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The Curious Sexual and Asexual Habits of the Mighty Mite

Mites are everywhere and they know how to reproduce. Some use sperm bombs, while others have done away with males entirely.

Have you ever heard of pancake syndrome? No, it’s not the medicalization of having breakfast for dinner, nor is it a skin rash reminiscent of those delicious flat cakes. It’s a severe allergic reaction that follows exposure to wheat flour. But there’s a twist.

In the 1990s, a 14-year-old boy saw a doctor after multiple episodes of sneezing, severe itching of the eyes and face swelling. The symptoms had started within minutes of accidentally inhaling pizza dough mix that had been stored in his cupboard for months. An allergy test revealed that he was not allergic to newly purchased pizza dough mix, but to the contents of the box that had sat in his kitchen for some time. The problem was not the flour itself, it turns out; it was what had been growing in it.

Dermatophagoides farinae. A type of mite.

Pancake syndrome is anaphylaxis caused by wheat-containing food that has been contaminated by mites, and it is one of many health problems that these tiny creatures can create for us. Mites are all around us. One of the reasons why they are so common is that they have some pretty interesting ideas about reproduction.

Tiny arachnids

Mites are small. Mighty small. In fact, the word “mite” can also mean “a very small amount.” At most, mites can grow to a few centimetres, but the majority are smaller than a millimetre and to see them requires a microscope. If we try to spot them on the tree of life, we have to look up our very own animal kingdom. Humans and mites are both animals, yes, but we are chordates (meaning animals that have a spine or something like it), whereas mites are arthropods. They are thus in the company of horseshoe crabs and scorpions, centipedes and millipedes, barnacles, brine shrimp and insects. Within arthropods, mites belong to the arachnid class, like spiders and a scorpion-like creature called a vinegaroon. Fun fact: the vinegaroon, described in a PBS documentary as a “mutant land lobster,” sprays concentrated vinegar from the base of its tail as a defense mechanism.

Much like Washington Irving’s terrifying Headless Horseman, a mite does not technically have a head. Imagine if we didn’t have a neck and our head was directly fused to our torso: that is the mite’s cephalothorax, which is followed by its abdomen. Like spiders, mites have jaw-like pincers at the front of their non-heads. Biologists have had a field day plundering the Latin and Greek languages to name special body parts found throughout the animal kingdom. The mouthparts of mites and spiders are known as chelicerae (pronounced kuh-LI-suh-reye). They can be used to attack and stab predators, or the mite can use them to grab hold of a larger animal and hitch a ride. Further down from the chelicerae are the pedipalps, a pair of sensing limbs that help the mite make sense of its environment.

Mites are everywhere: in the water, on land, in trees, on fur and feathers. They hatch from eggs, then go through a larval stage, numerous immature nymph stages, and finally a full-fledged adult stage. But when it comes to making more eggs to give rise to more mites, these tiny arachnids have a secret: they have come up with many ways to procreate, including a sperm bomb, and some of these acts don’t even require males.

“Life, uh, finds a way”

In their 2013 textbook on mites, David Evans Walter and Heather C. Proctor write about the “bewildering diversity of sperm transfer methods” seen among mites. “In males,” they continue, “every appendage that can be modified for reproduction has been.” One type of mites has what biologists refer to as a “sperm finger” on its jaws, which it uses like a needle to inject its seed. It seems that life, as Jeff Goldblum famously states in Jurassic Park, does indeed find a way.

Those chelicerae jaws that mites have are sometimes used to pick up a sperm packet from the mite’s genital opening and transfer it, like a gift, to the female’s own reproductive opening. Arthropods like mites produce discreet capsules of sperm known as spermatophores. This capsule can thus be directly transferred to the female or it can be left in the environment for a female mite to pick up later. Some male mites seem to be fans of Tom Jones and Mousse T.’s 1999 chart-topping hit: their sperm packet generates carbon dioxide which, like a bomb, creates pressure that drives the contents of the packet outward.

If mite sex seems a mite too dangerous, fear not: some females have done away with their male counterparts entirely. These female mites reproduce through a process known as parthenogenesis, meaning “virgin birth.” (And, yes, it’s the same word that gave us the Parthenon, literally the Temple of the Virgin Goddess.) To return to Jurassic Park, the biologist arguing with Jeff Goldblum that a group of exclusively female animals can’t breed didn’t seem to know about mites and other invertebrates, as well as vertebrates like fish, reptiles, amphibians and birds, as some members of each of these groups can indeed reproduce asexually.

How do mites do it? Sometimes, an unfertilized egg gives rise to a female mite, often identical to its mother. Other times, this unfertilized egg results in a male offspring that only has half of the DNA a child normally has. Either way, sperm need not apply.

Mites are fascinating bugs, but because of their omnipresence, they can negatively impact our health, from allergies all the way to disease epidemics that felled more soldiers in the Pacific than the war they were fighting.

Mighty diseases

It’s called Tsusugamushi disease or scrub typhus. It can give you a fever, muscle pain, hemorrhages, even multiorgan failure. The culprit is a bacterium that is carried by a mite, specifically a type of mite colloquially known as a chigger. A large number of soldiers deployed to the Pacific theatre of World War II contracted scrub typhus, and many more died of the disease than were killed by enemy forces. Antibiotics subsequently came to the rescue and made the disease much less deadly.

There are more than 250 species of mites that are known to cause health problems in humans and animals. Some parasitic mites called Demodex live in our eyelashes and they can cause inflammation. If you’re allergic to house dust, your immune system is actually freaking out over mite droppings, specifically some of the digestive enzymes that they shed in their feces. High enough humidity inside a house allows fungi to thrive and mites will feed off of these organisms: ergo a lot of mite poop and potential allergies. These mites also love our mattresses, especially because of our dead skin cells which they eat. With the right conditions, you can find 5,000 mites in every gram of mattress dust. What else do they feed on in our beds? A small Scottish experiment in the 1980s exposed one source of nutrients in a presentation titled “Human semen as a dietary supplement for house dust mites.” Food for thought.

Some of the larger mites that feed on blood are known as ticks. The fact that their feeding habits are rather slow—from many hours to several days—means that these ticks have plenty of time to transfer a disease-causing agent into the animal whose blood they are sucking. Lyme disease is one such tick-borne infection: the cause is a bacterium called Borrelia and it hitches a ride inside of a tick. Mites can also cause scabies, an itchy skin disease, and mange, a deterioration of the skin sometimes seen in animals like cats and dogs.

All this talk of microscopic biting bugs may make you a little bit paranoid. The fear of creepy-crawlies creeping and crawling on your skin is known as acarophobia, which translates to “fear of mites.” In more serious cases, a person may think they see millions of bugs infesting their home and body, leading to delusory parasitosis.

Coming back to pancake syndrome, there are ways to prevent pantry mites from growing in your dry goods. Keep dry goods dry, away from sources of moisture like kettles and stovetops. Consider storing them in glass or plastic containers and waiting until they are empty before refilling them, so as not to combine newer goods with older, potentially contaminated ones. Clean your containers regularly. If your goods acquire a brownish dust that smells of mint, you are dealing with a significant mite infestation.

Pancakes are tasty, but when they’re full of mites it makes breakfast a mite awkward.

Take-home message:
- Mites are tiny arachnids, related to spiders and scorpions
- Mites have evolved to reproduce in a wide variety of ways, including a process known as parthenogenesis in which a female’s unfertilized egg becomes an offspring without the need for sperm
- Mites can cause diseases either directly, such as in allergies, or by being vectors for disease-causing organisms, such as the tick that carries a bacterium that causes Lyme disease


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