Paleoclimatic Influences in the Evolution of Periodical Cicadas (Insecta: Homoptera: Cicadidae: Magicicada spp.)

Abstract

We present a paleoclimatic model of Magicicada evolution that predicts increased probabilities of survival for broods having longer cycle lengths and periodi- cal emergences. This model assumes a hypothetical probability that summer maxi- mum temperatures in deciduous forest refuges during Pleistocene glacial stades failed to sustain threshold temperatures required for cicada flight and copulation. If hybridization of individuals of differing cycle lengths produced adverse effects on the timing accuracy of the cycle lengths of the offspring, then less frequently hy- bridizing prime-valued cycle lengths would have been characterized by more precise periodicity and higher population densities. If 13-yr and 17-yr life cycle forms are the only prime-valued forms in a sequence of life cycle lengths, the sequence will ulti- mately be reduced to only these life cycle lengths. Hybridization of 13-yr and 17-yr broods can produce new broods temporally re- moved from the parental brood by 4 years. Environmentally triggered 4-yr accelera- tions and decelerations of sympatric, temporally offset broods may have developed due to the selective advantage of high population densities during emergences of con- specifics. This paleoclimatic model predicts the climatically zoned distribution of the two