The Fork-tailed Dragons

by Joseph DeSisto

Meet Furcula borealis, the caterpillar of the white furcula moth:

Furcula cinerea, the caterpillar of a medium-sized, gray moth. Photo by Joseph DeSisto.

Furcula borealis, the white furcula caterpillar. Photo by Joseph DeSisto.

It’s a weird-looking caterpillar already, but even weirder when viewed head-on:

The fork-tailed dragon caterpillar, Furcula borealis. Photo by Joseph DeSisto.

The fork-tailed dragon caterpillar, Furcula borealis. Photo by Joseph DeSisto.

Like all insects, caterpillars have only six legs, and these are located near the front of the body, just behind the head. The sticky, climbing appendages along the rest of the trunk are not true legs but “prolegs,” which are lost during metamorphosis. In Furcula caterpillars (and the related Cerura, shown below), the last pair of prolegs are modified into long, rigid “tails.”

Cerura scitiscripta, showing six true legs near the head (upper left), four typical prolegs on the trunk, and the special modified pair of prolegs at the rear of the body -- the forked

Cerura scitiscripta, showing six true legs near the head (upper left), four typical prolegs on the trunk, and the special modified pair of prolegs at the rear of the body — the forked “tail.” Photo by Joseph DeSisto.

The furcula moths belong to the strange and beautiful family Notodontidae. These are sometimes called prominent moths, despite mostly being brown, gray, or some combination thereof. Recall, however, that in my last post about caterpillars I referred to notodontid caterpillars as the dragon caterpillars. Furcula, then, are unofficially dubbed the fork-tailed dragon caterpillars.

In case you’re wondering what F. borealis looks like as a moth, here’s an example specimen:

The white furcula moth, Furcula borealis. Photo by Tom Murray, used with permission.

The white furcula moth, Furcula borealis. Photo by Tom Murray, used with permission.

So what is the forked tail for? To find out, we simply tap our little dragon on the head. This is what happens:

Furcula borealis. Photo by Joseph DeSisto.

Furcula borealis, beginning its display. Photo by Joseph DeSisto.

Bright red tentacles begin to emerge from the  modified prolegs. In less than a second they are fully everted:

Don't mess with the fork-tailed dragon! Photo by Joseph DeSisto.

Furcula borealis, tentacles emerging from modified prolegs. Photo by Joseph DeSisto.

The fork-tailed dragon then waves these tentacles about, and even attempts to rub them onto the offending party (i.e., my finger). The “strike” reminds me of a scorpion trying to sting, if scorpions dressed up as clowns and went to birthday parties. After tapping me, the tentacles disappear as quickly as they emerged, while the caterpillar tucks its head and braces itself for another attack.

Furcula borealis. Photo by Joseph DeSisto.

Furcula borealis. Photo by Joseph DeSisto.

In a less dramatic fashion, many insects use eversible organs to rub toxins on their predators. Rove beetles are an example — this explains the way many of them run, with abdomens held high in the air like scorpions. If you grab one, it will use its flexible abdomen to rub a cocktail of nasty chemicals on you, some of which can cause blistering. But in Furcula and Cerura caterpillars, the tentacle-rubbing is actually a harmless show designed to scare off predators. I must admit, were I a foraging bird, I would think twice about attacking a caterpillar with such a bizarre and intimidating display.

And yet, in an ironic twist, these caterpillars are not harmless. If sufficiently disturbed, they can fire off a burst of formic acid — the stuff fire ants sting you with — from glands behind the head. In F. borealis, these are stored in spiky, poisonous-looking projections:

If you're mean enough, this caterpillar might just spray formic acid at you. Photo by Joseph DeSisto.

If you’re mean enough, this caterpillar might just spray formic acid at you. Photo by Joseph DeSisto.

Don’t mess with the fork-tailed dragons!

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2 responses to “The Fork-tailed Dragons

  1. Jerianne Hoddes Berman

    Amazing photography and highly informative.

    Like

  2. I saw one at my camp

    Like

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