Final answer:
The speaker is surprised to see a louse on the lady's bonnet because lice are not typically found on clothing or bonnets.
Step-by-step explanation:
The speaker in the text is surprised to see a louse on the lady's bonnet because lice are not typically found on clothing or bonnets. Lice infest hair and lay their eggs at the base of hair fibers, so it is unexpected to find them on a piece of clothing. The presence of lice on the bonnet indicates that the person wearing it has lice in their hair.