Some plants and animals can reproduce either sexually or asexually. A good example are the aphids, (e.g. greenfly) which multiply parthenogenetically during the height of summer, but turn to sexual reproduction as the weather gets worse. At different times of the year, they can be viviparous (live birth) or oviparous (eggs). During spring and summer, aphids usually produce live young (nymphs) parthenogenetically. These female aphids may or may not have wings. Males only appear in any number at the close of the season. Females then give birth sexually during autumn, laying eggs. Therefore aphids are said to undergo 'cyclical parthenogenesis'.