This occurs in chapter 25 when Scout and Jem are out on the porch to spend the night. Scout begins to narrate about Jem's chastisement of her getting ready to smash a roly-poly:
He was certainly never cruel to animals, but I had never known his charity to embrace the insect world.
'Why couldn't I mash him?' I asked.
'Because they don't bother you,' Jem answered in the darkness.
Jem makes the same comment about a simple insect that Atticus made much earlier regarding mockingbirds. Mockingbirds don't do anything to harm anybody. This comparison between the two innocent creatures is important at this juncture because the story is transitioning from the innocent Tom Robinson being killed to the re-introduction of another innocent character: Boo Radley. Fortunately for Boo, although the town would like to criticize and hurt him, Scout remembers her lessons and treats him without harm.