Looking very much like a creature from a Dr. Seuss story, the horned spanworm features odd structures known in entomological terms as ‘eversible tentacles.’
Good Natured: Horned Spanworm
May 12, 2023
I’m a fan of Dr. Seuss. Always have been, always will be.
His magical rhymes helped get me hooked on reading almost 50 years ago, and the fantastic creatures he dreamed up made me curious about our animal friends in a way that’s still relevant today. I love looking at Seussian creatures and trying to figure out from which species he borrowed its shapes and parts.
Sneetches, for instance, look to me like a cross between an ostrich and a duck; and tweetle beetles, from Fox in Socks, to my eye anyway, look immature–more like larvae than full-grown adults. (Which may explain their juvenile behavior of battling with paddles in a puddle in a puddle on a noodle-eating poodle…)
At any rate, I thought of Dr. Seuss yet again the other day, when a most whimsical beast appeared quite literally out of nowhere. I was at Norris Woods in St. Charles, wrapping up a program and talking with my friend Kelly, when he/she/it dropped from the sky-or, more likely, a tree-and landed on Kelly’s shoe.
It was a caterpillar, for sure, but one unlike any we had seen before. It had teensy mouthparts made for munching, six legs up front and little prolegs behind, and moved in classic inchworm style-scrunch, straighten, scrunch, straighten…
But what really caught our eyes, and what I’m still talking about with my nature nerd friends (and anyone else who will listen), are the strange projections emanating from the tiny creature’s back. In Seussian terms, I’d describe them as “Hootable, scootable, infinitely inscrutable; scriggly, wiggly, feeler-like thinglies.”
In entomological terms, though, the description reads more like this: Eversible tentacles extending from the dorsal surface of the abdominal segments. Normally slightly curled, these structures can be extended to more than twice their resting length when the caterpillar is disturbed.
Watching the creature scooch along Kelly’s finger (for it was way too interesting to leave down on her shoe), we emitted a collective “Wow!” and reached for our cell phones to snap some photos. When Kelly noticed that the caterpillar was furling and unfurling the “thingies” on its back, we both let out a “Whee!” and switched our phones to video mode.
After several minutes of such excitement, we realized we were both getting worn out, and figured the caterpillar must be too. Looking at the vegetation surrounding us and keeping in mind the rather stiff breeze that was blowing, we made an educated guess as to which tree the critter had fallen from and placed it gently on a leaf.
From that point on, it was a race to get home and try to identify the little Whozit. I briefly considered flipping through Dr. Seuss’s “If I Ran the Zoo,” but instead opted for my other favorite reference, BugGuide.net. Using the search term “inchworm,” I soon was scrolling through hundreds of pictures of inchworms of virtually every color and configuration-including one with some odd projections emanating from its back. Two more clicks and I’d come up with a tentative ID: Nematocampa resistaria, the horned spanworm.
Reading the BugGuide entry as well as information from other websites, I learned that the species has two other common names, the filament bearer and the bordered thorn. The caterpillar feeds on a wide variety of trees, including Douglas fir, hemlock, willow and hazelnut, as well as herbaceous plants in the strawberry and carrot families. The various pages noted that the function of the “filaments” is unknown, but at least one source noted that they might be meant to mimic dead flower parts or plant debris.
With so many unique adornments in its larval phase, the adult spanworm must surely be even more ostentatious, right? Well…
While not wholly plain, the horned spanworm moth isn’t exactly flashy either. Their light-colored wings are adorned with varying patterns of brown, which I suppose are meant to help them camouflage against tree bark. But to me, sitting here in my perpetual state of hunger, I swear they look like moth-shaped sugar cookies drizzled in chocolate.
Flipping back to the caterpillar descriptions, I laughed more than once at the adjectives that were used to describe the caterpillar’s appearance. “Bizarre” was one, “peculiar” was another. One BugGuide naturalist simply noted, “Strange!”
To these remarks I’ll add my own, in verse inspired by one Theodor Geisel:
With tentacles that spread,
It can impress a small crowd.
The horned spanworm’s a species
That would make Dr. Seuss proud.
Pam Otto is the outreach ambassador for the St. Charles Park District. She can be reached at firstname.lastname@example.org.