Mount Diablo abounds with insect and arachnid life. One stunning example is the dragonfly Sympetrum corruptum, or the variegated meadowhawk. Dragonflies are ancient creatures. They existed even before the oldest rocks on Mount Diablo formed.
An open-eyed hiker can spot lots of well-camouflaged insects, like this European mantis (Mantis religiosa). Although they blend in extremely well, these mantids aren't natives. They hitched a ride from Europe to North America in 1899 on a shipment of nursery plants. Mantids have extremely good eyesight, which helps them detect movement of prey. They have a fully articulated head which they can pivot and swivel 180 degrees. They use their antennae for smell. These defenses help the mantis avoid predators like the tarantula, which would consider it a tasty morsel.
Mantids don't develop wings until their final molt, and some mantids never grow wings at all, or may have small flightless wings. The only time mantids fly is when the adult female begins to emit pheromones to attract males for mating. Contrary to popular belief, not all males become meals for females. Male mantids fly at night, as they seem to be attracted to artificial lights.
These silky globs are a common sight on summer trails, the homes of funnel web spiders. These arachnids add to their webs all year. By late summer the webs are really something to behold. If you peer into a funnel, you might find a timid (and completely harmless) spider hiding at the bottom.
The black widow (Latrodectus hersperus) is the only spider on Mount Diablo that is poisonous to humans. Its famously messy web is made of the strongest known spider silk. During World War II the Americans used the threads of these webs in their telescopic gun sights. The black widow bite is not fatal to healthy people, but medical attention should be sought. Black widows are not aggressive and will only bite if touched. The widow's larger cousin, the tarantula, is comparatively tame when it comes to people.