To investigate how seasonal insects respond to changing environments, nymphal skins of the two cicadas Cryptotympana facialis (Walker) and Graptopsaltria nigrofuscata (Motschulsky) (Hemiptera: Cicadidae) were monitored weekly from late July to August over 12 yr in a suburban habitat of central Japan. Based on over 8,000 skins collected from 1995 to 2006, the authors estimated the period during which temperature or precipitation impacted skin abundance and phenology. Adults of C. facialis tended to emerge earlier than those of G. nigrofuscata, for which total skin counts fluctuated up to seven-fold among years. The effective accumulated temperature from the previous 3.0–3.5 mo to the most recent 1.0–1.5 mo at a threshold of approximately >18°C showed the best fit to the cumulative skin counts within a season. Temperature explained 47 and 64% of the total variation in the skin counts for C. facialis and G. nigrofuscata, respectively. The point at which temperature had this effect was consistent between male and female cicadas. Conversely, accumulated precipitation accounted for <16% of the variation in the skin counts for both species. In summary, this long-term study revealed that late-spring temperature plays a key role in predicting the molting phenology of C. facialis and G. nigrofuscata but does not necessarily explain a large amount of the abundance fluctuation.