Mites

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Southern California Mites (Class Arachnida: Order Acarina)
© W.P. Armstrong 15 March 2011
  1. Velvet Mite: Family Thrombiddidae
  2. Spider Mites: Family Tetranychidae
  3. Parasitic Mites: Family Macronyssidae
  4. House Dust Mites: Family Pyroglyphidae  
  5. Hair Follicle Mites: Family Demodicidae
  6. Bot Flies That Bore Into Your Skin
  7. Sucking Lice That Live In Your Hair

Mites and ticks belong to the order Acarina (Acari) in the large arthropod class Arachnida (spiders). Some modern references treat the Acari as a subclass rather than an order. Approximately 50,000 species have been described, although there are an estimated 1 million species currently living. This is an ancient group of arthropods dating back in the fossil record to the Devonian period nearly 400 million years ago, long before the age of dinosaurs. Mites are extremely abundant and variable in habitat, but are not commonly seen by casual observers due to their small size. In fact, some mites are invisible to the naked eye. Many parasitic forms are vectors of diseases and some are serious agricultural pests. An entire branch of biology call acarology is devoted to the study of these remarkable organisms.

1. Velvet Mite Family (Thrombidiidae)

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.

Red velvet mite (Angelothrombium), possibly A. pandorae.


2. Spider Mite Family (Tetranychidae)

Citrus red mite (Panonychus citri): A serious crop pest of citrus trees, especially lemons (Citrus lemon).

Left: Mite in a willow leaf gall. It is an 8-legged spider mite similar in size to the 2-spotted mite (right).

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.

A minute, deceased mite taken from the underside of a Dorstenia leaf.


A Mite Hitchhiking On A Harvestman

A harvestman from Anza-Borrego Desert. It belongs to the family Phalangidae (possibly the genus Leiobunum).


3. Parasitic Mite 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 a 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.


4. House Dust Mite Family (Pyroglyphidae)

Note Regarding Photography of House Dust Mites: These minute, translucent arthropods are only about 250 micrometers in length, smaller than an average grain of table salt. They are not visible without at least 10x magnification; ideally with 100x. They have 4 pairs of hairy legs, no eyes, no antennae, and a mouth region at the anterior end that resembles the head. Long setae (bristlelike hairs) extend from the outer margins of the body. Because of the lack of depth of field, all of these structures are difficult to capture with a camera attached to an ordinary light microscope. In addition, when preparing them on a microscope slide, the legs and setae are seldom all extended and visible. Ideally you need a scanning electron microscope or a good scientific illustrator. The following images of dust mites were taken with a compound microscope and a Sony W-300 digital camera (magnification 400x). Because of the limited depth of field, the bristlelike setae are not visible in most of the images. Left drawing modified from the Ohio State University Extension Fact Sheet on House Dust Mites: (http://ohioline.osu.edu/hyg-fact/2000/2157.html)

House dust mites are among the smallest arthropods. Because of their minute size and pale color, they are generally invisible to the naked eye. Although they vary in size, the typical length of an adult body (excluding legs) is approximately 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 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 the underside (ventral side) of a dust mite compared in size with a grain of ordinary table salt (NaCl). Photographed with an Olympus laboratory grade compound microscope and Sony W-300 digital camera. Magnification 400x.

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. Magnification 400x.

House dust mites photographed with an Olympus laboratory grade compound microscope and Sony W-300 digital camera. Magnification 400x. The setae were retouched with Adobe Photoshop. The lower right object might be dust mite excrement.


5. Mites On Your Face (Class Arachnida: Order Acarina)

Hair Follicle Mite Family (Demodicidae): Ubiquitous Obligatory Ectoparasites

Of all the many thousands of plant and animal images I have taken and uploaded to Wayne's Word, this is truly one of the most amazing. It even surpasses the dust mites I found in my pillow. I didn't have to travel that far or spend a lot of money on gasoline or airline tickets. I didn't even have to look any further than my nose to find such a remarkable animal. In fact, I actually found this microscopic relative of spiders deep in a sebaceous gland of my nose. Most of the images were taken with a compound microscope and a Sony W-300 digital camera. Although the backlit, high magnification images appear two-dimensional with limited depth of field, you can clearly see the slender, fusiform body and eight stubby legs, each tipped with a pair of pincerlike claws. Illustration (left) modified from T. Ross and photos of the dorsal side of Demodex brevis by W.P. Armstrong.

Note: These minute mites are invisible to the naked eye: See Cell Size Comparison Table On Wayne's Word: Demodex.

Hair follicle mites of the genus Demodex are among the smallest multicellular animals. They were first described in humans in 1841 by Frederick Henle who reported this minute parasite from "miliary glands" of the ear canal. He was uncertain about the parasite's taxonomic position in the animal kingdom. [Henle is also known for the Henle's loop in the vertebrate nephron.] Another scientist from this time period by the name of Berger (1845) thought it was a member of the phylum Tardigrada. Tardigrades are minute animals often found on lichens and mosses. According to Desch and Nutting (1972), there are two species of follicle mites on humans. Demodex folliculorum measures 0.3 to 0.4 mm in length and typically occupies hair follicles. It is also called an "eyelash mite" because it commonly occurs in follicles at the base of eyelashes. Demodex brevis is about half that size (0.15 to 0.2 mm) and typically lives in sebaceous glands adjacent to hair follicles. The latter mite is only about the size of a unicellular Paramecium and appears to be the species I found in my nose.

