Arthropods 2
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Southern California Arthropods (Mostly) #2: Spider Allies
© W.P. Armstrong 25 February 2021
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Scorpions: Class Arachnida Order Scorpiones (Scorpionida)

Arizona desert scorpion (Hadrurus arizonensis).

  More Images Of Arizona Desert Scorpion  

Anuroctonus phaiodactylus (Burrowing Scorpion)

Pseudoscorpions: Class Arachnida Order Pseudoscorpionida

Pseudoscorpion perfectly preserved in 30 million-year-old
transparent tomb of amber from the Dominican Republic.

A modern-day pseudoscorpion sitting on the head of a U.S. penny.

A modern-day pseudoscorpion from Palomar Mountain in San Diego County.

  Pseudoscorpion From Supersition Mtns, Arizona  

The Insect Order Psocoptera

Bark Lice Belong To The Arthropod Class Insecta Order Psocoptera

A modern-day pseudoscorpion and its favorite food--a bark louse. This tiny cousin of the scorpion lives under the bark flakes of pine trees. "Scholarly" ones that live between the pages of books are called "book scorpions." They feed on book lice and dust mites.

I seldom encounter members of this fascinating insect order. The order Psocoptera contains the booklice and barklice. They are related to the hemipteroid orders including true bugs (Hemiptera) and thrips (Thysanoptera). Psocoptera (Psocids) are often regarded as the most primitive hemipteroids alive today because their mouthparts show the least modification from the primitive mandibulate condition.

Barklice generally live in moist terrestrial environments (in leaf litter, beneath stones, on vegetation, or under the bark of trees) where they forage on algae, lichens, fungi, and various plant products. They may have wings during the adult stage. The barklice and booklice I have observed are only about 2 millimeters or less in length

Many years ago I discovered book lice in my cereal. In fact, the tiny grains on my spoon appeared to be moving. I ate the cereal anyway! I also introduced pseudoscorpions into my cupboard to control the book lice.

Winged Psocoptera Caught In Pitfall Trap (Not A Chalcid Wasp!)

I found this minute insect in my yard. I originally called it a thrip and then changed my ID to chalcid wasp, but was corrected by entomologist James Trager at Bug Guide. I have seen bark lice and book lice, but didn't realize there were psocid species with 4 wings. In fact, I included above image of wingless bark louse and pseudoscorpion from ponderosa pine on Palomar Mountain.

Sun Spiders: Class Arachnida Order Solpugida (Solifugae)

A ferocious appearing sun spider or solpugid in San Diego County. Although nonvenomous, this predator has a pair of massive jaws (chelicerae). Unlike most arthropods, the jaws open and close vertically rather than horizontally. Like a pair of powerful pruning shears, the jaws work independently and literally tear the prey to shreds. The long pedipalps in front of the body are tipped with adhesive organs which are used in climbing smooth surfaces, feeding, drinking and battling. Like spiders, the head and thorax are fused into a cephalothorax. At the top of the head are a pair of closely-spaced eyes. This arthropod has a voracious appetite and is a formidable opponent to other predators of similar size.

A solpugid on Owens Peak north of Palomar College. It is also called a "sun spider" or "camel spider" by troops in Iraq.

Mites & Ticks: Class Arachnida Order Acarina

Velvet Mite Family (Class Arachnida Order Acarina FamilyThrombidiidae)

Red velvet mite (Angelothrombium), possibly A. pandorae. Called "angelito" in Mexico.

Compared with most mites, this is a large species, roughly the size of your little finger nail (6-8 mm long). The adults are bright red with a velvety coating of fine hairs. They typically emerge from the ground after spring rains, in this case after a February rain in the Anza-Borrego Desert of southern California. They remain in the soil most of the year and only spend a few hours above ground, probably to feast on other prey which also emerge in great numbers after spring rains. The larvae are known to be parasitic on grasshoppers, while the adults feed on subterranean termites.

