The Adult (or Imago)
Butterflies, like all insects, have an external skeleton (or exoskeleton) to which muscles are attached. The exoskeleton provides the butterfly's body with support and reduces water loss through evaporation. The respiratory system does not have a pumping mechanism. The sides of the thorax and abdomen have tiny pores (or spiracles) through which air enters and leaves the body via tubes (tracheae). The insect's blood (hemolymph) is pumped as it passes through a long, thin heart, and bathes the organs inside the body cavity.
Two large compound eyes, made up of hundreds of tiny units (ommatidia), cover much of the butterfly's head and allow a wide field of vision, including partially backwards. However, the eyes cannot distinguish much detail or determine distance, although they can readily identify color and movement, both of which are vital for survival. Colors aid in the identification of flowers, larval food plants, and the opposite sex of the species, while detecting movement may save butterflies from attacks by their predators.
The long, coiled proboscis (tongue) of the butterfly is projected into the center of flowers while searching for nectar (a liquid, sugar-rich food). Above the eyes and on either side of the head are two antennae covered with microscopic sense organs. The antennae are often incorrectly called "feelers," but "smellers" would be more accurate because it is through these organs that the butterfly sniffs out its favorite foods and potential mate. The antennae can detect pheromones, which are specific chemical signals released by the opposite sex that are detectable over a great distance.
The thorax of a butterfly is divided into three segments, each having one pair of legs. Each leg ends in a claw, enabling the butterfly to hold on while feeding and egg laying. Sensory receptors on the leg just above the claw detect the chemical makeup of appropriate food plants.
The wings are a spectacular part of the butterfly, and are extremely large compared to its body. The wings attach at the two rear segments of the thorax. Tough veins provide a framework which supports the wing membrane, which is covered by millions of microscopic scales arranged in rows like shingles on a roof. It is these scales (about 99,000 per sq in [15,000 per sq cm]) which form the color and pattern of the wing. Brown, orange, and black tones are due to the presence of chemical pigments. However, the brilliant, iridescent blues and greens are parts of the light spectrum separated by tiny facets on the surface of the scales, creating colors in the fashion of a prism. The wing colors provide camouflage against predators and aid in identifying mates of the correct species. Scales also cover the segmented abdomen, in which the digestive and reproductive organs are located.