In William Wordsworth’s poem “To a Butterfly,” the butterfly seems to exist as both a literal creature the speaker is observing and as a metaphor for an idyllic childhood. In examining his chosen words, one can see that the speaker is likely thinking of his past and looking back to a more carefree time. Note the fourth line of the third stanza:

Historian of my infancy!

The above is a way of expressing that, in the butterfly, the poet sees his own history recorded and preserved.

To go back to the beginning of the poem now, the words in the first stanza speak to the butterfly’s appearing to be completely still:

How motionless!—not frozen seas
More motionless!

Here, Wordsworth seems to be thinking in a wishful manner—he is wishing that the butterfly, which, to him, represents the simple joys of childhood, would remain there for him.

Then, consider the lines:

What joy awaits you, when the breeze
Hath found you out among the trees

He promises that an adventure waits for the butterfly once it is in motion again.

In the second stanza are the lines:

Come often to us, fear no wrong;
Sit near us on the bough! We’ll talk of sunshine and of song

With these words, the speaker is indicating that he would like to be visited by these memories more often and that he welcomes them to come whenever possible.

However, there is a slightly less light-hearted tone, in the second half of the poem, especially. Look at the second part of the third stanza for an example of this. In the first part of this stanza, the speaker is requesting that the butterfly stay with him longer, and then this line appears:

Dead times revive in thee

Followed by:

A solemn image to my heart,
My father’s family!

These words he has chosen—“dead,” “solemn,” and “My father’s family”—indicate the sense of loss he feels as a part of experiencing his more joyful days. He is perhaps saying that the loss and the joys go together.

In the last stanza, the tone changes slightly again. The poet describes himself as a bold “hunter,” while the words he has chosen to describe his sister make her seem more hesitant and more worried about hurting the butterfly:

But she, God love her, feared to brush
The dust from off its wings.

For more about this poem, see the section of analysis on this site devoted to it, linked below.

Why is the poet entranced by the butterfly?

The speaker in this poem is so entranced by the butterfly that he has been watching it "a full half-hour" when the poem opens. He seems, at first, to be particularly intrigued by the mysteries of the butterfly, repeating the word "motionless!" in an exclamation, curious as to whether the still creature sleeps or feeds.

The poet addresses the butterfly directly in this poem, exhorting it to "stay near me." He describes the butterfly as a "historian of my infancy," suggesting that it reminds him of summer days when he, as a child, saw similar butterflies. In the final stanza of the poem, the speaker confirms this to be true: he repeats the word "pleasant" to emphasize his feelings about earlier days with his sister Emmeline, when the two had chased butterflies. As a child, the poet was a "hunter" in his pursuit of the butterfly, which had enthralled him even then, although his sister was less boisterous in her approach to the creature.

The poet in the modern day, then, is entranced by this creature for dual reasons. On the one hand, he is reminded by the butterfly of happier days with his sister in his youth and how he felt compelled to follow the butterfly then. He is also intrigued by the butterfly because, as an adult, he is curious as to all the things he does not understand about it, such as how, or whether, it feeds or sleeps, and where it goes when it flies out of sight.