A red spider mite collected on a navel orange 4 January 2010. It is compared in size with the head of an ordinary straight pin (1.5 mm in diameter) and a cuboidal grain of ordinary table salt (NaCl). The width of the mite's body is about 0.6 mm, roughly equivalent to two average grains of table salt placed side-by-side. Spider mites resemble miniature spiders and actually belong the same arthropod class as spiders (Arachnida). They are placed in the subclass Acari along with other mites and ticks. There are 1600 species of mites in the family Tetranychidae, so assigning a genus or species to this image would be purely conjecture on my part. Some of the best known red spider mites belong to the genus Tetranychus. The common red spider mite on citrus trees is Panonychus citri. Like hymenopterans (bees and wasps) and some homopterans (aphids), sex determination depends on whether the egg is fertilized or not. Dipolid (2n) females develop from fertilized eggs and haploid (n) males develop from unfertilized eggs. Unmated females lay parthenogenetic eggs and their offspring are exclusively male, a term called arrhenotokous. Red spider mites are polyphagous and feed on many different species of plants, including vegetables and ornamentals. They suck the contents of leaf cells, leaving minute spots or scars where the epidermal cells have been destroyed. Although individual lesions are very small, commensurate with the small size of mites, infestations of thousands of spider mites can significantly damage the leaves and reduce their photosynthetic ability.

  See Galls On Willow Leaf Caused By Eriophyid Mite   

Predatory Mite Family (Class Arachnida Order Acarina Family Parasitidae)

Predatory mite with "Popeye" forelegs (family Parasitidae) found in Forelius mccooki ant midden in front of my home. Unlike spider mites in your garden and parasitic mites on animals, these beneficial mites feed on pest mites, nematodes and other minute arthropods. See Wayne's Mite Page and Wayne's Face Mites

Record-Breaking Discovery At Walnut Grove Park: Twin Oaks Valley

Whirligig Mite Family (Class Arachnida Order Acarina Family Anystidae)

Wayne's Trivia Note #671 (26 June 2020)

I noticed a very fast little Forelius pruinosus ant running across a patch of bare ground at nearby Walnut Grove Park. Suddenly, a reddish mite ran past the ant like it was standing still. This lilliputian speed demon turned out to be the fastest land animal on Earth based on body lengths per second. Another record for Twin Oaks Valley. The cheetah does 16 body lengths/second (60 mph) while the mite does 322 body lengths/sec. For a human this rate would be about 1300 mph, the speed of a jet plane! New World Land Speed Record!

Two cropped frames from the 10 frames taken in Burst Mode with 16x optical zoom (Sony DSC-HX9V). This large cat runs very fast in short bursts up to 60 mph. Once considered the fastest land animal, they were running in their enclosure in my direction on a hillside at the San Diego Zoo Safari Park.

The specific epithet "macropalpis" of this unusual mite (Paratarsotomus macropalpis) refers to its conspicuous palps that flank its tiny jaws (chelicerae).

On 26 June 2020 I was observing some tiny Forelius pruinosus ants (2 mm in length) running very fast across a dirt clearing in Walnut Grove Park near my home. I thought to myself that it would be interesting to calculate their relative speed in miles per hour based on their small size (body length per second). Suddenly, a tiny reddish mite shot across the clearing like the Forelius were standing still. With great difficulty I attempted to collect a few of the mites by adhesion to my thumb wetted with ethanol. When I returned home I did some Google searching and to my surprise a student at Pitzer College and his adviser at Pomona College already published an article about this remarkable mite in a scientific journal. In fact they concluded that this endemic southern California mite is the fastest land animal on our planet, surpassing the previous record of an Australian tiger beetle! I was very excited about this new discovery right in Twin Oaks Valley and began photographic trials on the mite. My best results came from my trusty old Nikon SLR with macro lens & 3 extension rings (see above images).
Samuel Rubin at Pitzer College, Claremont, CA (LA County), his advisor Professor Jonathan Wright of Pomona College, and their colleagues used high frame-rate video cameras to record the mite's sprints in the lab and in their natural environment. They were very surprised to find that the Paratarsotomus macropalpis speed is equivalent to a human running roughly 2,100 km per hour. They were also surprised to find the mites running on concrete up to 60 degrees Celsius, a temperature significantly higher than the upper lethal temperature of most animals.
Samuel Rubin et al. 2014. "Exceptional Locomotory Performance in Paratarsotomus macropalpis Mites. The FASEB Journal, vol. 28, no. 1, Supplement 878.1.

