There are several studied processes that are responsible for modifying the generic pattern elements found in the Nymphalid ground plan into the spectacular diversity we have come to expect from these inspirational insect designers. Each individual wing cell (the space between the wing veins) are capable of customizing the pattern elements found within. This kind of cell-by-cell customization gives the freedom to resize, shape and color individual elements. An additional mechanism worth noting is known as pierellization where pattern elements become so dislocated from their expected neighboring wing cell element that they align to other elements. This freedom of pattern manipulation has allowed species like the Indian Leaf Butterfly to simulate a very convincing leaf pattern on the ventral wing surfaces, complete with venation that mimics a leaf rather that a butterfly wing. (continued…)

Another interesting observation about wing patterns is referred to as Oudemans’ principle. As you study the ventral wing pattern of a resting butterfly, you’ll notice the pattern often smoothly translates from the hindwing to the visible tip portion of the forewing. In contrast, the covered portion of the forewing lacks the patternation and is often more brightly colored making for a disorienting flash of color as the butterfly launches into flight. Oudemans’ principle can also be observed on the forewing patterns where design element align between the fore and hind wings when the butterfly is displaying its dorsal surfaces.
A fair volume of research has been conducted into the analysis of pattern element along with exploration into the developmental mechanisms of pattern formation. A good portion of this article was inspired by H. Frederik Nijhout’s book, “The Development and Evolution of Butterfly Wing Patterns.” Studying the commonality between the bewildering diversity of patternation found on the wings of butterflies and moths resulted in a generalize model of pattern elements and symmetries known as the Nymphalid ground plan. Although no single butterfly species exactly manifests all the pattern elements established in this model, it forms a useful reference framework to discuss the specific elements of any particular design. This model was initiated back in the 1920′s by B.N. Schwanwitsch and F. Seffert. The model identifies regional bands of symmetry that radiate out into the wing plane from the root where the wing attaches to the thorax. Reviewing the pattern elements of the model from the root out to the wing tips we find a wing root band, a basal symmetry system, a central symmetry system, a discal spot, border ocelli complex, parafocal elements and marginal bands. The model is enhanced with the recognition of venous strips as a major pattern element. (continued…)

If we take a closer look at the individual scale cells, we notice they vary considerably in size, shape and structure. Scale cells are generally held at a 45 degree angle to the wing membrane. The exposed top surface of these scale cells have an elaborate extra cellular structural architecture known as fenestration. These micro structures play an important roll in the iridescent color characteristics of various butterflies. The tiny structures interfere with light wavelengths and usually result in brilliant shimmering blues and greens. The multitude of other colors found on the scales of butterflies and moths comes from pigmentation. Each scale cell holds a single color pigment that include melanins, ommochromes, pterins, and flavonoids derived from plants.
Observing patterns as a whole, we notice that the left and right wing designs are generally symmetrically along the axis of the body. The dorsal (top) forewing, dorsal hindwing, ventral (underside) forewing and ventral hindwing represent the four wing surfaces that carry a unique pattern on each butterfly species. The dorsal wing surfaces typically display bold, simple and colorful designs, whereas the ventral surfaces are notably more detailed. This phenomenon corresponds with the fact that the dorsal surfaces are generally visible during the rapid undulating wing beats of flight compared to the detailed and often cryptic ventral surfaces which are encountered in plain view as the stationary butterfly assumes a resting position. (continued…)