You as an entomologist...khaki shorts and shirt, fisherman's vest stuffed with vials and bottles, butterfly net. Pith helmet optional but recommended.

Her as a butterfly...any uniformly coloured pants/shirt/tights combo (or corset and stockings if it's an adult party), big wings made out of posterboard (glue several layers together or reinforce with yardsticks or coat hangers) (or you can buy some inaccurate ones at any Halloween shop), head piece with antennae out of those chenille pipe cleaners from any craft department.