Wednesday, September 29, 2010

Mighty Mites

[ Fair warning -- if thinking about creepy crawlies makes your skin itch, you might just want to scroll on down and read the next post instead. ]

mating dragonflies (c) John Ashley
Mating dragonflies
Ah, the delicate and lovely dragonfly. A reminder of summer days spent playing in the water. Here, the red male dragonfly clasps the yellow female by the back of her head, and she reaches forward to collect a spermatophore from his mid-abdomen storage compartment -- the dragonfly version of sex.

But what's this, all these little golden guys clinging to the female dragonflys' abdomen? Could they be? Are they? Parasites? As one invertebrate biologist described it, these guys are like tiny versions of bloated ticks on a dog.

Welcome to the wonderful world of mites.

You already know some mite species by their common names. Chiggers, mange and scabies mites. Rust mites, dust mites and -- maybe the creepiest -- eyelash mites (more juicy details on them later).

Most mites look like tiny versions of spiders or ticks, their close relatives. Their fossil history goes back almost 400 million years, making them one of the earliest animal groups to crawl out of the ancient seas. Currently, the 50,000 species that have been described represent only about 5% of the estimated total. Mites are the most common animals in nearly every kind of habitat, but they are rarely seen because they are so darn small.

As arachnids, all mite species have a two-part, segmented body -- and that's about as definitive as it gets. They might have 1, 2, 3 or 4 pairs of hairy legs. The might have 1-5 eyes, though most are eyeless. Some species live in fresh water, some live in salt water, and others are terrestrial. Most are external parasites, but other species or age groups may be predators, vegetarians, or detritivores. Their life cycle typically follows as: egg, pre-larval stage, larval stage, then a series of 1-3 nymph stages.

On our delicate dragonfly, the mites are in their parasitic larval stage. They are living off the dragonfly's internal juices, but they probably won't adversely affect their lovely host. When this female dragonfly returns to the lake to deposit her eggs, the larval mites will drop off into the water. After a series of nymph stages, adult female mites will lay 20-400 fertilized, red eggs in the mud, under a rock, or attached to submerged vegetation.

Red mites on a blue damselfly (c) John Ashley
Red mites (genus Arrenurus) on a blue damselfly
When a red egg hatches, a pre-larval mite will wiggle about in the water until it finds a suitable host to grab hold of -- like a dragonfly nymph. When the larval dragonfly climbs out of the water, splits its exoskeleton, and spreads its new wings to dry in glory, the tiny parasitic hitchhikers crawl aboard. In some areas, water mites parasitize 20-50% of the aquatic insect population.

After finding a new host (or meal), two segmented appendages ("pedepalps") grab hold from the mite's head end. Then the mite stabs with two retractable, snake fang-like mouth parts ("chelicerae"). Through these holes, digestive enzymes are injected into the host (or prey) item. The mite hangs on, sucks up the juices, and bloats up like a tiny tick. Mmmmmm.

Various mite species have adapted to thrive in hot thermal vents, deep sea trenches, and arctic extremes. And closer to home, there are 65 microscopic mite species that have specialized to live in the hair follicles of mammals -- including two species that live in grandpa's eyelashes.

Demodex folliculorum (first described in 1842) lives in hair follicles, while Demodex brevis lives in the sebaceous oil glands that are connected to the hair follicles. Both species are found primarily on the face, especially around the nose, eyelashes and eyebrows. These semi-transparent mites eat dead skin cells and extruded oils. And while they don't defecate on the skin (or even at all), they do crawl out at night to mate. In each follicle colony of about 10 mites, adults frolic at the follicle opening and then the females lay about 25 eggs inside. The eggs hatch in 3-4 days, then 7 more days to reach adulthood, with a total lifespan of 14-18 days.

Hosting these mites is quite common and usually there are no symptoms. Eyelash mites live in about 1/3 of all children, 1/2 of adults and 2/3's of the elderly. (Our skin might produce more cebum, or oil, as we get older.) You can even look for these mites by plucking an eyelash and examining it under a microscope.

Or, if that idea makes your skin itch, you can also look for mites in lots of other places.

Dragonfly with mites (c) John Ashley
Engorged mite hitchhikers
Though small, mites are very common. One square meter of the muddy debris at the bottom of a lake or pond may contain over 2,000 mites, while the same amount of cobble substrate from a mountain stream may contain 5,000 mites. A square meter of forest soil might hold a whopping 1,000,000 mites of 200+ species. And yet, how many times have you seen a mite in a lake, stream or forest? They may be everywhere, but they are mighty small indeed.

In fact, the name "mite" originates from the 12th century Dutch word for a small, golden coin. The huge family of tiny mites is very adaptive, and adaptability is the golden coin of evolution -- even the evolution of creepy crawlies.

Behind the lens: Following active insects from perch to perch renders most tripods and macros useless. I photographed these dragonflies with a short extension tube on a 70-200mm lens, hand held. This gave me a flexible close-focus zoom that still kept me 1-2' away from these shy suitors. Also, by removing the lens hood, I still had the option to use the on-camera flash for fill. 

Behind the bugs: Thanks to Bryce and "invertebrate guru" Dave, at the Montana Natural Heritage Program, for pointing me in the right direction to solve what had been a baffling mystery.