Reproduction in butterflies, as in many holometabolous insects, is usually constrained by the amount of nutrients the animals can collect as juveniles. In polyandric species the females can also supplement their larval–derived reserves with protein–rich donations, so–called nuptial gifts, delivered by the males at mating. Recent findings also indicate that females have access to nitrogen from the histolysis of flight muscles in the thorax. This field study examined how butterflies of the polyandric gift–giving species Pieris napi (Lepidoptera: Pieridae) use body resources in their reproduction and how the male donations affectthe females use of stored reserves. The results support earlier studies, indicating that females use resources from the breakdown of thorax muscles to increase their reproductive potential and the results also indicate that males also use thorax material in their reproduction. The study also supports recent findings that the male donation increasesthe breakdown of body resources and thereby booststhe reproductive outputof the female.