Periodical cicada mass emergences are one of those once or twice in a lifetime kind of things.

蝉的周期性大规模出现一生中可能只有一到两次。

The nymphs of the periodical cicada have been biding their time.

布鲁德X蝉（一种周期蝉）的若虫为这个机会等待了很长时间。

To continue the species, he said billions of periodical cicadas will appear all at the same time.

劳普表示，为了种族延续，这种周期性的蝉会大批同时出现。

Well, two different broods or groups of periodical cicadas, which only emerge every 13 or 17 years, are set to surface around the same time.

因为蝉群有两个不同的群体分属不同的周期，它们每 13 或 17 年才会出现一次，今年将同时出现。