It may be a symbolic poem during which the grasshopper is a symbol of hot summer and therefore the cricket is of very cold winter.
Every poet has been attracted towards the sweetness of nature. John Keats is no exception to this and finds nature beautiful in all seasons not excluding the hot summer and cold winter.
He says that the earth is always singing. When the birds stop singing within the hot summer finding cool place, we discover the grasshopper singing and flying from hedge. He sings tirelessly and when tired, rests beneath some weed.
When the birds are silent in very cold the world never stops and expresses its pleasure through different beings just like the cricket. It sings the beautiful songs from the stones. It seems to be increasing in warmth every moment and half-asleep person feels it to be a grasshopper’s song coming from grassy hills.