as if no terrible thunderbolt had fallen among them
The Dales try to continue on with their daily routine as if the “thunderbolt” of the news that Lily and Crosbie’s engagement was ended had never been announced. This is a reference to Zeus’ traditional control over the power of thunder in Classical myth, and is particularly relevant in the context of Trollope since The Jupiter, which is a Roman name for Zeus, is a popular newspaper in the world of his novels. [EB 2006]
wolf into their flock
Bernard feels as though Crosbie was a “wolf” in the “flock” of the Dale family. This phrase is a reference to Aesop’s fable about the wolf in sheep’s clothing, which carries the lesson that appearances can deceive. This is certainly the case with Crosbie, whose seemingly dashing nature hid his faithlessness. [EB 2006]
Sources: Link to the story of The Wolf in Sheep’s Clothing at Laura Gibbs’ Aesop site.