London Plane Trees (sycamore) – like the ones on Jefferson Street — are important as habitat for the Western Tiger Swallowtail butterfly (papilio rutulus).
This pretty butterfly, which is native to the western part of North America, lays its eggs on the underside of the leaves. The caterpillars eat the leaves when they hatch, and finally the caterpillars become pupae there, forming a green chrysalis. In summer, they hatch quickly; in the fall, they turn brown like wood, and only hatch the following spring.
Here’s an eye-witness account of a new butterfly from a London Plane Tree on Market Street from Dennis Dooley:
Last June as I was setting up on Market Street under a tree (same species as Jefferson?) in front of Noah’s Bagels, I spotted a butterfly perched motionless on the sidewalk. Not even a whisker twitch. Then a bus pulled up, door opened, and a Muni driver stepped down. I had to warn him not to step on the creature. We both marveled at its delicate beauty, vibrant color. The driver closed the door and went for coffee, as this was his rest stop. Several passers by stopped to examine the wonder, and I warned them not to touch. After a half hour or so, patches of sun began to appear on the bricks, warming the sleeping beauty. I noticed an antenna twitch; it was not dead. I turned to finish setting up, and when I felt a full blast of sun on my face, I looked down to see the butterfly gone, having flapped away to its destiny (hopefully).
Liam O’Brien, lepidopterist (sfbutterfly.com) confirmed it was a newly-hatched female Western Tiger Swallowtail butterfly. It needed time for its wings to harden and for it to warm up, a time during which it was very vulnerable. By protecting it during that critical period, the observer gave it a chance to live and maybe produce more Tiger Swallowtails for our city.
The Trees of Jefferson Street, San Francisco 
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