Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley RegionsButterflies are such a common sight that we can be excused for thinking we understand these beautiful insects. They fly around on big floppy wings, visit flowers, and their larvae are called caterpillars. It’s all pretty simple, right?

The truth is that the lives of butterflies are far more interesting and complex than this. Even our popular notion of what separates butterflies from moths breaks down upon closer inspection. One lepidopterist recently commented that butterflies are simply colorful day-flying moths.

We also tend to think that butterflies fly around when their favorite flowers are in bloom, yet there are many exceptions to this rule. The West Coast Lady (Vanessa annabella), Red Admiral (Vanessa atalanta), and other species can be observed all winter long. And the Pipevine Swallowtail (Battus philenor) is active during the scorching heat of summer when its host plant, the California pipevine, has long ceased flowering.

California’s diverse landscapes provide excellent opportunities to study butterflies and learn about their lives. Whether you visit fog-enshrouded coastal chaparral or fields of alpine flowers in the Sierra Nevada, you will find many species of butterflies. And once you find butterflies you can start hunting for caterpillars – look for them chewing on just about any plant, even scraggly weeds in a vacant city lot.

It is surprising, however, that the butterflies of our populous and well-studied state are so poorly known. Very few of California’s unique botanical and geologic sites have been surveyed for their butterfly fauna, and of those sites that have been surveyed many have revealed new subspecies or new records. Much remains to be learned.

Consider the Pipevine Swallowtail. New genetic studies have revealed that this large, glittering blue-black butterfly is a recent (post-Pleistocene) arrival from the tropics. It has the strangest life cycle of any California butterfly because its early spring larvae either pupate until the following spring, hatch immediately into adults, or wait to hatch in late summer. Adults may fly during the summer but they will die without laying eggs unless they find pipevines that are actively resprouting after being burned or cut back – talk about an obscure niche!

Even the brilliant colors that we see on butterflies hide deeper secrets. It is now known that these colors mainly serve to communicate species status and sex over a distance. For instance, the glittering orange tips on the wings of the Sara Orange-tip (Anthocharis sara) glow different colors in the ultraviolet spectrum depending on whether the butterfly is male (purple) or female (orange), yet both appear as orange to the human observer. And in a different example, it is now known that the whites (Pieridae) hold their reflective wings partly open so that the angle of their wings reflects warming sunlight onto their cold bodies.

After decades of surveys it is becoming clear that many butterfly populations are responding to human-induced changes in California, and that observations by amateur naturalists play a critical role in understanding these changes. Formerly common species such as the funereal Mourning Cloak (Nymphalis antiopa) and stately Lorquin’s Admiral (Limenitis lorquini) have become notably scarce, while other species hover on the edge of extinction or local extinction. There is no better time to wander afield to appreciate the beauty and mystery of these delightful insects, and to contribute your observations to our scant body of knowledge.

David Lukas, co-author of Sierra Nevada Natural History