As I understand it, caterpillars are usually ugly, while butterflies always have beautiful wings, so caterpillars must be male and butterflies female. At the same time, Vietnamese also has the same problem: "Swimming" and "sailing" are the same thing. There is a song like this : I go to swim/play the boat (Em đi bơi/chơi thuyền).

>>1
To be even more precise, the usual Russian word for caterpillar is the female гусеница (gusenitsa). But the word used in this film is the male червячок (chervyachok), which means "worm" - usually "earthworm", but potentially a caterpillar too (I have the impression that in the past it was used like that more often than now - another example is Tsehanovskiy's 1929 cartoon "Post").

Seems like maybe Suteyev specifically chose the word that would cause the more transformational character change.

As for the original story, it had a beetle instead of a caterpillar.