Lepidopterology is the part of Forestry that deals with Butterflies.
The player first encounters Butterflies by breeding Bees near Trees. Forestry Tree leaf blocks (with genetic information) have a very low chance to spawn in a Butterfly.
Additionally, a config option can enable the generation of Cocoons in the world.
When the Lifespan of a mated Butterfly ends it will die and respawn a Cocoon on the underneath of a Leaf block, and some time later the new Butterfly will emerge.
When Butterflies land on the leaf blocks of Forestry Trees they have a chance to take up Pollen carrying the genetic characteristics of that Leaf block or pollinate the leaf block with Pollen they are carrying.
Butterly genetics and mutations
Butterflies, Butterfly Larvae, Cocoons and Serum all carry the genetic traits of Butterflies, can be scanned using the Portable Analyzer, and are combined using Mendelian inheritance rules. Butterflies in the world can be gathered using a Scoop (right-click) and Butterflies that are not "MATED" can be combined in a Crafting Table with Butterfly Serum to produce a mated Butterfly that can be released in the world by simply dropping it.
As with Bees and Trees, the species trait can trigger mutations to introduce completely new Species with entirely new traits.
Butterfly and Caterpillar product
It is not known how Caterpillars and their products are obtained in base Forestry. Right-clicking a Caterpillar on a leaf block will convert the leaf block to a Forestry leaf block if it was not before, and probably cause that tree to start spawning that specific butterfly. Right-clicking a leaf block with a Cocoon will place it somewhere on the same tree.
The Escritoire can be used to perform research on any Butterfly specimens (butterflies, Serums and caterpillars), using Honeycombs to provide clues. The yield will be a Research Note, and a Serum for the Butterfly being researched. Research Notes can be consumed to improve the chance of the Player getting certain Mutations.