Maintenance of the Polymorphism for melanism in the moth Phigalia pilosaria in rural north Wales

A mark–release–recapture study of male Phigalia pilosaria was performed in a 0.75 km2 area of north Wales in early 1978. No flight was observed at temperatures below 0 °C, but above this temperature there was activity. Of 2372 moths handled, 59% were non-melanic, 3% were intermediate melanic and 38 % were the extreme melanic monacharia. Maximum population density was 3000 km-2 and daily survival rates showed that nonmelanics were at an advantage of up to 20 % compared with monacharia. Most loss from the population was due to death and little emigration took place, with average movement being 200–300 m per night. Selection against the melanic and the remote chance that this form could move in from populations where it is at high frequency indicate that other selective forces must be implicated in the maintenance of the polymorphism. Frequency-dependent and non-visual (physiological) selection may be involved.