Here come real stars to fill the upper skies,
And here on earth come emulating flies,
That though they never equal stars in size,
(And they were never really stars at heart)
Achieve at times a very star-like start.
Only, of course, they can't sustain the part.
Nothing Gold Can Stay 
 People inevitably yield to time, to age, and to reality. Consequently, they tend to lose their initial fascination with fireflies. 
 However, the fascination lives on in the next generation of children, in the next  “star-like start.” 
 
 Frost, in other words, may be using these “emulating flies” as symbols of human lives, human intellects, or human artists, who must inevitably be humbled in their attempts to “equal” the brilliance and power of the stars.
Fireflies in the Garden