Symbolism, Imagery, Allegory
Throughout his childhood, Vladimir develops an obsession with butterflies, and by the time he's an adult, he's a full-scale lepidopterist. In nonfiction, you may be unsure about calling a real thing a symbol, but Nabokov is a clever writer, and he knows what he's doing, prattling on about butterflies while he's telling the story of his family more or less losing everything.
Vladimir is entranced in the Vyra pavilion, listening to Mademoiselle read to him in French:
Coming from nowhere, a Comma butterfly settled on the threshold, basked in the sun with its angular fulvous wings spread, suddenly closed them just to show the tiny initial chalked on their dark underside, and as suddenly darted away. (5.5.3)
Through his trance of listening, Vladimir is learning to see the beauty of the natural world.
By the time Vladimir is seven, his imagination is ruled by the wings and movements of butterflies and moths:
If my first glance of the morning was for the sun, my first thought was for the butterflies it would engender. (6.1.4)
When his father is briefly jailed for writing an unsanctioned criticism of the Russian police, a letter home reads:
Did V. get any 'Egerias' [Speckled Woods] this summer? [...] Tell him that all I see in the prison yard are Brimstones and Cabbage Whites. (9.1.4)
Both of Vladimir's parents support their son's hobby (and have dabbled in it themselves), and speaking of butterflies is a way of reaching out and connecting when more sinister things are afoot. Plus, let's not forget the fact that butterflies are free to fly, ignorant of man-made borders and government rules.
Later, Vladimir tells us of Ustin, the St. Petersburg town house janitor, who, after the family had fled, led the communists to the family's hidden jewels, thus becoming a hero in the communists' eyes. Nabokov adds sarcastically that it must have been "adequate recompense for the Swallowtail he had once caught for me." (9.4.4) Sure, he's joking, but the fact that he made the connection at all signifies that butterflies mean more to him than a hobby.
Finally, let's not forget the key part of a butterfly's life, wherein it turns from a lowly little crawling caterpillar into a great and beautiful butterfly. Metamorphosis! In evolving, a butterfly gains beauty, agency, and freedom, and isn't that all a little boy-turned-adult émigré wants from life?