UC Davis Natural Reserve System
Stebbins Cold Canyon Reserve

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The diversity of arthropods (Phylum Arthropoda) worldwide can only be guessed, but it is known to be enormous. The importance of arthropods in ecosystems is, in many cases, not well understood; however, ecologists believe that they often play pivotal roles in ecosystem processes, such as nutrient recycling. Although arthropods are quite diverse in appearance, ranging from lobsters to millipedes to spiders to butterflies, they all have certain characteristics in common. For example, "arthropod" means "jointed foot," which hints at their jointed appendages. Also, all arthropods have tagmata, which are specialized body segments, such as the head, thorax, and abdomen in insects.

Chelicerates : Arachnids 

The class Arachnida, which includes the spiders (order Araneae), harvestmen (order Opiliones), mites and ticks (order Acari), scorpions (order Scorpiones), pseudoscorpions (order Pseudoscorpiones), and wind-scorpions (order Solifugae), has long attracted the attention of naturalists for its amazing diversity in morphology, ecology, and life history. Arachnids have conquered most of the earth's terrestrial and marine environments and play major roles as predators, herbivores, parasites, and decomposers in ecosystems worldwide. With more than 108 families and 40,000 species, spiders are the second largest order of arachnids and exhibit some of the most interesting biology known among invertebrate animals. Only in the Insecta can another assemblage of such diverse organisms be found.

Arachnids can be differentiated from other small, hard-bodied creatures in the reserve by their numbers of legs and antennae. Arachnids have four pairs of walking legs and no antennae, whereas insects have three pairs of walking legs and one pair of antennae. Many arachnids also have the startling characteristic of multiple pairs of eyes, which may be one reason for their success as predators.

Because of its numerous microhabitat types, Cold Canyon provides an excellent representation of Californian arachnid diversity. Most of the major orders occur in the reserve, including spiders, scorpions, mites and ticks, harvestmen, pseudoscorpions, and wind-scorpions. A current list of recorded families is included, as well as a table associating them to microhabitat type (Table **). To date only the spiders have been surveyed, leaving significant gaps in the understanding of what should prove to be a remarkable fauna. Arachnid researchers are encouraged to expand on this work through additional surveys or taxonomic work.

Mites and ticks (Order Acari)

Mites and ticks are both small, flattened, hard-bodied animals that occur in multiple habitats on the reserve. Mites are minute, almost microscopic, creatures that may either feed on plants, other insects, mammals, or birds, depending on the species. They are ubiquitous and abundant, and can often be spotted by looking for red dots on plants in the reserve. Ticks, on the other hand, are not as common as mites, but because they have the unpopular habit of feeding on blood, they attract more attention. After the adult tick has fed on enough blood to nourish her eggs, she drops off her host. Then her offspring work their way up a leaf of grass and wait to hitch a ride on the next passerby. Because the bite of a tick is not detectable, it is important to carefully examine your skin for ticks after hiking anywhere in California.

Scorpions (Order Scorpiones)

Scorpions occur throughout the Coast Range and are an exciting if somewhat startling sight. They have stout bodies, large pincers, and a tail that curves over its body to deliver a poisonous sting to its prey. Their stings, though painful, are not toxic enough to be life-threatening to human beings. Scorpions usually lurk in dark, narrow places, so they should be sought under logs or rocks.

Spiders (Order Araneae)

With 20 of the 108 recorded spider families, Cold Canyon, despite its relatively small size, is home to an exciting amount of worldwide spider diversity. Furthermore, California as a whole is a center of spider diversity and endemism with 55 families represented and over 1,500 species. Cold Canyon provides a good representation of California's interior flora, with oak woodland, riparian, and chaparral habitats all within short hiking distance. Many of the spider families discussed below are restricted to these microhabitats in the reserve which greatly improves the possibility of reliable observation and identification. In general, the descriptions that follow emphasize web construction because webs are often the most straightforward means of identifying spiders. For a list of spider families and habitats, look at checklist.

Funnel-web spiders (Family Agelenidae). These spiders construct funnel-shaped webs leading into a shallow retreat near the ground or into dense vegetation. The funnels of grass spiders (Agelenopsis spp.) are found along the trail during spring, especially in tall, grassy vegetation. Grass spiders tend to be yellowish to brownish in color and have eight eyes. Hololena is among the most common spiders in the reserve and occurs in virtually every habitat although it is less abundant in riparian areas. In addition to the typical funnel, Hololena constructs an adjoining sheet that serves to signal the spider of approaching prey.