A Mite Hitching A Ride On A Harvestman!

A harvestman from Anza-Borrego Desert. Its left rear leg has a minute parasitic mite.

Parasitic Mite Family (Class Arachnida Order Acarina Family Macronyssidae)

When I first collected these mites I assumed they were house dust mites in the family Pyroglyphidae. My original assumption was based on their minute size and infestation in a house in southern California. The typical length of an adult body (excluding legs) is approximately 0.5 mm, slightly larger than an average grain of table salt (NaCl) that measures about 0.3 mm on a side. House dust mites are even smaller and are generally not visible without magnification. There are several reasons why I changed my original assumption, and have now concluded that they are parasitic mites of the family Macronyssidae and not house dust mites. Parasitic mites are very difficult to identify and require detailed microscopic examination. Based upon illustrations in Insects of the Los Angeles Basin by Charles H. Hogue (1993), they resemble bird mites of the genus Dermanyssus or rat mites of the genus Allodermanyssus. The fact that a rat was observed behind the wall in the same area where the mites were discovered is a strong argument in favor of rat mites.

  1. Large numbers of mites were crawling on the upper wall and ceiling, apparently entering the room through a crack. House dust mites typically are found in mattresses, pillows, carpets, furniture and bedding where they feed on flakes of shed human skin.

  2. A resident of this house received numerous bites from an unknown arthropod. House dust mites do not bite.

  3. These mites are reddish-brown and house dust mites are pale, cream-colored.

  4. Most references give 0.9 - 1.0 mm for the length of bird and rat mites. The mites in my images have a body length of 0.5 mm (excluding the legs). With the legs completely extended, their overall length is almost 1.0 mm.

  5. Most authors state that house dust mites are invisible to the naked eye and their publications usually contain drawings or electron microscope images, but not camera photos. Length measurements for house dust mites are generally 0.5 mm or less. If these measurements include the legs, then their body length is much smaller than bird or rat mites, as small (or smaller) than grains of salt. The mites in my images are visible with the naked eye, although their bodies are only slightly larger than grains of table salt. They are much smaller than Argentine ants, another common household nuisance in southern California.

  6. My images compare favorably with several other images of bird and rat mites on the Internet. They could be bird mites, but there is no proof that birds were nesting inside the wall. As I stated above, a rat was observed behind the wall where the mites were discovered.

  7. According to Occam's razor (principle of parsimony), when there are several tentative explanations (hypotheses), the one that makes the fewest new assumptions is probably the best explanation. In other words, do not generate a hypothesis any more complex than is demanded by the data. In this case, since a rat was observed where the mites emerged from the wall, the unknown mites are probably rat mites.

   Note: Do not confuse bird mite with bird louse. See Bird Louse  

Parasitic mites (possibly rat mites) compared with an ordinary straight pin and grains of table salt (black arrow). Photo taken with a hand held Nikon D-90 and Phoenix RF46N Ring Flash using a 60mm Micro Nikkor AF-S F/2.8G ED Macro Lens.

The Size Of A Grain Of Table Salt (NaCl)
See The Head Size Of Ordinary Straight Pin
  See Nikon D-90 Used To Take The Above Photo  

The following two images were taken with a Sony W-300 digital camera mounted on a Bausch & Lomb stereo dissecting microscope and an Olympus compound laboratory grade microscope. The two microscope images were superimposed to increase the depth of field and show detail in the legs and dorsal surface of the body.

Magnified view of a parasitic mite (probably a rat mite) compared in size with an average cuboidal grain of table salt (NaCl). The salt grain is 0.3 mm on a side. This is the average size of grains in a salt shaker. Note the bristlelike hairs (setae) and triangular back plate on dorsal surface.

