To be as silent as a cicada in late autumn; describes being too scared to speak up or say a word.

to remain silent to avoid trouble, like a horse at a ceremony or a cicada in winter

Cicadas in winter and horses in a royal guard; used to describe someone who keeps their mouth shut and dares not speak for fear of getting into trouble

The chilling effect of a climate of fear or legal pressure in which people are afraid to speak their minds.