Conjuring Nature’s Ghosts
In my last post, I pondered what survival strategies the hickory horned devil caterpillar could be using
in the middle stages of its larval life. On the one hand, they are not huge and
threatening yet, and clearly not toxic to predators (no toxic foodplant, no
stinging spines). They also are an inconvenient color (brown) among the green
leaves they feed on during the day. This should be a recipe for disaster…
But they do have these freaky looking horns – harmless
spines that are kind of like antlers. These can’t be easy to handle when you’re
a caterpillar crawling around a tree… which means there must be some advantage
to having them.
Looking at them a bit more, I remembered something from my
own past experience. I actually had seen a hickory horned devil just once
before raising them. Seven years ago, I was new to Charlotte, and out walking
my dog in my neighborhood. Watching the
dog sniff things, I noticed some large caterpillar droppings on the pavement. I
looked up, and there, just above my head, was a fifth instar hickory horned
devil clinging to some sweet gum leaves. I jumped back, which, for me, was an
odd reaction to spotting a spectacular caterpillar. As you’ve probably figured
out from reading this blog, I think caterpillars are cool and I’m not at all
frightened of them.
But I am, inexplicably, frightened of spiders (I always have
been) and there was something spiderlike about this caterpillar. My spider fear
is literally instinctual (I say it’s “hard-wired”) – I generally have no fear
of other bugs, even ones that bite. This is actually a common human fear
(psychologists call it “arachnophobia”) and I’ve read scientists who theorize
that many of us come with it built into our brains because, in the distant
past, our ancestors lived in places where spiders were frequently dangerous, so
evolution has favored a built-in fear. I believe this, because it is definitely
something primal in my brain, like a fear of barking dogs or of snakes.
So, look again at the photo of the fourth instar caterpillar
above and look at these orb-weaving spider photos:
Notice the similarities between the spiders’ conspicuous
legs and the caterpillar’s spines? There are eight large spines, and even in
the fourth instar, they look distinctively like these native, long-legged
spiders. Yuck.
And I can tell you (with an inner quiver) that we have a lot
of these spiders in Charlotte, hanging their webs from tree branches. In
August, about the time the hickory horned devil caterpillars reach their fourth
and fifth instars, orb weavers also mature and come down out of the treetops
where they have been growing in obscurity and hang their webs out in the open.
At this time the hickory horned devils are becoming so large that they too
become conspicuous.
Why would a caterpillar mimic a spider? Spiders bite, but
these spiders are not dangerously venomous like, say, the black widow spider…
one wouldn’t expect birds to be afraid of them because of that. Of course, they
are dangerous to other insects, particularly
to flying insects…
So here may be our answer – you may remember that I
mentioned in earlier posts that parasitic wasps are one of the biggest dangers
that giant silk moth caterpillars face, destroying large percentages of every
brood. A big spider is a dangerous predator to a little wasp. The hickory
horned devil’s spider mimicry may frighten away the parasites.
But, thinking about it, there still may be an effect here on
bird predators as well. I’ve always found it puzzling that big orb weaving spiders
(like those whose photos you see above) are so conspicuous – large, brightly
colored, hanging in the middle of the web with no cover, where a flying bird
could easily pluck them. Yet birds
don’t. I did a search of the literature on bird predation on orb weavers and
found that biologists find that bird predation appears to be uncommon. In
nature, bright coloration is sometimes a sign that an insect is toxic and not
good to eat. Yet again, there is no mention of this in the literature – surely
someone has looked into these big arachnids
being bad tasting… They are not poisonous in bite or taste, yet they advertise
their presence… hmmm.
So let me propose an alternative –perhaps birds leave orb
weavers alone because they are hard-wired to be instinctively frightened of
large spiders just like I am, though there is no longer a real threat in these
particular bugs. In the tropics there
are large orb weavers that are more
venomous and sometimes prey on small birds. In other words, evolutionary
history has given large spiders a fearsome reputation, and, though they may not
still deserve the fear, they are still using it for protection. And so does the
hickory horned devil… Deep instinctive fear is a force and it has echoes.
See -- totally harmless!
History has meaning to humans because we learn lessons from
it. In the battle for survival that we
call nature, some lessons have value for other species as well, and they too find
ways to preserve the history.
Anyway, it’s a hypothesis.
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ReplyDeleteHere is another idea. I encountered a fourth instar (or thereabouts) hickory horned devil yesterday, and was struck by its defensive posture of displaying those black spots and splaying out its large spines. It was a distinctive posture and reminded me of another caterpillar - the saddleback slug caterpillar. These caterpillars sting like the dickens and I always seem to brush against one in the late Summer. Could the hickory horned devil be a Batesian mimic of those caterpillars?
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