At the end of July, after Sam and I came back from a walk, S_ said, “I have a surprise for you!” and showed us this:

That’s my wrist it is crawling on, by the way. It was just a bit over three inches long. S_ said, “I found it crawling on the porch in front of the house. Isn’t it huge and ugly? I saw it and immediately thought of you!”[1].

Handling it was a bit disturbing. Imagine picking up a raw sausage, and then having it suddenly thrash violently from side to side, and that was about what it was like to pick it up.

It had an interesting feature at the tail end, a structure that kind of resembled an eye. Not an insect eye, but a vertebrate eye:

The size, the color, the lack of hair, and the pseudo-eye on the rear were all pretty distinctive, and it was easy to narrow it down to the Abbott’s Sphinx Moth caterpillar, Sphecodina abbottii. It is a bit different from other sphinx moth caterpillars, which mostly have a horn or spike on the rear. The thing is, the Abbott’s Sphinx changes quite a bit as it grows. They start out green, with a horn on the end segment like other sphinxes. When they are about half-grown, they turn blue-green with an orange knob on the end. Finally, in the last stages before they pupate, they either look like this one, or they turn brown with a set of ten large, pale green “saddles” running down their backs. The larvae eat either grape leaves or virginia creeper leaves[2], and there is some speculation that the color phase with the ten green saddles is adapted to look like a bunch of unripe grapes.

The pseudo-eye makes the tail kind of resemble a snake head viewed in profile. A lot of the things that prey on caterpillars are kind of cautious about snakes, so this could scare off most of the smaller predators. The lashing behavior is pretty darned startling, too, so anything that tries to pick it up and eat it is likely to drop it. Supposedly they also hiss (which is an odd thing for a caterpillar to do), but I didn’t hear any hissing from this one.

An interesting point is that it has evolved a form that mimics a snake in profile, which means that it can get away with simulating only one eye instead of two. But, two pseudo-eyes would be a much more convincing snake mimicry, so why didn’t it go for two? Well, the structure it developed the pseudo-eye from is the “horn” that is characteristic of sphinxes. And, its ancestors only had the one horn. So, they were constrained – if there was only one horn, then they could only fake up one eye. This comes up a lot in evolved systems: a structure gets co-opted for a new purpose, but it is constrained by its starting structure and can’t be formed into exactly what is needed. Instead, it ends up being turned into something that is not perfect, but is good enough. Kind of like the way the human backbone gives a lot of problems, but functions well enough for us to get by most of the time.

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[1] And then she said, “Whoops, I didn’t mean for it to sound like that”.

[2] This one was probably on virginia creeper, seeing as how we don’t have any significant grape vines around. The virginia creeper (or “five-leaved ivy”) is pretty plentiful on the hill leading down to the swampy area, to the point where it is engulfing and dragging down some of the smaller trees.