The life story of a tick.

News and commentary about environmental issues.
June 24 2008 8:04 AM

A Tick's Life

The first in a series on revolting creatures.

Adult deer tick. Click image to expand
An adult deer tick

Ticks, which live on blood and nothing but blood, are loathsome to us. We strongly prefer not to share our blood, unless the act is voluntary and we get juice and doughnuts afterward.

It's good that ticks are loathsome, because getting them detached from us as quickly as possible is the best way to avoid being infected by the diseases they carry. Ticks not only extract blood, they ooze pathogens from their salivary glands into the wound they've sliced with their tiny claws and penetrated with their barbed mouthparts. Normal human beings don't sit around and watch with interest for days and days as this process takes place. (Undisturbed, a tick could happily sup for up to a week.)

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It's generally known the danger these small creatures pose, particularly the deer tick— Ixodes scapularis. This tick's saliva is the medium for delivery of a particular spirochete, or corkscrew-shaped bacterium, called Borrelia burgdorferi—famous for causing Lyme disease. I managed to locate Willy Burgdorfer, the scientist who identified the Lyme spirochete in 1982, and asked, "Why did God make ticks?"

"I don't have the answer," Dr. Burgdorfer said. "There are a lot of things we assign to the good Lord and we ask the question, why? All I can advise is to check yourself for ticks and remove them fast."

We know about the tick's danger to us, but we haven't thought much about how the ticks themselves survive life's competitive drama—how they reproduce and how they die.

The tick's life is simple, fairly boring, but urgent. No host, no blood meal, and the tick dies. There's only one blood meal for each of the three stages of a tick's life—larva, nymph, and adult. At each stage, every one of the tick's behaviors has been honed by evolution to sense a victim and latch on.

Deer ticks don't find the mice, deer, or us by sight; they have no eyes. On the tips of their front legs they have sensors, the Haller's organs, that allow them to detect, from as far away as a few yards, the heat given off by warm-blooded animals and the molecules of carbon dioxide that we mammals exhale. Blow into a tube of ticks at a lab, and you'll see them begin to wave like excited fans at a Justin Timberlake concert.

Exhale on those other bloodsuckers, fleas, and they jump. The good news about ticks is that they cannot jump or fly. The bad news is that if you find a tick on your scalp, it has probably crawled up your body from about sock-top level. Ominously, the Centers for Disease Control and Prevention advises us to check for ticks between our legs and then in the belly button.

When the "mammal nearby" message is received, a tick's two front legs, equipped with claws that act like grappling hooks, thrust into the air while its three pairs of back legs hold on to a blade of grass, a twig, or a leaf. (Ticks are arachnids, with eight legs, in the family of spiders, scorpions, and mites. Insects have six legs. If you want to impress tick researchers, tell them that you know a tick in larva stage has only six legs.) A host brushes against a tick, and the tick hitches a ride.

A tick mouth. Click image to expand
A tick mouth

He or she roams for a few hours looking for the right spot to attach. Then the two claws make the incision, and in goes the hypostome, shaped like a harpoon, with backward-facing barbs.

The tick then secretes a cementlike substance from its mouth, which glues it to the host and dissolves days later when the tick is sated and ready to drop off. Tick saliva also contains an anticoagulant to keep a host's blood flowing.

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