Insects, spiders and other arthropods form the great majority of animal species on Earth. The number of described species of arthropods worldwide is almost 900,000 (out of just over a million animal species), and there are thought to be 5 million to 50 million yet to be described! Arthropods are integral to many ecosystem functions, such as pollination by bees, butterflies, flies and beetles. breakdown and recycling of organic matter by spiders, mites, and insect larvae, and predation on herbivorous insects by bugs, beetles, dragonflies, wasps and flies. Insects also are a key food item for many birds, mammals, reptiles, and amphibians54.

Honey Bee; Photo courtesy of Joyce Gross © 2002 Joyce GrossAn ongoing study at Quail Ridge and other nearby localities highlights the ecological role of arthropods. Bees are important pollinators not only of wildland flowers that produce habitat and food for other wildlife, but also of commercial crops. Recently there has been concern about declines in populations of the European honeybee and other important crop pollinators. But a survey of farms and natural habitats along the western edge of the Central Valley shows that wildlands, such as Quail Ridge, support a rich diversity of native bees that provide pollination to bee-dependent crops such as tomatoes and melons on nearby farms46.

Despite the importance of arthropods, we do not have a full list of the species for Quail Ridge or for virtually anywhere else on Earth. Many groups of species have not yet been fully classified, and even most nature reserves have not been thoroughly surveyed. However, Professor Lynn Kimsey and her Entomology 107 class made a good start in 1997, 1999, and 2001 by compiling a list of the insect families they collected at Quail Ridge (Insect List). With 132 families, it falls short of the 221 recorded at Stebbins Cold Canyon, probably because there are fewer streamside habitats at Quail Ridge, but also because there has not been as much collecting yet.Anise Swallowtail; Photo courtesy of Gerald and Buff Corsi © California Academy of Sciences

How many arthropod species might there be at Quail Ridge? We can get an estimate by considering the survey of butterflies and moths (order Lepidoptera) conducted at Quail Ridge by Greg Kareofelas and Bill Patterson. In over ten years of netting the day-flying species and capturing the nocturnal ones by blacklighting (where moths and other night-flying insects come to rest on a white sheet hung next to an ultraviolet light), these dedicated lepidopterists found just over 500 species (Moth List, Butterfly List). Since Lepidoptera typically comprise about 15% of all insect species in a locality or region41, we can predict there to be about 3,300 insect species at Quail Ridge. Add another 400 mites, spiders, and other arthropods, and we get a total of nearly 4,000 arthropod species – nearly 80 times the number of mammal species thought to occur at the Reserve.

In this section we will briefly describe the major kinds of arthropods likely to be found at Quail Ridge, emphasizing insects since they are the largest and best-studied group. We conclude with some suggestions on how to go and see interesting insects at the Reserve.

What are Arthropods?

Arthropods are invertebrate animals with a hard chitinous exoskeleton, jointed appendages, and a body that is composed of segments. These segments are themselves grouped into major body regions (tagmata), for example the head, thorax, and abdomen of insects. All arthropods undergo periodic molting to grow out of the confines of their rigid exoskeleton. There are four major groups of arthropods:

Spiders, mites, and relatives (chelicerates) have two tagmata, usually 4 pairs of legs, and no antennae.

Sow bugs, pill bugs, and their aquatic relatives (crustaceans) have many legs and two pairs of antennae.

Centipedes and millipedes (myriapods) have two tagmata, many legs, and one pair of antennae.

(hexapods) have three tagmata, three pairs of legs, one pair of antennae, and usually one or two pairs of wings.

There are also other less conspicuous invertebrates at Quail Ridge, such as earthworms (annelids) and snails and slugs (molluscs). See The Natural History of Stebbins Cold Canyon Reserve45 for more about these non-arthropod invertebrates.

More About Insects

Insects are easily the most diverse and abundant group of arthropods. They are the only invertebrates to evolve wings, which perhaps helps explain why they are so successful. Also, most insects have complex metamorphosis that allows their immature and adult life stages to occupy different habitats and use different resources. The most advanced insects, including beetles, wasps, butterflies, and flies, have a “holometabolous” life cycle with four stages (egg, larva, pupa, and adult). During the pupal stage a remarkable transformation occurs as the maggot or grub-like larva becomes a winged adult. Other insects have a “hemimetabolous” life cycle with three life stages (egg, nymph, and adult); the nymphal stage may have several substages (instars), each one a larger version of the one before.

Holometabolous Development; Courtesy of Hickman et al. 2001, see Literature Cited
Example of holometabolous development, with four life stages (egg, larva, pupa, adult), as seen in endopterygote insects.

Insects in strongly seasonal environments may produce one or several generations every year, but typically become dormant during the unfavorable season. In California’s Mediterranean climate, winter and spring are the time of feeding, growth, and mating for most species, followed by dormancy in summer and fall. Depending on the species, the dormant stage may be the egg, larva, pupa, or adult. A few species, such as the tortoise-shell butterfly (Nymphalis californica), undergo spectacular migrations to take advantage of spring conditions at higher elevations after summer has begun at lower elevations.

Hemimetabolous Development; Courtesy of Hickman et al. 2001, see Literature Cited
Example of hemimetabolous development, with three life stages (egg, nymph, adult), as occurs in exopterygote insects.

A good reference for learning basic insect identification is the Peterson Field Guide to Insects42; try the pictorial key to orders on the front and back endpapers. Other useful field guides include California Insects49; National Audubon Society Field to Guide to North American Insects and Spiders47; Peterson Field Guide to Western Butterflies; Sierra Nevada Natural History53; and The Natural History of Stebbins Cold Canyon Reserve45. More technical keys to insect families can be found in An Introduction to the Study of Insects41.

Photo Credits: Title, Giant Water Bug (Mike Benard), Honey Bee (Joyce Gross), Swallowtail (Gerald and Buff Corsi), Life stages (Hickman et al. 2001). For more pictures see:

This page last updated: July 5, 2005  

Contact: Dr. Virginia Boucher
John Muir Institute of the Environment
109 The Barn, University of California, Davis, CA 95616
Phone: 530-752-6949; email:

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