Eclosions in populations of some species of holometabolous insects only occur during a restricted time each day, often within as little as an hour or two. We have described the daily pattern of eclosions in a desert population of the Pipevine Swallowtail (Battus philenor) so that we might better understand the daily pattern of this species' mating behavior. Our anecdotal impression that these animals eclose only around sunrise was confirmed both in a greenhouse with a natural light cycle and in an incubator with a semi natural 16:8 light:dark cycle. Our experiments indicate that on the day of eclosion, levels of short wavelength light at sunrise reach a point that triggers eclosion and that an internal clock contributes to the pattern of when eclosions occur during the day. Finally, we show that the internal clock is set by the light cycle to which the pupa is exposed during the last few days of the 11 to 12 day pupal stage.