Ask The Entomologist

Our inchworms and their violent kin

Mike found this inchworm in the woods recently.

Inchworms are moth caterpillars, specifically members of family Geometridae. Their family name means something like “the earth measurers”, after how they fold up and then stretch themselves out, like a surveyor’s measuring rope.

As you can see, this little fellow nearly lives up to the name.
About 6 millimeters too short, though.
And its classic gait. – note that it has all three pairs of real legs right by its head, at the left. The footlike things on the right are just extensions of the abdomen.

Ours around here aren’t all that interesting. They’re pretty standard caterpillars – standard herbivores, or opportunistic omnivores. And, like most larvae, they are very hard to identify beyond family until they molt to adulthood.

Hawaii, however, has an interesting lineage of carnivorous inchworm caterpillars. The prevailing thought is that when the Hawaiian islands were first colonized by insects, few of those pioneering species were predatory. After all, carnivores require a healthy prey population to do well. As time went on, a certain group of inchworm caterpillars adopted a predatory role – after all, there wasn’t much competition.

A good videoclip of an ambush hunter inchworm.

As time went on, the caterpillars radiated into different species with different preferred prey and hunting tactics. Some mimic twigs, others mimic leaves. Some rely on ambushes alone, while others bind up their prey with silk before consuming them. The ones I first read about prefer hunting flies, while others eat things as strange as snails.

Looking at a caterpillar move, it’s initially hard to fathom it ever moving with sufficient speed and dexterity to grab a fly, but amazing things can happen with time and the right selective pressures.

And another brief video of one of the fly-hunting species.

While we in the lower 48 do have a few omnivorous caterpillars, I’m not aware of any strict carnivores. Omnivory is easy to understand – say you have a passel of siblings all competing for the same leaves… sooner or later that sibling rivalry will result in somebody taking a bite out of a brother, and realizing that he’s pretty decent food.

Many insect mothers lay extra unfertilized eggs, just in case some of the early hatchers are in the mood to eat potential siblings. Gives the slow larvae a bit longer to escape their hungry siblings.

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