Several quite large taxa (groups of organisms) use cyclical parthenogenesis. This is when several generations are born by virgin birth, and then a generation occurs with normal sexual reproduction. Examples include aphids, and cladocerans (small crustacea called "water fleas"). Aphids usually operate as follows: when the weather is good, and their plant hosts are at their best, they use parthenogenesis. At the end of the season, when the weather gets worse, they use sexual reproduction. This system of reproduction is called "apogamy".