orange, the veins and margins are black, and two series of small white spots occur in the margins. Monarch forewings also have a few orange spots near their tips. Wing undersides are similar, but the tips of forewings and hindwings are yellow brown instead of tawny orange and the white spots are larger. The shape and color of the wings change at the beginning of the migration and appear redder and more elongated than later migrants. Wings size and shape differ between migratory and nonmigratory monarchs. Monarchs from eastern North America have larger and more angular forewings than those in the western population. In eastern North American populations, overall wing size in the physical dimensions of wings varies. Males tend to have larger wings than females, and are typically heavier than females. Both males and females have similar thoracic dimensions. Female monarchs tended to have thicker wings, which is thought to convey greater tensile strength and reduce the likelihood of being damaged during migration. Additionally, females had lower wing loading than males, which would mean females require less energy to fly. Adults are sexually dimorphic. Males are slightly larger than females and have a black spot on a vein on each hindwing. The spots contain scales that produce pheromones that many Lepidoptera use during courtship. Females are often darker than males and have wider veins on their wings. The ends of the abdomens of males and females differ in shape. The adult's thorax has six legs, but as in all of the Nymphalidae, the forelegs are small and held against the body. The butterfly uses only its middle and hindlegs when walking and clinging. Adults typically live for 2–5 weeks during their breeding season. Larvae growing in high densities are smaller, have lower survival, and weigh less as adults compared with those growing in lower densities. Vision Physiological experiments suggest that monarch butterflies view the world through a tetrachromatic system. Like humans, their retina contain three types of opsin proteins, expressed in distinct photoreceptor cells, each of which absorbs light at a different wavelength. Unlike humans, one of those types of photoreceptor cells corresponds to a wavelength in the ultraviolet range; the other two correspond to blue and green. In addition to these three photoreceptors cells in the main retina, monarch butterfly eyes contain orange filtering pigments that filter the light reaching some green-absorbing opsins, thereby making a fourth photoreceptor