Magnified view of the dorsal (upper) side of a parasitic mite (probably a rat mite). Note the bristlelike hairs (setae) and triangular back plate. These structures are not present on house dust mites. These mites superficially resemble minute ticks in overall shape and color.

Magnified view of the dorsal side of a younger (smaller) parasitic mite (probably a rat mite). Note the bristlelike hairs (setae) and triangular back plate. Setae are not visible on the dorsal surface because this image was taken with an Olympus compound microscope (magnification 100x) with only substage illumination. The close-up view of mouth region (magnification 400x) shows the piercing-sucking mouthparts between the two pedipalps.

   Sony W-300 Mounted On Bausch & Lomb Microscope   

A minute rat mite between two pillars of the Lincoln Memorial on the reverse side of a 2000 Lincoln penny. Photo taken with a hand held Nikon D-90 and Phoenix RF46N Ring Flash using a 60mm Micro Nikkor AF-S F/2.8G ED Macro Lens.

House Dust Mite Family (Class Arachnida Order Acarina Family Pyroglyphidae)

House dust mites are among the smallest arthropods. Because of their minute size and translucent body, they are invisible to the naked eye. Although they vary in size, the typical length of an adult is 0.25 mm, smaller than an average grain of table salt (NaCl) that measures about 0.3 mm on a side. In fact, the specimen I photographed is only about twice the length of a pine pollen grain! Dust mites of the genus Dermatophagoides are a serious household pest in southern California. Although they do not bite humans, they are a common cause of bronchial asthma and other allergies. Their chitinous exoskeltons and feces containing digestive enzymes are readily inhaled in airborne dust and serve as potent allergens that induce antibody and cell-mediated immune responses in hypersensitive people. Typical symptoms include itchiness, inflamed eczema, watering & reddening eyes, conjunctivitis, sneezing repeatedly, runny nose, wheezing and clogging of the lungs.

A typical mattress may contain tens of thousands of house dust mites, not to mention pillows where you place your face. Dust mites prefer warm, moist areas where you are sleeping, although they are also common in carpets and sofas. Dust mites feed primarily on flakes of dead skin shed from your body each day. In fact, your bathtub ring is composed of dead skin cells in addition to grime and dirt. It is debatable whether dust mites and their fecal pellets add significantly to the weight of your favorite old pillow. Basic knowledge of dust mites gives new meaning to sleeping in motel beds. It might be reassuring to carry your own pillow, unless it is already mite infested. Dust mite exoskeletons and fecal pellets are the primary allergens involved in severe allergic reactions, so in a motel bed it probably doesn't matter whose skin cells the dust mites were feeding on. However, there are other concerns for hypochondriacs, including pathogenic organisms (viruses & bacteria), lice and bedbugs.

The following image shows how incredibly small these mites really are. They are compared in size with the reverse side of a U.S. Lincoln penny (one cent) and an ordinary grain of table salt (NaCl):

Left: An average cuboidal grain of ordinary table salt between two pillars of the Lincoln Memorial on the reverse side of a 2000 Lincoln penny. The salt grain is 0.3 mm on a side. Right: Magnified view of salt grain and a minute dust mite (red arrow). The dust mite body (excluding legs is about 0.2 mm long. Photo taken with a hand held Nikon D-90 and Phoenix RF46N Ring Flash using a 60mm Micro Nikkor AF-S F/2.8G ED Macro Lens.

Magnified view of dust sample vacuumed out of Mr Wolffia's couch. The image shows a dust mite, a pine pollen grain and flakes of dead skin, undoubtedly shed by Mr. Wolffia. The mite's body is approximately 0.2 mm in length. Image taken with an Olympus laboratory grade compound microscope with a Sony W-300 digital camera. Magnification 400x.

   See More House Dust Mite Images   

Hair Follicle or Eyelash Mite Family (Demodicidae)

Hair follicle mites or eyelash mites of the genus Demodex are among the smallest multicellular animals. In fact, D. brevis is only about the size of a unicellular Paramecium! These microscopic spider relatives (arachnids) live in the hair follicles and adjacent sebaceous glands of human eyelashes, noses, cheeks and foreheads. The following Wayne's Word Mite Page contains images of this minute removed from the author's nose!

