Where have all the cicadas gone, long time passing...

Apologies to Pete Seeger for the misquote.

Two days ago suddenly almost all of the cicadas in the trees near us have either vanished or just ceased to "sing" to one another. There's just one or two that occasionally chirrup, but it's now quiet all day. The weather hasn't changed appreciably, it's still very warm, in the low/mid 30's, so that doesn't seem to be the reason. A search through internet articles about cicadas hasn't given me any clues about their "singing" season. Do they know something about the season that we don't?