These microscopic spider relatives (arachnids) live in the hair follicles and adjacent sebaceous glands of human eyelashes, noses, cheeks and foreheads. In heavily infested cases, the posterior ends of D. folliculorum can be seen around the bases of eyelashes. Their mouthparts resemble those of spiders, scorpions and other arachnids, but are much smaller. They are designed to feed on skin cells (cytoplasm) and the oily sebum produced by sebaceous glands. Males come out at night and move around on the skin surface to look for potential females to mate with. Unsubstantiated reports state that these mites can move on your skin at a rate of 8 to 16 mm per hour. Their digestive system is reportedly so efficient that they have very little excretory waste and no excretory orifice. This is reassuring news to those of us who worry about them defecating on our faces and in our eyes.

Most people have these minute parasites living relatively harmlessly in their skin pores and hair follicles, although people with oily skin, or those who use cosmetics heavily and don't wash thoroughly, reportedly have the heaviest infestations. They are transferred by facial contact and are apparently more common in adults than in children. In some people, high mite populations can cause demodicosis and blepharitis, possibly due to a suppressed immune system. Theses conditions are characterized by itching and inflammation of the skin and eyelids. High densities of Demodex have also been found in people with the skin disorder rosacea. A cell-mediated immune response involving cytotoxic "killer" T-cells may be involved in the inflammation associated with acne rosacea. There are numerous species of Demodex that live on many different mammals, including rodents, monkeys, cattle and bison. In fact, they have been found in 11 of the 18 orders of eutherian (placental) mammals, with most species harboring two or more species of Demodex (Desch, 2009). One species Demodex canis lives on domestic dogs and causes a condition called "demodectic mange." There is a plethora of information about these amazing animals on the Internet. You might start with an excellent summary of Demodex on Wikipedia.

All species of Demodex are obligate ectoparasites that cannot live independently of their host. In parasitism, the parasite benefits by obtaining nutrients from the host's body, which is often harmed by the relationship. In most humans, these ubiquitous parasites may not cause any harm to their host. In fact, some author's refer to the relationship as commensalism rather than parasitism. In commensalism, one organism benefits from the relationship while the other neither benefits nor is harmed. The dorsal fin of the remora is modified into a sucker which forms a temporary attachment to the shark. The shark does not seem to be inconvenienced by this and makes no attempt to remove the remora. When the shark feeds, the remora is in a good position to pick up scraps of food left by the shark. Marine mammals, including whales and manatees, often carry harmless hitchhikers called barnacles on their backs. The barnacles benefit from the ride through nutrient-rich waters. In my opinion, the term parasite is more appropriate because these mites are definitely feeding on human cells and glandular secretions (sebum). A few mites feeding on dead skin cells and sebum deep within hair follicles are probably harmless unless their populations get out of control. Recent peer-reviewed articles accessible through Google Scholar indicate there is considerable dermatological research on human follicle mites and different opinions regarding their detrimental effects.

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Images From Prepared Microscope Slides

Prepared microscope slide from the dermis layer of human skin showing a hair follicle and sebaceous gland. Deeply embedded within the hair follicle is a slender, elongate follicle mite (Demodex folliculorum). Magnification 400x. Inset: Close-up view of hair follicle containing a mite. Several legs are visible at anterior end of mite (yellow arrow). Part of the mite has been sectioned during the preparation of slide. Magnification 1000x.

Fresh Mites From Mr. Wolffia's Nose

The following images of a folicle mite came from my own nose. Based on its size and presence in sebum, it appears to be Demodex brevis. I squeezed a skin pore and collected the whitish sebum from sebaceous glands. With a toothpick I carefully transferred the sebum to a drop of olive oil on a clean microscope slide. [Other salad oils, such as soybean oil, will work just as well.] I added a cover slip and examined the slide under low magnification (40x) with a compound microscope using substage illumination (not a dissecting microscope). When I spotted a small fusiform body with eight stubbly legs, I switched to higher magnifications of 100x, 400x and 1000x. This may seem a little gross to some readers, but it is not nearly as shocking as the bot fly larva that emerged from a colleague's arm after returning from Central America.

Ventral view of follicle mite (Demodex brevis) removed from Mr. Wolffia's nose. Several squamous epithelial cells are also included in the image. The epithelial cells have an average diameter of about 60 micrometers (0.06 mm). Magnification 300x.

Dorsal view of follicle mite (Demodex brevis) removed from Mr. Wolffia's nose. Magnification 400x.

A follicle mite (Demodex brevis) compared in size with an average grain of table salt (NaCl). The mite is about 1/2 the length of one side of the grain. Magnification 400x.

Demodex brevis compared in size with the freshwater protozoan Paramecium bursaria. Magnification 400x.

Dorsal view of follicle mite (Demodex brevis) from Mr. Wolffia's nose under high magnification (1000x). The bifid, pincerlike claws and mouth are clearly visible.

References:

  1. Desch, C.E. 2009. "Human Hair Follicle Mites and Forensic Acarology." Experimental and Applied Acarolgy 49 (1-2): 143-146.

  2. Desch, C. and W.B. Nutting. 1972. "Demodex folliculorum (Simon) and D. brevis Akbulatova of Man: Redescription and Reevaluation." The Journal of Parasitology 58 (1): 169-177.

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

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

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