   See Images Of Minute Hair Follicle Mite On Wayne's Word Mite Page   

Hard Tick Family (Class Arachnida Order Acarina Family Ixodidae)

Pacific coast tick (Dermacentor occidentalis).

The head and mouthparts of this tick are embedded in Mr. Wolffia's abdomen. Coating the tick's body with oil did not encourage it to let go! It had to be carefully pulled out with forceps placed close to the head. See the following two close-up images.

The Pacific coast tick (Dermacentor occidentalis).

Bighorn sheep tick (Dermacentor hunteri). Photographed in Fossil Canyon north of Ocotillo, Imperial County, California by Vince Balch.

Male deer tick of the genus Ixodes removed from Mr. Wolffia in Monterey County (May 2006). Two views are shown: Dorsal (left) and ventral or underside (right). This tick is smaller than the Pacific coast tick shown in the previous image. Deer ticks are known to cary the spirochaete that causes Lyme disease.
Another view of a male deer tick (Ixodes) removed from Mr. Wolffia. There are more than 200 species in the genus Ixodes, but this may be the deer tick or western black-legged tick (X. pacificus).

Deer tick of midwestern and eastern U.S. (Ixodes dammini female = I. scapularis).

  More Images Of Pacific Coast Tick  

Harvestmen (Class Arachnida Order Opilones)

Harvestmen superficially resemble spiders, but they actually belong to a different order (spiders belong to the order Araneae). They are also called "daddy long-legs," but this name is also used for crane flies (Tipulidae) and cellar spiders (Pholcidae). Unlike spiders, they have no fangs and do not inject venom. In addition, their abdomen is fused to the cephalothorax. They typically have only two eyes on a single turret-like tubercle at the top of their head region. The second pair of legs are longer than the others, and are used like tactile antennae. Harvestmen are typically scavengers that feed on dead insects, spiders and pillbugs (isopods). Apparently the first species to be described were seen in the fall during harvest time.

A harvestman from the Merriam Mtns. of San Marcos in northern San Diego County. Two eyes are visible on a turret-like tubercle at the top of the head region (white arrow).

A harvestman from Anza-Borrego Desert. It belongs to the family Phalangidae and possibly the genus Leiobunum. In the lower image the harvestman is feeding on a cricket.

A harvestman from Anza-Borrego Desert. Its left rear leg has a minute parasitic mite.

Whiptail Scorpion or Vinegarroon (Class Arachnida Order Thelyphonida)

A whiptail scorpion (Hypoctonus rangunensis) native to Thailand. Whiptail scorpions are also known as uropygids and were formerly placed in the order Uropygida. Although they appear ferocious, these scorpion relatives have no venom glands. They have glands near the rear of their abdomen that can spray a combination of acetic and octanoic acids when disturbed. The spray has a vinegar-like smell, giving rise to the common name "vinegarroon." The powerful front pincers (modified pedipalps) are used to grasp and kill insects and millipedes. Vinegarrons typically are found in dark, humid places under rocks, soil and leaf litter. One species (Mastigoproctus giganteus) is native to the southern United States, from Arizona to Florida.

Tailless Whipscorpion or Cave Spider (Class Arachnida Order Amblypygi)

Featured In The Movie "Harry Potter and the Goblet of Fire"

West African tailless whipscorpion (Titanodamon johnstoni).

Tailless whipscorpion or African cave spider (Damon variegatus). It differs from whiptail scorpions (order Thelyphonida = Uropygida ) in having a rounded abdomen lacking a tail and spray glands. This relative of the whiptail scorpion has a conspicuously flattened body. It differs from true spiders in having long, whip-like front legs, strong, armored pedipalps and no spinnerets. The unique 1st pair of legs are modified into long, slender "feelers" called antenniform legs. These are used to sense surroundings and find prey in dark caves of eastern Africa. The modified pedipalps are used to grasp prey. One species (Paraphrynus mexicanus) is native to the southern Arizona.

