Acmon Blue (Plebejus acmon) (?) on Yellow Star-thistle (Centaurea solstitialis). Irene Brown says: "Looks like female Acmon blue to me--- given place and timing ---and what I can see of forewing. However, cannot completely rule out female Lupine blue. Don't have Jerome's bfly list here, would be good to check it, although it is not comprehensive. Tell Dan nice shot!" The Yellow Star-thistle is an invasive exotic, introduced after 1848 as a contaminant with alfalfa seeds. Contains a poison that can sicken and kill grazing livestock. Shapiro & Manolis, "Field Guide to Butterflies of SF Bay..." claims Acmon Blue as regularly present throughout the Bay Area; Lupine Blue (P. lupini) only in old, vague, or suspect record, and then only in Santa Clara & Santa Cruz Counties, not San Mateo County. This book also says "Butterflies are members of the order Lepidoptera that belong to the superfamilies Papilionoidea [Swallowtails & Parnassians] & Hesperioidea [Skippers]. All other members of the order Lepidoptera are called moths by default." -- then goes on to refer to the family Lycaenidae, which contains Blues, this way: "This is the largest family of butterflies, with over 6,000 species worldwide -- and counting." So I guess it is OK to call this an Acmon Blue Butterfly, even though it is not in the families Papilionidae or Hesperiidae. (The book is somewhat inconsistent re family/superfamily.)