The ability of the lime aphid to produce sexuals (males and females) as an alternative to parthenogenetic females is regulated by a timing mechanism ('interval timer') which restrains the appearance of these morphs early in the year. In this species there are two 'interval timers': one controls the production of sexual females, and the other controls the production of males. Both 'intervals timers' are sensitive to day-length and temperature but they respond in different ways.With the approach of autumn the waning effect of the 'interval timer' inhibiting female production combined with the short day-lengths results in an increasing proportion of the aphids developing into sexual females. The restraining effect of the 'interval timer' results in a gradual transition from parthenogenetic to gamic reproduction over a period of several generations and is still operational in the autumn. However, in this species even relatively long day conditions (17 h) can induce the development of oviparae. This low threshold of response to day-length combined with the short generation time results in the sexual morphs appearing very early in the year. This is of considerable adaptive significance in years when, as frequently happens, the aphids disappear locally before the onset of autumn.