Autumn is a delightful time to hike the golden hills of Mount Diablo. But unsuspecting hikers are often startled to find themselves sharing the trails with some rather formidable eight legged walking companions! "What are the critters doing?" "Where are they going?" "Will they hurt me?" 

These are the common questions park staff and park volunteers must answer this time of year. Fall is tarantula time on Mount Diablo. Mount Diablo's tarantulas (Aphonopelma sp.) have long inspired fear and fascination. One 19th century visitor described our local tarantula as "attaining the size of a small bird, possessing fangs the size of a rattlesnake's, and delivering a bite generally considered fatal!" In reality, the tarantula is one of Mount Diablo’s most innocuous animals—a terror to small insects and not much else! Outside of horror movies, no person has ever been killed by a tarantula. Tarantulas have very small venom glands and the bite of our local tarantula is no more painful that a bee sting.

Harmful spider bites generally come from poisonous spiders that are too small to notice. The tarantula, being so conspicuous, gets the blame. For example, in Renaissance Italy, the bite of a tarantula was thought to cause convulsions. The only known treatment was to sweat the poison out by frenetic dancing. This was the origin of the tarantella, the dance named for a spider! In reality, a European black widow, and not the innocent tarantula, was the culprit doing the biting. 

The tarantula's main weapon against larger creatures is defensive. If a bobcat or fox is harassing it, the spider rises up on its front legs and with its back legs scrapes off a cloud of barbed, porcupine-like hairs from its abdomen into the face of its tormentor. This tactic sometimes gives the tarantula time to escape. It also gives rise to another common visitor query, "Why does that tarantula have a bald butt?" 

Although most commonly seen wandering the roads and trails in late summer and early fall, tarantulas are on the mountain all year. They are seldom seen at other times because they live in underground burrows and are nocturnal in their habits. Typically, a hunting tarantula waits patiently near the opening to its burrow until an unsuspecting insect (usually a cricket) crawls by. The spider rushes out, bites the prey, and drags its victim back into the burrow. In the dark of night, this activity goes unnoticed (except by the cricket!). This secretive existence ends in late summer, when male tarantulas that have reached about 7 years of age shed their exoskeleton for the last time. They have finished growing. The mature spiders leave their burrows and begin to search for female tarantulas. 

It is this horde of love struck males, searching for females with which to mate, that forms the annual "tarantula migration" park visitors witness each year in September and October. Male and female tarantulas are difficult to tell apart until the last molt, when the male spider develops little stirrups on its front legs. Why does the tarantula need these strange appendages? When the male finally locates a female tarantula and entices her out of her burrow, her thoughts are not on love but on dinner. 

In order to safely mate, the male spider must hook and secure the female’s fangs using the stirrups on his front legs. After mating, the male scurries away, and the female usually allows him to leave. It is a myth that female tarantulas always kill the males after mating. A female will consume the male only if she is famished and needs a meal to be able to lay eggs. Otherwise, she allows her paramour to retreat in safety. Though free to live another day, the roving male spider never returns to his burrow. Rather, he wanders around searching for other females until he finally dies with the onset of cold weather. 

The stay-at-home mother tarantula, by contrast, may live to the ripe old age of 20 years or more. After mating, the female retreats to her burrow and lays about 100 eggs on top a sheet of silk spun from her spinnerets. Momma spider shapes the silken sheet into a basket and guards the eggs inside until they hatch. Soon after hatching, the tiny spiderlings crawl out and leave the burrow. Of the hundred or so eggs laid, perhaps one or two spiders will survive to adulthood. It's not easy being a tarantula. 

In fall, pesky yellow jacket wasps are the bane of Mount Diablo picnickers. The picnickers should consider themselves lucky, for the wandering tarantula must contend with a far more formidable flying foe. The spider's antagonist is a large black and orange wasp, known as a tarantula hawk (Pepsis sp.) The female wasp flies around searching for a tarantula. When she locates one, the wasp attacks and stings the spider under a leg. This does not kill the tarantula, but it does paralyze him. The wasp drags the spider off, scraps out a hole, and pushes him in. Before she covers the tarantula, the wasp lays a single egg on the helpless spider. When the egg hatches, the wasp larva dines on fresh tarantula meat! 

For all their fearsome reputation, Mount Diablo’s tarantulas are really gentle souls that play an important part in the web of life on our island mountain. So the next time you encounter a tarantula on the trail, remember the old adage, "if you wish to live and thrive, let a spider run alive!"