Western Thatching Ant (Formica obscuripes)

I still remember the first time I discovered this ant, decades ago in a subalpine meadow in Colorado. I stepped on what I thought was an odd patch of dried bits of last year’s meadow grass. My foot sank into it and my leg was instantly covered in angry ants! Fortunately, I managed to brush them off before I received many bites.

I was astounded by how large that anthill was. Little did I know at the time that that particular colony was only a small-to-medium-sized one as this ant goes. After I moved to the Pacific Northwest, I started regularly encountering colonies of this ant. One I spied in the Cascades of Oregon was five feet high and a dozen feet in diameter!

The anthill pictured below is a fairly average size for this species’ colonies (note my bicycle helmet for scale). As always for this species, it is constructed of plant debris, not soil or sand. This anthill is near a road and I first noticed it when riding my bicycle past it.

Formica obscuripes colony.

It was teeming with activity; every square inch of it contained at least several ants.

Formica obscuripes.

These ants have an undeserved reputation for being aggressive biters. Bite they do, and after their mandibles break the skin they add insult to injury by spraying formic acid into the wound, causing further irritation and pain. But unless one is one of the few unfortunates who is abnormally sensitive to formic acid, the bite is minor and the pain quickly vanishes.

Moreover, in my experience these ants tend to be reluctant to bite. I spent ten to fifteen minutes acquiring the photographs in this article, and received but a single bite for my efforts, despite being in the immediate proximity of a hill teeming with thousands of ants and even accidentally bumping that hill and doing minor damage to it with a leg of my tripod.

Formica is the Latin word for “ant,” and this genus is the type genus for entire ant family, Formicidae. The plastic used for kitchen countertops that goes by the same name has nothing to do with ants, being so named because it was originally envisioned as being a substitute material for mica. Formica is, however, how formic acid (first discovered in ants) and chemically similar compounds like formaldehyde got their names.

These ants have quite a profound impact on the plants in the areas they inhabit. This is one of the ant species that tends and spreads aphids, feeding off the honeydew that the aphids secrete. They also dislike plants in the immediate neighborhood of their colonies, and will chew through the bark and spray formic acid on the resulting wounds, which eventually kills the attacked plants.

Their impact on the plant world is far from entirely baleful, however: many of our woodland wildflowers, such as trilliums and bleeding hearts, are dispersed by ants. Their seeds bear oily, nutrient-rich bodies called elaiosomes whose purpose is to attract ants, which usually carry the seeds for some distance before removing the elaiosome and discarding the rest of the seed.

These ants are plentiful here on Bainbridge Island, and our woods are full of blooming trilliums in the spring. I have never seen these ants inside the city limits of Seattle, and even in Seattle’s larger wooded parks, trilliums are an unusual sight. I do not believe these facts to be mere coincidence.

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