Abstract
The life cycles of aphids are among the most remarkable of any animal group. They include parthenogenetic and sexual generations, elaborate polyphen isms, and obligate shifting between unrelated host-plant taxa. These and other unusual life-cycle traits occur in a variety of combinations among the approx imately 4000 extant species within the Aphidoidea (46). Although the complexity and the diversity found in aphid life cycles are often daunting, the study of these insects repeatedly draws both entomologists and evolutionary biologists, for several reasons. First, a few aphid species are agricultural pests, and studies of life cycles can be essential to effective control measures (e.g. 71). Second, aphids, especially their life cycles, are intrinsically fascinating. Finally, aphids are good study organisms for addressing outstanding problems in evolutionary biology. For example, the partially clonal reproductive mode of aphids is useful for testing hypotheses for advantages of sexuality (18, 21, 36, 115, 124, 186) and also facilitates estimation of the genotypic component of fitness traits (e.g. 54, 102, 124, 129, 131, 148, 166, 167, 176-179). The occurrence of several other unusual phenomena, including extensive polyphenism (129), complex life cycles with seasonal alternation between two disjunct sets of hosts (29, 82, 89, 90, 109, 126, 128), soldier castes (3-5, 86), and sex-ratio control (130, 190) have
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