The sunflower moth,Homoeosoma electellum(Hulst), has a distinct eclosion rhythm in either a light–dark constant temperature photoperiod, or in constant darkness, or in constant light with alternating temperatures. Eclosionis arrhythmic in either constant light or constant darkness with constant temperatures. With a 12:12 light–dark, constant-temperature regimen, there is a broad eclosion peak occurring over ≍6 h before lights turn off as well as over the first part of the dark phase. Some individuals eclose during all hours of the day regardless of the regimen. With a regimen of 12 h each at 26° and 17°C. (both in constant light or darkness or light–dark with the low temperature during the dark), a bimodal periodicity of eclosion is initiated with a high peak immediately preceding the temperature drop and a low peak shortly after the temperature rise. The rhythm damps out rather quickly in constant darkness at constant temperatures.