A Tick's Life
The first in a series on revolting creatures.
There they are, imbibing and drooling; it's the drooling that puts us at risk. Perhaps we should sympathize because the tick itself is a host. (We, too, by the way, carry spirochetes—harmless ones, in our mouths.) The parasitic spirochete sits in the tick's midgut until the blood flows in. Then, stimulated by the blood's nutrients and warmth, the newly expanded crowd of spirochetes migrates to the tick's salivary glands.
Entomologists estimate it takes more than 24 hours for the spirochete to move up and out, thus the importance of checking your body for embedded ticks and removing them as soon as possible.
An adult tick isn't as dangerous to us as a nymph, which is tiny enough to be mistaken for a freckle on light skin. The tiny adolescent is also likely to have fed on a mouse, the most efficient reservoir for the dangerous spirochete. (Though heavily infested with B. burgdorferi, mice don't get sick. In any case, no one has heard mice complain of fever, aching joints, fatigue, rash, and mood disorders.)
The male adult tick expands his repertoire to include finding females. He looks, logically enough, on the biggest moving mammalian blood supply around—a deer. Deer are in one way relatively innocent in the Lyme disease story—they have components in their blood that prevent the spirochetes from surviving. But they are also orgy enablers; if there were fewer deer, there would be fewer ticks, because the ticks would have a harder time finding one another.
Male and female engage in an impressive combination of gourmandise and lust. Maybe not lust exactly; for the female, it's more like being interrupted at breakfast by the UPS guy, with a package of perishables. Here's the setup: The female has her hypostome planted in the deer, imbibing. A long meal is the cue to her body to produce her 2,000 or so eggs. The male approaches from below, then uses his mouthparts to pluck a packet of sperm called a spermatophore from his genital pore. He delivers the packet into his partner's genital pore with his hypostome, the same barbed hollow needle that he sticks into mammals. The male frequently remains attached, mouthparts locked in the female genital aperture, to prevent other males from linking up with his chosen mate. After the female drops to the ground, full of blood and sperm, she lays her eggs, and then the fun is definitely over. She begins to atrophy. Her intestines spill out in a yellow blob. "When does the male die?" I asked Durland Fish, who studies tick-borne pathogens at the Yale University School of Public Health. "When he runs out of energy or sperm, whatever comes first," Dr. Fish replied.
So death for the tick comes from starvation, dehydration, egg-laying, or old age, rather than from predation. We don't seem to have any natural allies in tick control. It's not well known what kind of animal eats ticks, though the larvae are vulnerable to fungi. Dr. Fish scornfully dismissed the guinea hen as a form of pest control—"a Christie Brinkley-ism." (The former model advocates buying a flock of the cackling black-and-white birds to clear your yard of ticks.)
I repeated the question of why God made ticks for Dr. Fish. He responded with a growl to what he took to be my facetious tone: "Nobody makes them. They're just there. Their object, like ours, is to make a living any way they can."
And the tick's place in the great web of life? "They transmit disease. They control population."
"Including us?" I asked.
"Whatever is susceptible to the disease."
Next installment: Vultures!
Constance Casey is a former New York City Department of Parks gardener and writes the monthly "Species" column for Landscape Architecture Magazine.
Photograph of deer tick by Scott Bauer/U.S. Department of Agriculture. Photograph of tick mouth by Rocky Mountain Labs, NIAID, NIH.