That is the Grasshoppers - he takes the lead in summer luxury he has never done -

the word " that " refere to? voice, bird, insect,hedge ?

As said that "And hide in cooling trees, a VOICE will run Form hedge to hedge, about the new mown mead THAT is the grasshopper's" Therefore "that" refers to the voice that is continued by the grasshopper so that the poetry of the earth never stops .