San Francisco seems to have fewer insect-watchers than bird-watchers, but we’re trying to post what we encounter in Sutro Forest. (This page isn’t strictly for insects; it also includes arthropods and other invertebrates.)
Butterflies: We’ve seen Red Admirals, West Coast Painted Ladies, and Cabbage Whites. And we got this lovely Anise Swallowtail picture taken in March 2010 by Keith McCallister (posted with permission).
A Pumpkin Spider. Pumpkin spiders are interesting. Most of the year, they’re tiny and inconspicuous, but in the late fall they increase in size to these dramatic orange creatures, and build large webs. The picture below is the Halloween version!
There are various other insects and invertebrates, including a banana slug.