cell sensitive to longer-wavelength light. The combination of filtered and unfiltered green opsins permits the butterflies to distinguish yellow from orange colors. The ultraviolet opsin protein has also been detected in the dorsal rim region of monarch eyes. One study suggests that this allows the butterflies the ability to detect ultraviolet polarized skylight to orient themselves with the sun for their long migratory flight. These butterflies are capable of distinguishing colors based on their wavelength only, and not based on intensity; this phenomenon is termed "true color vision". This is important for many butterfly behaviors, including seeking nectar for nourishment, choosing a mate, and finding milkweed on which to lay eggs. One study found that floral color is more easily recognized at a distance by butterflies searching for nectar than floral shape. This may be because flowers have highly contrasting colors to the green background of a vegetative landscape. On the other hand, leaf shape is important for oviposition so that the butterflies can ensure their eggs are being laid on milkweed. Beyond the perception of color, the ability to remember certain colors is essential in the life of monarch butterflies. These insects can easily learn to associate color, and to a lesser extent, shape, with sugary food rewards. When searching for nectar, color is the first cue that draws the insect's attention toward a potential food source, and shape is a secondary characteristic that promotes the process. When searching for a place to lay its eggs, the roles of color and shape are switched. Also, a difference may exist between male and female butterflies from other species in terms of the ability to learn certain colors; however, no differences is noted between the sexes for monarch butterflies. Courtship and mating Monarch courtship occurs in two phases. During the aerial phase, a male pursues and often forces a female to the ground. During the ground phase, the butterflies copulate and remain attached for about 30 to 60 minutes. Only 30% of mating attempts end in copulation, suggesting that females may be able to avoid mating, though some have more success than others. During copulation, a male transfers his spermatophore to a female. Along with sperm, the spermatophore provides a female with nutrition, which aids her in laying eggs. An increase in spermatophore size increases the fecundity of female monarchs. Males that produce larger spermatophores also fertilize more females' eggs. Females