Orb-weaving spiders (Family Araneidae). Anyone who remembers Charlotte from childhood will instantly know the classic shape of an orb-weaver's web. The two genera with the largest body sizes in this family (0.8 to 1.2 in., or 2-3 cm, body length) in the reserve are Araneus and Argiope. Both genera consist of species that tend to be yellow to brown, but members of the genus Argiope are covered with silvery hairs. These spiders are abundant in the riparian areas of the reserve and are generally observed in their webs at dawn or dusk. In late summer their young are responsible for gossamer-the tangled strings of web often seen floating through the air. Gossamer results from ballooning juvenile spiders that abandon their mode of transport upon finding suitable new habitat to colonize.

Ground spiders (Family Gnaphosidae). Ground spiders are among the most diverse spider families in California. They can be found in virtually any microhabitat on the ground-in rotting logs, under stones, or in leaf litter. Most members of this family are dark and uniformly colored, not mottled, like many of the other spiders covered in this chapter. Sergiolus montanus is found under stones in the reserve, especially in shaded areas. It has a contrasting black and white pattern on the abdomen and a distinctive "jerky" motion when disturbed. Micaria spp. is found in the drier areas of the reserve and a constriction in its abdomen causes it to be an exceptional ant mimic.

Wolf spiders (Family Lycosidae). Wolf spiders are nocturnal predators and are notable for their pairs of eyes of unequal sizes. They are best observed at night (not on the reserve!) with a headlamp because their eyes reflect light and produce a distinctive iridescence. In most genera, the female carries the egg sac, and when the young emerge, they hitchhike on her abdomen for a considerable length of time. Two genera of wolf spiders occur in the reserve and are identified by size differences. The smaller (0.4-0.8 in., or 1 to 2 cm, body length) and very abundant thin-legged wolf spiders (Pardosa spp.) are common in riparian areas where they can be seen running along rock surfaces searching for prey. They acquired their name because of their relatively long, thin legs. The much larger and generally uncommon Schizocosa spp. is most likely to be found among leaf litter in moist areas, though it may be difficult to spot due to its excellent mottled camouflage.

Jumping spiders (Family Salticidae). Jumping spiders have the best vision of all spiders and can be observed in the open, slowly stalking their prey and suddenly pouncing. Their brightly colored appearance and complex behavior have captured the hearts of many would-be arachnophobes. Habronattus americanus is arguably the most beautiful of California's spiders and can be found in exposed sunny areas. Only the males of this species exhibit the wonderful contrast of red, white, and iridescent blue colors which have made this species famous. The larger Phidippus audax is the most abundant jumping spider in the reserve and is easily recognized by the white spots on its abdomen, its iridescent blue-green chelicerae, or biting mouthparts, and its extreme hairiness.

Long-jawed orb weavers (Family Tetragnathidae). Tetragnathid spiders are easily located by the small, horizontal (not vertical as in Araneidae) orb webs that are constructed over shallow streams or pools. When disturbed, these spiders align their legs vertically in the center of their webs, effectively mimicking twigs or other vegetation.

Comb-footed spiders and widow spiders (Family Theridiidae). Theridiid spiders are a large and disparate family of spiders much maligned due to the poisonous widow spiders (Latrodectus mactans in California). While L. mactans does occur in the reserve, Theridion spp. and Steatoda spp. are more commonly encountered. Theridion spp. is a very small spider (less than 0.2 in., or 5 mm, body length) of variable coloration which builds irregular webs on vegetation and in crevices. Steatoda very closely resembles the black widow although it has no ventral hourglass marking and does have a diagnostic white U on the back of the abdomen. Both Steatoda spp. and black widows spin irregular webs and live in dark crevices, so it is critical to pay attention to detail when trying to distinguish between these two genera!

Crab spiders (Family Thomisidae). Crab spiders, which do indeed look like miniature crabs, are common throughout the reserve but are most abundant among trailside vegetation and riparian areas. They are sit-and-wait predators that build no webs. Coriarachne spp., which sports amazing bark-like camouflage, is a spectacular sight. The camouflage is so effective that the inexperienced or impatient eye will often not see this spider. Coriarachne spp. is common on the bark of oak trees, especially at the edges of riparian areas.

Credits: All pictures were obtained from Calphotos (http://galaxy.cs.berkeley.edu/photos/fauna/).

Last Updated 03/09/06