Today, while moving some furniture on our balcony, which overlooks lovely Lake Merritt, I disturbed several interesting spiders. Both are species that live mostly unseen by humans, except when we move piles of debris or look under tree bark or in similar hideaways.
Distinguishing spiders ain’t particularly easy, at least below the level of their genus, and sometimes not even to that level. Spiders that inhabit similar habitats often look similar, just as bumble bees of different species evolve to look alike when they live in the same area.
One of these spiders, a fat, gravid female, might be one of our 22 species of Callobius, a so-called “hacklemesh weaver” because of the type of web these spiders make. But she could also be one of our 23 species of Amaurobius. As one of my spider books puts it, “The differentiation of these genera often requires examining their reproductive structures.” That requires dissection, a microscope and expertise that I don’t have.
The second spider seems like it could be in the genus Sergiolus — they have the distinctive patterned abdomens like this one. But none of the five known Sergiolus species on the Pacific Coast have this exact pattern. Another problem. But often among spiders there are tiny local populations that don’t look exactly like others of their species, or like the pictures in spider field guides! Spiders in this genus are ground hunters, and this spider’s eye pattern seems to match theirs — lots of forward eyes for spotting prey.
But this spider could also be another ground hunting species, or even a male Steatoda grossa, a completely different type of spider known as a “false black widow” (because the females resemble Latrodectus heperus). S. grossa spiders are commonly found in houses in this area.
Steven E. F. Brown 