Why does Keats specifically use the grasshopper and the cricket for his sonnet "On the Grasshopper and the Cricket"?

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.