When and how do 17-year periodical cicada nymphs decide to emerge? A field test of the 4-year-gate hypothesis.

The 17-year cicadas (Magicicada spp.) are renowned for the longest, strictly regulated juvenile period in insects, yet how they control their life cycle is unknown. Here, we test our hypothesis of adult emergence decision points at developmental gates of 4n years based on critical body weight (CBW). We studied growth and gene expression in 11-16-year-old last instar nymphs of two Magicicada species at multiple locations in autumn. The decision to emerge was reliably indicated by eye-colour change from white to red. We found that almost all 16-year-old nymphs were red-eyed (thus decided to emerge) with large body weights probably exceeding critical body weight, and a small but appreciable proportion of autumn 12-year-old nymphs having larger body weights than white-eyed nymphs similarly made the decision to emerge. However, nymphs of other ages were not red-eyed even if they had large body weights, except for very few cases. Red-eyed nymphs showed elevated expression for genes involved in response to external stimuli, especially light, and for genes facilitating adult morphological development, but genes for adult metamorphosis and moulting were expressed only after overwintering, at 17 years old. Thus, 17-year cicadas likely make the decision to emerge principally at 4-year gates, if they have achieved critical body weight as hypothesized.