This species is featured in the movie "Harry Potter and the Goblet of Fire." In one scene Mad-eye Moody tortures this animal with the Imperius Curse until Hermione Granger begs him to stop. As the whipscorpion is tossed onto different students in the classroom, Moody makes the following two contradictory statements: "Don't worry, completely harmless....If she bites, she's lethal." In case you are wondering, this spider relative is nonvenomous.

Tailless whipscorpion or African cave spider (Damon variegatus)

Centipedes (Class Chilopoda: Family Scolopendridae)

The name centipede is derived from the Latin words "centi" (100) and "pede" (foot). Actually, adult centipedes in North America may have fewer or more than 100 legs, typically one pair per body segment. Centipedes are the only animals with legs modified into fanglike "poison jaws" (toxicognaths) that inject poison for subduing and killing prey. Modified forelegs, called prehensors, are located under the head. Glands inside the prehensors release venom into ducts that lead into the fangs. The last pair of legs are longer than the others. Some species use these to subdue prey or as defensive pincers. The posterior body segment with its unique pair of legs superficially resembles the head. This "pseudohead" found in some species may serve to confuse potential predators by misdirecting their attacks to a less vital part of their body, thus leaving the head free to bite the attacker. A common question that students ask is "can centipedes bite?" I can personally testify that they indeed have powerful fangs and can inflict a painful bite, particularly larger species shown in the following images. I once took a nap on a boulder at the summit of Owens Peak near Palomar College. My back was apparently pressing against a large centipede who decided to alert my presence with a painful bite.

Scolopendra polymorpha: This colorful southern California centipede may be 3 to 4 inches in length.

Magnified view of the underside of a centipede showing a pair of fanglike "poison jaws" or toxicognaths (forcipules). (red arrow). These are modified forelegs that inject venom and can produce a painful bite.

Be Careful When You Check Ant Pitfall Traps in Arizona!

This 4 inch centipede fell into a pitfall trap! I carefully released it.

House Centipede (Class Chilopoda: Family Scutigeridae)

House centipede (Scutigera coleoptrata), originally from Mediterranean region.

Scutigera coleoptrata is a very distinctive centipede that commonly lives in houses, especially damp areas in bathrooms and basements. It can run surprisingly fast with it 15 pairs of long legs. They have a shorter body and longer legs than other species, preventing them from tripping over themselves as they run. Their legs progressively get longer towards the rear of the body. This allows the rear legs to cross the legs in front of them, going above and to the outside, preventing entanglement. The rear-most legs are actually twice as long as the front legs. Their fast-moving legs and coordination is quite remakable with such a tiny brain.

The house centipede is a voracious predator, eating worms, snails, cockroaches, silverfish, fly larvae, and other arthropods. It senses its prey using its long antennae which have scent and touch receptors on them. Then they use their fangs to hold the prey while injecting poison with the modified front legs. Its body is frail compared with other more robust centepedes shown above. House centepedes are considered beneficial and relatively harmless to people, but I wouldn't recommend holding one against its will between your fingers.

Business end of a house centipede (Scutigera coleoptrata). The front pair of legs are modified into sharp, jaw-like mandibles called forcipules that inject venom into prey. By Kevincollins123 - Own work, CC BY-SA 3.0,

Millipedes (Class Diplopoda: Family Spirobolidae)

A large southern California millipede (probably Hiltonius pulchrus), a member of the class Diplopoda in the phylum Arthropoda. Diplopoda means "double-footed," and refers to two pairs of legs on each body segment. The name millipede is derived from the Latin words "milli" (1000) and "pede" (foot). Actually, adult centipedes in North America may have up to 750 legs, typically two pairs per body segment. Unlike centipedes, most millipedes are harmless detritis feeders (detritivores) and feed on rotting vegetation.


  1. Evans, A.V. 2007. Field Guide to Insects and Spiders of North America. Sterling Publishing Co., Inc. New York, New York.

  2. Hogue, C.L. 1993. Insects of the Los Angeles Basin. Natural History Museum of Los Angeles County.

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