The meaning of Keats' poem "On the Grasshopper and Cricket" and the reason he particularly chooses the grasshopper and the cricket derives from the opening line ("The poetry of earth is never dead") and is mirrored in the ninth line ("The poetry of earth is ceasing never"). Composed in one octave (eight lines) and one sestet (six lines) and having the rhyme scheme abbaabba (octave) cdecde (sestet) without an ending couplet, this poem is structured as a Petrarchan sonnet of fourteen lines. In the Petrarchan sonnet, the ninth line turns the poem to a new subject matter in what is called the sonnet volta; all sonnets require a change of subject matter to usher in the resolution to the problem or idea introduced in the first eight lines.
In this poem the ninth line volta repeats the first line with a variation and turns the subject from summer and the grasshopper to winter and the cricket. In Keats' poetic imagination, imagery and vision, these two small creatures are the voices of nature--which is often commemorated in Romantic period poetry--one speaking in summer when all else is "faint with the hot sun" and quiet "in cooling trees" and the other speaking in winter when "the frost has brought silence" except for "The Cricket's song" from "the stove." Keats' is praising these two small yet insistent voices of nature that are never silent even when all else is and that have the power to make summer seem to spring from frozen winter. In other words, Keats specifically uses the grasshopper and cricket because they are the voices of nature that continue when all other voices in their season are stilled.