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.

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):

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!

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, https://commons.wikimedia.org/w/index.php?curid=4088673. 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. References: Evans, A.V. 2007. Field Guide to Insects and Spiders of North America. Sterling Publishing Co., Inc. New York, New York. Hogue, C.L. 1993. Insects of the Los Angeles Basin. Natural History Museum of Los Angeles County. 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