There is here an allusion to the strange and terrible fact, that all life--and particularly the life of man--is maintained only by the destruction of other life.

In order to live we must kill--perhaps only plants, but in any case we must kill.
Wordsworth has several poems on butterflies, but only one of them is really fine.

It is fine, not because it suggests any deep problem, but because with absolute simplicity it pictures the charming difference of character in a little boy and a little girl playing together in the fields.

The poem is addressed to the butterfly.
Stay near me--do not take thy flight! A little longer stay in sight! Much converse do I find in thee, Historian of my infancy! Float near me; do not yet depart! Dead times revive in thee: Thou bring'st, gay creature as thou art! A solemn image to my heart, My father's family.
Oh! pleasant, pleasant were the days, The time, when, in our childish plays, My sister Emmeline and I Together chased the butterfly! A very hunter did I rush Upon the prey: with leaps and springs I followed on from brake to bush; But she, God love her, feared to brush The dust from off its wings.
What we call and what looks like dust on the wings of a butterfly, English children are now taught to know as really beautiful scales or featherlets, but in Wordsworth's time the real structure of the insect was not so well known as now to little people.

Therefore to the boy the coloured matter brushed from the wings would only have seemed so much dust.